Contributed Posters

Emilie Dressaire, Trinity College, CT, USA
Title: Drop impact on fibers: elasticity-enhanced capture
Abstract: The impact of a drop on a solid substrate results in rich short-time dynamics that involve spreading, splashing, receding, and/or bouncing. For some industrial and agricultural applications, such as the treatment with pesticides, the drop size is comparable to the width of the solid structure, which has a fiber-like geometry. In this situation, the impact can result in the capture of the drop by the fiber or its release with a reduced momentum. Previous studies have shown that the efficiency of drop capture by a rigid fiber depends on the impact velocity and defined a threshold capture velocity below which the drop is captured. However, to achieve a better modeling of biological substrates, the coupling of elastic and capillary effects is necessary. In the present work, we study experimentally the dynamics of drops impacting on a flexible fiber. We investigate the effects of the fiber's elasticity and geometry on the capture mechanism. Our study shows that the flexibility of fibers increases the threshold capture velocity. We conclude that tuning the mechanical properties of fibers improves the efficiency of droplet capture.
Elise Lorenceau, IFSTTAR and Laboratoire Navie'r, France
Title: Coalescence of particle-laden interfaces
Abstract: We discuss the ability of hydrophobic particles to prevent coalescence between droplets. Indeed, two particle laden interfaces do not coalesce when pressed again each other if the applied pressure is below a critical value. To understand this threshold, we provide a quantitative characterization of the mechanical properties (surface tension and bending modulus) of the coated interface considering surface wave propagation. We discuss the results obtained as a function of the diameter of the particle and the shape of the particle (various shapes can be synthesised using continuous-flow lithography techniques ). We eventually give the critical pressure of coalescence and compare it to the impalement transitions on superhydrophobic micropatterned surfaces.
Mayuko Murano, Ochanomizu University, Japan
Title: Bursting of a bubble confined in between two plates
Abstract: In recent years, many studies in confined geometries have revealed physical mechanisms of the dynamics of small drops and bubbles. As for bursting of a liquid film sandwiched in between another liquid immiscible to the film liquid, it is reported that the thin film bursts at constant speed and the speed depends on the viscosity of the surrounding liquid when the film is less viscous [1]. In this research, we study the bursting speed of a thin film sandwiched in air to seek different scaling regimes. By measuring the bursting velocity of an air bubble with a high speed camera, we have found several dynamic regimes of the bursting. We clarify physical mechanisms of the bubble bursting by theoretical and experimental examinations.
This work is done in collaboration with Ko Okumura and Natsuki Kimoto.
Reference: [1] Ayako ERI and Ko OKUMURA, Phys. Rev. E Rapid Communication, 82, 030601 (2010).
Marie Tani, Ochanomizu University, Japan
Title: Imbibition dynamics on surfaces of legs of a small animal and on artificial surfaces mimicking them
Abstract: Recently, imbibition of textured surfaces covered with homogeneous micro-pillar arrays has been actively studied partly because of the potential for transport of a small amount of liquids. In most cases, the dynamics is described by the Washburn law, in which the imbibition distance scales with the square root of elapsed time, while different scaling laws have been recently found, for example in [1]. In this study, we studied imbibition on legs of a small animal that absorbs water via its legs [2] to find yet another scaling law. Furthermore, imbibition of artificial surfaces mimicking the leg surface was found to be described well by a composite theory.
This work is a result of collaboration with Daisuke Ishii, Shuto Ito, Takahiko Hariyama, Masatsugu Shimomura and Ko Okumura. [1] Obara and Okumura, Phys. Rev. E 86, 020601R (2012).
[2] Ishii, Horiguchi et al. Sci. Rep. 3, 3024 (2013).
P. J. Park, Korea National University of Transportation, South Korea
Title: Dynamics of a Polymer near a Fluctuating Membrane
Abstract: Steric interaction between a fluctuating membrane and a polymer is calculated using the equilibrium statistical mechanics. By employing the Fokker-Planck equation, we study the dynamics of a polymer near fluctuating surfaces. The diffusion controlled reaction dynamics between a receptor on the membrane and a polymeric ligand is investigated analytically, and the role of membrane fluctuation is identified. Our results will be useful in understanding the dynamics near biomembranes and the role of thermal fluctuations therein.
T.-M. Hong, National Tsing Hua University, Taiwan
Title: Curling edge: A problem that has plagued scrolls for millenia
Abstract: Scrolls are a form of membrane that shares many properties with interface. In this work we study the curling edges that have plagued scrolls for thousands of year. The Chinese phrase, Qi-Wa, refers to the up curl on the lengths of handscrolls and hanging scrolls, which has troubled Chinese artisans and emperors for as long as the art of painting and calligraphy exists. This warp is unwelcome not only for aesthetic reasons, but its potential damage to the fiber and ink. Although it is generally treated as a part of the cockling and curling due to moisture, consistency of paste, and defects from the mounting procedures, we demonstrate that the spontaneous extrinsic curvature incurred from the storage is in fact more essential to understanding and curing Qi-Wa. In contrast to the former factors whose effects are less predictable, the plastic deformation and strain distribution on a membrane are a well-defined mechanical problem. We study this phenomenon by experiments, theoretical models, and Molecular Dynamics Simulation, and obtain consistent scaling relations for the Qi-Wa height. This knowledge enables us to propose modifications on the traditional mounting techniques, that are tested on real mounted paper to be effective at mitigating Qi-Wa. By experimenting on polymer-based films, we demonstrate possible relevance of our study to the modern development of flexible electronic paper.
Debasish Saha, Raman Research Institute, India
Title: Instabilities at interfaces between a Newtonian fluid and an aging non-Newtonian fluid in quasi-2D geometry
Abstract: The elasticity of glassy colloidal suspensions builds up as the system ages, and the mechanical properties of soft glassy suspensions change dramatically as time progresses. Colloidal suspensions of Laponite are loaded into a quasi-2 dimensional cell consisting of two glass plates separated by a thin gap (a radial Hele-Shaw cell). An immiscible Newtonian fluid, having low viscosity compared to Laponite, is injected at a constant flow rate through a hole in one of the glass plates of the cell to replace the aging Laponite suspension. The fractal behavior of the interfacial patterns has strong dependence on fluid injection rate, age of the colloidal suspension and surface tension between the two fluids. A transition from viscous to viscoelastic fingering is predicted and observed as the age of the colloidal suspension increases. For a miscible fluid, crack propagation patterns are observed at higher ages of Laponite where the colloidal suspension develops considerable elasticity due to aging. High speed imaging is used to investigate the similarities of such crack propagation patterns with those seen in solids. The fractal nature of the resulting interfacial patterns is monitored to distinguish the dependence of the transition behaviors i.e. from viscous to viscoelastic and viscoelastic to crack propagation on the rate of injection of the Newtonian liquid phase.
Etienne Reyssat, PMMH-ESPCI, France
Title: Drop and bubbles in wedges
Abstract: Constrictions or widening of porosity result in non-balanced capillary forces acting at the interface between two fluid phases in porous media. Gradients of confinement can thus be used to produce, break up or manipulate drops and bubbles. We investigate experimentally the motion of oil drops and air bubbles confined between two quasi-horizontal plates forming a sharp wedge. The confinement gradient drives the migration of drops of wetting fluid toward the apex of the wedge. The capillary driving force is balanced by viscous dissipation occurring both in the bulk of the drop and along the contact lines. We provide a minimal model that quantitatively explains the migrations dynamics. In particular, we observe and explain two asymptotic regimes associated to both dissipation modes. We also present various possibilities to trap, expel or transport fluid using confinement gradients.
Ildoo Kim, Seoul National University, South Korea
Title: Temperature effects on droplet size distribution
Abstract: Atomization of a liquid through the pressure-driven injection is not only of fundamental interests but also crucial to many applications such as internal combustion or air-breathing engines, flow cytometry and etc. Due to the phenomenon’s real-life importance, many studies have been conducted to reveal the influence of the injecting nozzle shape, ambient air pressure, surface tension of the fluid and etc. In this study, we focus on the effect of the temperature to the atomization process. We varied the temperature of a liquid from the room temperature 300K to approximately 600K. Such hot liquids are injected through a simple orifice injector, and the size distribution of the spray was measured by optical measurement technique. The study indicates that the higher temperature of the liquid yields the smaller median droplet size.
Jung Chul Kim, Seoul National University, South Korea
Title: Capillary rise within folded papers
Abstract: Liquid imbibition into papers is so mundane phenomenon that the process has been used in writing, cleaning, or other various works in human daily lives. The liquid flow is fundamentally due to capillarity caused by the interactions between porous structures of the paper and liquids. Recently, the imbibition into porous media has been studied intensely using hydrophilic micro pillar arrays and real papers as substrates where liquid film deposited. Almost all studies have addressed film flow or bulk flow only; however, film flow with bulk flow has not been studied yet even though the latter case is much more common in nature and laboratory. Here, we use a paper as an example of porous medium, which is folded so that bulk flow occurs on the corner as well as film flow through the paper fabrics. We propose a new theory, showing different power laws from that at the corner formed by impermeable walls (A. Ponomarenko, D. Que're', and C. Clanet, J. Fluid Mech. 666, 146-154, 2011). The difference is caused by the fact that Laplace pressure of the folded paper is independent of liquid height while it increases with height at the corner of impermeable walls. And experimental data show good agreement with the theories.
Julia Lee, Brown University, USA
Title: An experimental analog for the study of waving marine grass in tidal currents
Abstract: Tidal currents passing through submerged vegetation mix the fluid and facilitate various environmental and ecological transport processes. This fluid-vegetation interaction, where the submerged grasses behave like those on the ground waving from wind, results from a shear instability of the surrounding flow. We devise a two-dimensional lab scale analog of the fluid-vegetation interaction using ABS plastic filaments immersed in a soap film to simulate the grass blades in a tidal flow. The array of filaments spontaneously waves in response to the flow of the soap film. Our experimental system makes direct flow measurement possible for a detailed comparison with theory.
Joshua McGraw, Saarland University, Germany
Title: Slip effectis in dewetting polymer microdroplets
Abstract: Spherical caps on a substrate with less than equilibrium contact angles contract as a result of capillary forces. Applying the classical no-slip condition at the liquid-substrate interface results in diverging stress at the contact line. This divergence can be alleviated, however, by allowing finite flow velocity at the substrate, corresponding to the slip boundary condition. Experiments have been conducted in which glassy polystyrene microdroplets are placed upon, as substrates, different self-assembled monolayers (SAMs). The spherical caps are prepared such that initial contact angles are much less than the equilibrium contact angle. Above the glass transition temperature, a capillary induced flow is observed; the droplet radii shrink while their heights grow. Furthermore, the intermediate height profiles are highly non-spherical. Different SAMs give rise to differing slip lengths, resulting in dramatic changes to the temporal and morphological path these tiny droplets take toward their equilibrium spherical cap shapes.
Kyle Welch, University of Oregon, USA
Title: Macroscale Boltzmann statistics: Recreating statistical mechanics with a mechanically derived temperature
Abstract: We use chaotic Faraday waves to to create a macroscopic 2D pseudothermal environment in which we study dynamics, self-assembly, and pattern formation in interacting buoyant particles.  The chaotic surface waves create an effective temperature that is proportional to the driving amplitude.  We use Boltzmann statistics to measure interparticle potentials by tracking the distribution of particle positions.  This allows us to study interparticle interactions without interfering with the dynamics of the system.  We explore various systems of multiple interacting particles, focusing particularly on systems of particles linked together in a chain with stiff links, in analogy to polymers.  We report on predictions and empirical results demonstrating entropic spring-like behavior of such systems and their response to changes in parameters such as effective temperature and maximum/minimum bond angle.
L.-T. Huang, National Taiwan University, Taiwan
Title: Using Spatially-Designed Heat Source to Control Flow Patterns Induced by Natural Convection
Abstract: Controlling the alignment or patterns of materials on a surface in a large scale can have applications in fabricating functional polymer materials or tissue engineering scaffolds. Herein, we used the idea of natural convection to create and control special flow patterns in the fluid and at the surface (or boundary) of a fluid.  The novelty of this work is to use spatially designed heating wires as heat source. We bended the wires to desirable shapes to control the temperature gradient in the system, which inducing natural convection flow with the corresponding pattern. We experimentally demonstrated how the wire-wire distance and the bended angle could influence the pattern resolution.  In addition, we used a mathematical model to simulate the system and found that the simulation results were comparable to our experimental observations. We conducted the dimensional analysis to reduce the system variables and numerically evaluated the range of possible scenarios of temperature and flow pattern in space.  The model suggests how we can adjust the pattern resolution by the system properties, which could provide us some insights into how this technique can be used in material and biotechnology applications.
Lucie Ducloue', IFSTTAR and Laboratoire Navie'r, France
Title: Rheological properties of suspensions of bubbles in a yield stress fluid
Abstract: We study the macroscopic response under shear of suspensions of bubbles in yield stress fluids. Well-controlled model suspensions are prepared by an original method that consists of mixing a monodisperse foam with a concentrated oil in water emulsion, both having the same continuous phase of a surfactant solution. The interstitial yield stress fluid (the concentrated emulsion) behaves as a solid visco-elastic material below a critical level of stress, and as a shear-thinning fluid above this yield stress. We measure the change in the macroscopic response (elastic modulus, yield stress, non-linear viscosity) due to the addition of air bubbles to the fluid. We find that for a given emulsion, the elastic modulus is a decreasing function of the gas volume fraction ϕ, this decrease being all the sharper as the bubbles are big. By contrast, we observe that the yield stress of most studied materials is not modified by the presence of bubbles, whereas the non-linear viscosity during flow increases with the bubble volume fraction. We show that the key mechanism at the origin of those apparently contradictory changes in the behaviour is the deformability of the bubbles in the fluid. To quantify this effect, we introduce capillary numbers which compare the stresses exerted on a bubble during a measurement to the stresses due to surface tension. We thus compute an elastic capillary number, relevant in the solid regime, a plastic capillary number relevant at the yield stress and a viscous capillary number relevant during flow. Those numbers are found to be very different in the solid and in the liquid regimes, explaining why the elastic, plastic and viscous behaviours under shear are not affected in the same way by the presence of bubbles. Our results are quantitatively well predicted by a micromechanical approach taking into account the experimentally measured capillary numbers.
Martin Brinkmann, Max Planck Institute for Dynamics & Self Organization, Germany
Title: Contact line dynamics on chemically heterogeneous surfaces
Abstract: Modeling the dynamics of liquid interfaces in contact to a heterogeneous solid involves a multitude of dierent length scales. A description of the unsteady motion of the three phase contact line on a substrate with spatially varying wettability remains a challenging task, even for highly viscous liquids where inertial eects can be safely neglected. To investigate this problem in the framework of continuum mechanics, we rst consider the motion of an effectively two-dimensional viscous droplet on a plane substrate using the boundary element method (BEM) to numerically solve the Steady Stokes equation [1]. On a smooth, sinusoidal wettability pattern, the droplets can be found in either a periodically moving or in a pinned state for the same value of the volume force driving the liquid parallel to the surface. This dynamic bistability is observed if the slip length is comparable to the height of the droplet [1] and, at the same time, the extension of the droplet is close to a multiple of the wavelength of the pattern. The complementary case of a liquid interface forced to spread on the substrate with a given average velocity is studied in the asymptotic limit of small contact angles and small amplitudes of the wetting energy employing a linear response formalism. As expected, we observe a continuous transition from a smooth contact line motion at high contact line velocities to a stick-slip motion at low velocities. We show that the time averaged microscopic contact angle can be directly related to the increase of the macroscopic dynamic contact angle that arises in the presence of the wetting heterogeneities.
[1] D. Herde, U. Thiele, S. Herminghaus, and M. Brinkmann, "Driven large contact angle droplets on chemically heterogeneous substrates", EPL 100: 16002 (2012)
Maria Fernanda Lugo Bolanos, Brown University, USA
Title: Photoelasticity method to measure the traction forces exerted by the foot in motion.
Abstract: When walking and running, the foot acts like a flexible viscoelastic object that cushions impact and stores elastic energy. To characterize the functioning of the foot as a spatially extended and flexible object, we require all components of the ground traction forces to be measured with sufficient spatial and temporal resolution. We present here the theoretical underpinnings of a method based on photoelasticity to measure these traction forces with millimeter scale spatial, and millisecond temporal resolution.
Mathieu Leocmach, Ecole Normale Supe'rieure de Lyon, France
Title: Wrinkling and nested buckling in a confined protein gel
Abstract: Syneresis is the expulsion of solvent during gel formation. It is an issue for the stability of food (yogurt, tofu, gelatin) and biomedical gels (agarose, tissue engineering). This process can be understood in terms of phase separation and surface tension: gelation is a phase separation where the concentrated phase (the gel network) is arrested. Surface tension within the gel drives the phase separation further, making the gel contract and expel the solvent. Here we induce the gelation and unidirectional synÊresis of a casein solution by slow acidification. The sample is confined in a thin cell thus forming a wall-water-gel-water-wall sandwich. Further decrease of the pH below the isoelectric point makes the gel layer swell inhomogeneously and wrinkle with a characteristic wavelength. The confined geometry then induces a cascade of successive buckling of the initial wrinkles. This novel nested pattern may have implication in morphogenesis and could be used as an optical filter with concentric annuli.
Niveditha Samudrala, Yale University, USA
Title: Adsorption of Sub-Micron Amphiphilic Dumbbells to Fluid Interfaces
Abstract: Geometrically and chemically anisotropic particles provide additional degrees of freedom to engineer the mechanical and thermodynamical stability of Pickering emulsions. We present direct measurements of the orientation of sub-micron amphiphilic dumbbell particles adsorbed to a fluid-fluid interface. In samples of varied geometry and chemistry, we find three recurring orientations of dumbbells and observe preferences within these states. We then show how the dominant orientation at the interface can be accurately predicted using a simple geometrical model. This calculation provides insight into how the shape and composition of dumbbells can be tuned to stand upright and pack efficiently on curved interfaces.  We then propose our hypothesis on the adsorption mechanism to the interface.
Oliver Baeumchen, Max Planck Institute for Dynamics & Self Organization, Germany
Title: Can Liquids Slide? - Capillary-Driven Flows as a Probe ofthe Slip Boundary Condition
Abstract: The precise control of the motion of small amounts of liquid on the micro- and nanoscale is of great technological interest, e.g. in lab-on-a-chip applications for pharmaceuticals, biological analysis or chemical reactions. For flows on such small length scales, the hydrodynamic boundary condition of a liquid at a solid surface plays an enormous role. In recent years much has been learned about this slip boundary condition from flows that are driven by internal, capillary, forces such as dewetting of thin liquid films [1,2]. In this case holes are nucleated in a flat film to expose some of the underlying substrate surface of lower energy and develop a characteristic fluid rim at their receding edges. In the course of this dewetting process the rim becomes unstable via an instability of Rayleigh-Plateau type. We explain how the development and macroscopic morphology of the instability are controlled by the slip of the film on the substrate: liquid slip not only affects the time scales and, hence, the velocities involved in flow processes or - e.g. in microfluidic channels - the throughput, it also has implications on the spatial morphology. In other words, by monitoring a retracting front of a liquid film through an optical microscope, one can judge whether or not slippage is at work [3].
In the second part, we show that also the opposite approach represents a powerful nanofluidic tool to elucidate the fluid dynamics on small lengths scales: Liquid surfaces with non-constant curvature are unstable and relax towards a flat free interface. In a combination of experiments and theory, we show that the capillary levelling of initially curved liquid surfaces, e.g. stepped polymer films, is sensitive to the nano-rheological properties of the liquid [4] and also the slip boundary condition at the solid/liquid interface. Thin film models comprising slip enable a quantification of the interfacial mobility of macromolecules and to access interfacial slip on the molecular level.
[1] O. Baeumchen, R. Fetzer, and K. Jacobs, Phys. Rev. Lett. 103, 247801 (2009).
[2] O. Baeumchen, and K. Jacobs, Soft Matter 6, 6028 (2010).
[3] O. Baeumchen, L. Marquant, R. Blossey, A. M¸nch, B. Wagner, and K. Jacobs, submitted (2014).
[4] J.D. McGraw, T. Salez, O. B‰umchen, E. RaphaÎl, and K. Dalnoki-Veress, Phys. Rev. Lett. 109, 128303 (2012).
Ryotaro Shimizu, University of Tokyo, Japan
Title: A new mechanism for coarsening in immiscible fluid mixtures
Abstract: In our daily life, after shaking a salad dressing, we see the coarsening of oil droplets suspended in vinegar. Such a demixing process is ubiquitous in nature and also of technological importance. The basic mechanism of the phenomena has been believed to be well understood and described by the so-called Brownian coagulation mechanism: thermal force noises exterted by molecules induce random Brownian motion of individual droplets, which leads to accidental collisions between them and the resulting coalescence driven by interfacial tension. Repetition of this elementary process leads to droplet coarsening with time. In this mechanism, which is reminiscent of the process of colloidal aggregation, a system does not know how to decrease the total free energy until accidental collisions take place. Here we question the validity of this well-accepted mechanism written in textbooks and show with a help of numrical simulation that the motion of droplets is not random but deterministic in the sense that it is under the control of the free energy. We demonstrate that the droplet motion is a consequence of a non-trivial coupling between diffusional and hydrodynamic degrees of freedom. Spatial inhomogeniety of the chemical potential reflecting inhomogeniety of droplet size induces the interface tension gradient in each droplet, which leads to the motion of a droplet towards a more smaller neighboring droplet via Marangoni effects and induces collisions between them. In summary, the coarsening process in bynary fluid mixtures of an asymmetric volume fraction has a stochastic yet deterministic pathway of material transport from smaller to larger domains.
S.-K. Hu, National Taiwan University, Taiwan
Title: The Transport Phenomena of Membrane Species in 2-D Lipid Membranes under Hydrodynamic Force above the Membrane.
Abstract: The transport phenomena of cell membrane species in the 2-D lipid membrane plays an important role in cellular processes and remains as an unclear issue. Developing an appropriate theoretical method to describe movement of membrane species in 2-D membranes can enhance the fundamental understanding of cell membrane dynamics. For cell membrane studies, supported lipid bilayer (SLB) is one of the most common model systems. We here present a theoretical model to describe the experimentally observed motion of membrane associated species when encountering the hydrodynamic force from the bulk solution above the lipid membrane. To obtain the experimental results, we used laminar focusing technique to form SLBs with the desired model membrane species originally locating at certain region. Then, we kept a steady flow above the SLB in a microfluidic channel, and the velocity of the fluorescently labeled target membrane species relative to the solid support can be observed by fluorescence microscopy. To obtain the theoretical predictions, we considered all of the possible forces applied to the species and setting the net force to be zero to obtain the steady state velocity. The hydrodynamic force applied by the bulk solution flow on the hydrophilic part of the species was calculated by Navier-Stokes equation. Evans-Sackmann model was incorporated to estimate the induced drag force inside the lipid membrane in our theoretical model. We tested several different model membrane species with various hydrophilic and hydrophobic sizes and found that their experimentally observed motions are consistent with the predictions from our theoretical model. The proposed model could provide us a method to predict the movement of membrane species under the bulk solution hydrodynamic force, based on the size of their hydrophilic head group and hydrophobic part embedded in the membrane.
Hunter King, Harvard University, USA
Title: Wrinkling and Crumpling of a Sheet on a Drop
Abstract: An ultrathin circular polystyrene sheet floating on the surface of a water drop stretches radially and compresses along its circumference as the curvature of the drop increases. The compression is at first fully relaxed by an axisymmetric wrinkle pattern extending inward from the edge, the extent of which agrees with predictions using a recently developed strategy of post-buckling analysis.  At larger curvature, axisymmetry is further broken as cusps, reminiscent of stress-focusing d-cones, gradually dominate the shape.  From optical data, we find a subtle transition from wrinkling to crumpling when the wrinkle length reaches half the sheet radius.  Using optical profilometry, we observe the transition locally, as wrinkle tips join to form crumples, and track the distribution of higher-order parameters such as gaussian curvature.  Surprisingly, the stress field in the central, unbuckled portion is not sensitive to the appearance of crumpled features. The transition shows little hysteresis and is smooth with respect to measured quantities.
Jason Wexler, Princeton University, USA
Title: Failure of liquid-infused surfaces
Abstract: Recent research has shown that rough or patterned surfaces infused with a lubricating liquid can display superhydrophobic properties. However, infused surfaces may lose their novel properties if the imbibed liquid film drains from the surface and exposes the underlying roughness.  If such a surface is subject to external flow, the shear induced by the outer phase can drain the lubricating layer, causing regions of the surface to transition to a hydrophilic Wenzel state. In addition, the high density of lubricating liquids means that this loss can  be driven by gravity alone.  We examine the shear- and gravity-driven failure modes of liquid-infused patterned surfaces experimentally, and develop a unified model to predict the dynamics of drainage via these two types of forcing.  Remarkably, the dynamic evolution of the two drainage mechanisms takes on a single functional form.  Under the influence of gravity, we show that a finite length of the pattern will remain filled indefinitely; this is a variant of the familiar capillary rise height.  Under the influence of external shear, a length of the pattern will also remain filled, this time depending instead on the shear parameters.
Khoi Nguyen, Brown University, USA
Title: Evaporation rate profile near the contact line of a droplet
Abstract: The coffee ring effect, the observation that coffee stains are darker at the rim than the center, is caused by the transport of the coffee particles to the edge due to an evaporation driven flow. Application of this transport has been proposed for small wire assembly, stretching DNA and for disease detection in blood. Because this transport is evaporation-driven, an experimental investigation of the evaporation rate will describe the transport dynamics. We present an optical system using plasmonics techniques to obtain a concentration profile of the bottom-most layer of a droplet. The space and time dependent evaporation rate of a liquid droplet can be inferred from this concentration profile. With an understanding of evaporation, we can characterize the convection patterns inside the droplet and model its’ behavior in various applications.
Kevin Roger, Lund University, Sweden
Title: Self-assembly in interfacial gradients induced by evaporation: structure and dynamics
Abstract: The structure chosen by an aqueous system that can self-assemble largely depends on water concentration and chemical potential. Bringing this system in contact with dry air leads to evaporation of water and thus the formation of a water concentration gradient in the interfacial layer. This gradient is controlled by the boundary conditions and the diffusion coefficients of the different species. A crucial point is to describe the coupling between structure and transport at such interfaces, essential in a problem such as skin hydration or gas exchange in lung alveoli.
Using (lipid or oil)/surfactant/water systems, I will discuss the changes in self-assembly structures due to this interfacial gradient. I will show that the time-scale and dynamics of formation are determined by the species and boundaries conditions. A key question I want to address is the importance of the non-equilibrium conditions: do we move in the equilibrium phase diagram or do we obtain new self-assembly structures?
Mohamed Gharbi, University of Pennsylvania, USA
Title: Elasto-capillary interactions between solid spheres at smectic thin films
Abstract: Colloidal particles organize spontaneously at fluid interfaces owing to a variety of interactions to form well organized structures that can be exploited to synthesize advanced materials. While the physics of colloidal assembly at isotropic interfaces is well understood, the mechanisms that govern interactions between particles at liquid crystal interfaces are not yet clearly established. In particular, smectic liquid crystal films offer important degrees of freedom that can be used to direct particles into new structures. In this work, we report the behavior of solid micrometric beads with homeotropic anchoring confined at interfaces of thin smectic films. We study the interactions and self-assembly of these particles in both supported and free standing films. When particles are captured in thin membranes, they induce distortions of the smectic interface to satisfy wetting properties at particle boundaries, leading to capillary interactions. These forces compete with elastic ones induced by the distortion of the smectic layers. The resulting potential drives assembly of the spheres into new different structures in a self-assembly process. Recent progress in understanding the mechanism of particle self-organization is presented.
Michael Miller, Brown University, USA
Title: Attractive behavior of the superficial: Cheerios effect for object with sharp corners
Abstract: Objects suspended on a fluid interface by surface tension are subject to attractive and repulsive forces towards each other.  These forces, dubbed the "Cheerios effect", are not well understood for objects whose contact lines contain sharp corners.  We employ optical refraction from the meniscus to obtain the shape of the liquid surface, and use the shape to calculate the forces acting on suspended object.  From these experiments, in concert with a boundary integral method applied to the Young-Laplace equation of a liquid surface, we find a scaling of the attractive force as a function of the contact line geometry alone.  For contact lines with corners, these forces are shown to concentrate at vertices, resulting in a net torque when forced by nearby objects.
Robert Botet, CNRS & Universite' Paris-Sud, France
Title: Drying suspension of nanoparticles: uncovering a novel interfacial phase
Abstract: Drying process is the appearance of a solid phase resulting from evaporation from a suspension of particles. This is an important process in paints, food processing and cosmetic creams, to cite only a few. Though known for ages, new data about drying processing are nowadays available, due to improvement in SAXS analysis and more powerful numerical simulations. Recently, we showed that drying aqueous dispersions of silica nanoparticles leads to formation of a thick (millimetric) interface between the liquid and the solid. This new phase has well-defined boundaries and corresponds to a specific range of solid volume fractions. Its structure is progressively anisotropic (as shown by SAXS analysis), with anisotropy following the uniaxial strain due to the draining flow of water toward the solidification front. This interfacial phase is now well characterized, but not fully understood. We will discuss here what we see experimentally and what we know from Brownian Dynamic simulations. It seems that a key point is the breakdown of ergodicity (compared with the liquid phase). At a definite value of the volume fraction, the particles are trapped in deformable "cages" and the system behaves like a soft glass. We study the dynamical organization of the matter in this intermediate phase by analyzing the anisotropies of the 3D Voronoi cells in the numerical simulations, and noting the coherence with the SAXS data. Actually, the distribution of the Voronoi-cells anisotropies is amazingly wide, suggesting that the interfacial phase is not homogeneous but could be coexistence of isotropic (liquid-like) domains and strained (gel-like) domains. It is probable that the dynamical heterogeneity of this interfacial phase is related to the appearance of cracks in the solid phase, though the proper mechanisms are not yet known.
reference: arXiv:1309.1048
Raphael Sarfati, Yale University, USA
Title: Interaction between colloids bound to lipid membranes
Abstract: The interaction of colloidal particles through deformations of a membrane is an interesting extension of the better known problem of colloids on a liquid interface. It is a simple model system to understand the aggregation of curvature-inducing membrane proteins in cells. The aim of this project is to measure the interactions between micron-sized colloids bound to lipid vesicles. Using a combination of micropipette aspiration and optical tweezers, we perform equilibrium and out-of-equilibrium experiments on systems of two particles as well as on systems of several particles to explore possible many-body effects. The statistics of the colloids trajectories over different timescales reveal their interaction profile.
Stephen Strickland, North Carolina State University, USA
Title: Spatiotemporal dynamics of molecular monolayers on Faraday waves
Abstract: The transport of insoluble surfactant by gravity-capillary waves is of interest for industrial patterning processes, containing oil spills, and oceanography.  The well-studied Faraday instability, in which such waves emerge above a critical oscillation amplitude, provides a way of probing the spatiotemporal dynamics of a molecular monolayer.  Due to the difficulty of measuring the spatial distribution of surfactant, theoretical predictions of these dynamics have not been investigated in experiments.  To make progress in this regime, we use stroboscopic fluorescence imaging of a monolayer of NBD-PC deposited on the surface of Faraday waves in water, with the fluorescence intensity providing a measure of the local surfactant concentration.  Simultaneously, we measure the wave profile using Moire-imaging, in which a patterned light source reflects off the surface of the water.  First, we find that Faraday waves, but not the meniscus waves present below their onset, homogenize the surfactant concentration.  This homogenization is most prominent for a dilute monolayer where the spreading would otherwise stall.  This finding suggests that Faraday waves could provide a new method for controlling the spreading of surface contaminants.  Second, by observing the wave profile while slowly increasing the driving amplitude, we find that the presence of surfactants significantly increases the critical amplitude.  We observe a non-monotonic trend of this critical amplitude as a function of surfactant concentration, an effect that cannot be accounted for by simply considering the fluid surface to have a lower surface tension.  By additionally considering the effect of viscous damping due to the inextensible surface, we find quantitative agreement with the observed trend.
Thomas Salez, ESPCI, France
Title: A direct quantitative measure of surface mobility in a glassy polymer
Abstract: The simple geometry of a polymer film on a substrate with a step at the free surface is unfavourable due to the excess interface induced by the step, thus allowing for a new nanoprobe of the melt state rheology. After describing the experimental technique [1], we demonstrate how the theoretical tools [2] enable to directly probe the surface evolution of thin polymer films below the glass transition temperature Tg  [3]. While above Tg the entire volume between the substrate and the free surface participates to the flow, below Tg only a near surface region responds to the excess interfacial energy. In the latter case, the developed thin film theory for flow limited to the free surface region is in excellent agreement with experimental data. Strikingly, the system transitions from whole film flow to surface localised flow over a narrow temperature region near the bulk glass transition temperature. The measurements and model presented provide a quantitative measure of surface mobility in a sample geometry where the confinement of polymer chains and substrate effects are negligible. Therefore, this study may contribute to solve further the ongoing controversy around glass transition in thin polymer films.
[1] McGraw et al., PRL 109 128303 (2012)
[2] Salez et al., PoF 24 102111 (2012)
[3] Chai et al., SCIENCE, in press (2014)
Marcelo Dias, Brown University, USA
Title: Swimming near elastic interfaces at low Reynolds number
Abstract: Microorganisms are rarely found in Nature swimming freely in an unbounded fluid. Instead, they typically encounter other organisms, hard walls, or deformable boundaries such as free interfaces or membranes. Hydrodynamic interactions between the swimmer and nearby objects lead to many interesting phenomena, such as changes in swimming speed, tendencies to accumulate or turn, and coordinated flagellar beating. Inspired by this class of problems, we investigate locomotion of microorganisms near deformable boundaries. We calculate the speed of an infinitely long swimmer close to a flexible surface separating two fluids; we also calculate the deformation and swimming speed of the flexible surface. When the viscosities on either side of the flexible interface differ, we find that fluid is pumped along or against the swimming direction, depending on which viscosity is greater.
Ko Okumura, Ochanomizu University, Japan
Title: Simple views on the dynamics of fluids in confined space

Abstract: Recently, the dynamics of fluid drops has acquired considerable attention. We have been working on the dynamics, in particular, in a space confined in between two parallel plates, to find scaling laws, which is the focus of this talk. In such a “Hele-Shaw cell” geometry, dimensional crossovers and role of liquid thin films appear as important issues. For example, we have found the dimensional crossovers in the dynamics of drop coalescence, together with a self-similar dynamics [1]. In a different type of coalescence, the effect of symmetry breaking in bursting of a thin film has been revealed [2]. Studies on rising and sinking motion of fluid drops [3] and on the lifetime of bubbles in the Hele-Shaw cell have unveiled various boundary conditions realized in thin films. In the dynamics of imbibition of a certain textured surface, the dynamics has been found to be governed by balance of three effects of capillarity, gravity and viscous dissipation [4]. If the time allows, we will further touch on a spectacular effect of electric field on drop coalescence and a jamming transition concerning drag friction in granular medium [5] and toughness of biomaterials [6,7].

[1] Maria YOKOTA and K.O., Proc. Nat. Acad. Sci. (USA), 108 (2011) 6395–6398; featured in “This week in PNAS”.
[2] Ayako ERI and K.O., Phys. Rev. E, 82, 030601R (2010).
[3] Ayako ERI and K.O., Soft Matter, 7, 5648 (2011); highlighted as hot article.
[4] Noriko OBARA and K.O., Phys. Rev. E, 86, 020601R (2012).
[5] Yuka TAKEHARA, Sachika FUJIMOTO and K.O., EPL, 92, 44003 (2010).
[6] Yuko AOYANAGI and K.O., Phys. Rev. Lett. 104, 038102 (2010); featured in Nature Mater. 9, 190 (2010).
[7] K.O. and Pierre-Gilles de Gennes, Eur. Phys. J. E 4 (2001) 121-127. 


M. Ramaioli, Nestle' Research Center, Laussane, Switzerland.
Title: Wetting of soluble coatings and its relevance to the dissolution of dehydrated beverage poweders.
Abstract: When a powder is brought in contact with a solvent, a complex interplay of several phenomena conditions its dissolution. The reconstitution of a beverage by pouring a soluble powder onto water represents a typical example. We study this complex problem by simplifying it into several model problems. In this study, we characterize the dynamic wetting of soluble coatings of malodextrin [1, 2], an example of "reactive" wetting system. The effect of contact line velocity, water content and coating thickness is discussed. The role of water diffusion and the effect of the glass transition of the coating [3] are elucidated. These results allow deriving an upper bound for the capillary impregnation rate in soluble powders, a phenomenon conditioning strongly their dissolution performance.
[1] J. Dupas, E. Verneuil, M. Ramaioli, L. Forny and F. Lequeux, "Dynamic Wetting on a Thin Film of Soluble Polymer: Effects of Nonlinearities in the Sorption Isotherm" Langmuir 29, 40 (2013).
[2] J. Dupas, E. Verneuil, L. Talini, F. Lequeux, M. Ramaioli and L. Forny, "Diffusion and evaporation control the spreading of volatile droplets onto soluble films" Interfacial Phenomena and Heat Transfer 1(3), 231-243 (2013).
[3] J. Dupas, E. Verneuil, M. Van Landeghem, B. Besson, L. Forny, M. Ramaioli, F. Lequeux and L. Talini, "Glass transition accelerates the spreading of polar solvents on a soluble polymer" in press, PRL 2014.
Denis F. Hinz, OIST Graduate University, Japan
Title: Moving passive particles with self-propelled particles: Patterns and diffusion properties
Abstract: Although many biological systems consist of both self-propelled and passive agents that may be crucial for overall functionality, there are but a few studies concerning the properties of such mixtures. Here we formulate a model for mixtures of self-motile and passive agents and show that the model gives rise to three different dynamical phases: a disordered mesoturbulent phase, a polar flocking phase, and a vortical phase characterized through two large-scale counterrotating vortices. We use numerical simulations to construct a phase diagram and discuss the relation between the statistical properties of the different phases and self-motile bacterial suspensions. Our findings afford specific insights regarding the interaction of microorganisms and passive particles and provide novel strategic guidance for the efficient technological realization of artificial active matter.
Amy Shen, OIST Graduate University, Japan
Title: Manipulating particle shapes: Deformation and solidification of molten wax drops at an immiscible liquid interface
Abstract: The controlled generation of varying shaped particles is important for many applications: food processing, consumer goods, adsorbents, drug delivery and optical sensing. Here, we investigated the deformation of millimeter size, molten wax droplets as they impacted an immiscible liquid interface. Spherical molten wax droplets impinged on a cooling water bath, then deformed and solidified into various shapes such as mushrooms, thin discs, ellipsoids and spherulites. The final morphology of the wax particles are governed by the interfacial, inertial, viscous and thermal effects, which can be studied over a range of Weber, Capillary and Stefan numbers. By systematically varying the initial temperature and viscoelasticity of the molten drop, drop size, impact speed, viscosity and temperature of the bath, and the interfacial tension between the molten wax and bath fluid, we constructed a regime map for the various particle shapes that resulted. We solved a simplified Stefan problem for a spherical drop and estimated the time required to initiate a phase transition at the interface of the molten wax and water after impact, and correlated this time with the final wax particle shape to understand the influence of temperature difference between the two phases. We conclude that the competition between the thermal energy with kinetic, potential and interfacial energy leads to the final morphology of the wax drop.
Amy Shen, OIST Graduate University, Japan
Title: Flow induced irreversible gelation of ionic and non-ionic surfactants in a microfluidic device
Abstract: Surfactant molecules can self-assemble into various morphologies under proper combinations of ionic strength, temperature, and flow conditions. In particular, cylindrical micelles in the presence of salts can form flexible and elongated wormlike micelles. We consider the flow of ionic and nonionic wormlike micellar solutions through a microfluidic device containing microposts, with focus on their microstructural and rheological transitions. One wormlike micellar solution consists of cationic surfactant cetyltrimethyl amonioum bromide (CTAB) and sodium salicylate salt (NaSal). The non-ionic surfactant solution is a mixture of polyoxyethylene (20) sorbitan monooleate (Tween-80) and Monolaurin. When subject to strain rates (O(10^4)s^(-1)) and total strain (O(10^3)) we observed the formation of a permanent flow-induced structured phase (FISP) in both micellar solutions. The high stretching and local micellar concentration fluctuations in the microdevice induce an increment in the end-cap energy, which lowers the energy required to form wormlike micellar connections. As a result, FISP with highly entangled and multi-connected wormlike micellar bundles form.
Doojin Lee, OIST Graduate University, Japan
Title: Ferrohydrodynamics for energy harvesting and particle separation
Abstract: Ferrohydrodynamic energy harvesting and particle separation based on magnetorheology were investigated. For the energy harvesting, a physical model was introduced to understand the ferrohydrodynamic system with four interactive forces including buoyancy, interfacial tension, drag, and electromotive yield forces. It was demonstrated that the magnetic flux density increases with increasing the external magnetic field, while the rate of change of the magnetic flux density decreases with increasing the yield stress due to retardation of the air droplet motion. The output voltage and current were generated according to Faraday’s law of induction. A numerical simulation was performed to understand the fundamental mechanism, especially one single up-and-down peak. Also, the particle separation was successfully achieved inside a microchannel owing to the magnetorheology. Once particles are focused at the center due to the elastic force by the viscoelastic nature of a medium, they undergo different migration velocities depending on the particle sizes on account of the external magnetic force. We also investigated the physics behind the ferrohydrodynamics driven particle separation by using a numerical simulation.
Tamoghna Das, OIST Graduate University, Japan
Title: Structure and dynamics of two-dimensional aggregates from competing interactions.
Abstract: A two dimensional system of mono-disperse particles with competing short range attraction and long range repulsion is numerically investigated. Keeping the competing interaction strength fixed at low temperature and density, a dynamical transition from an exponential to power-law behaviour of self-fluctuations has been observed as a function of the repulsion length alone. This is accompanied by a structural transition from non-compact to compact  aggregates. Whereas strong bonding is responsible for non-compact cluster formation, caging dynamics in compact clusters result in non-exponential relaxation characteristic of glassy behaviour. With increasing temperature, the non-equilibrium aggregation gives way to an ergodic liquid. With increasing density, the system undergoes a geometric transition into a percolating gel state, independent of temperature and repulsion length.
Aisa Biria, OIST Graduate University, Japan
Title: A soap film bounded by a flexible loop: the role of twist
Abstract: The equilibrium of a flexible loop spanned by a soap film is investigated, in which the surface tension of the soap film competes with the flexural and twisting resistances of the bounding loop. In addition to the inherent physical and mathematical beauty of this problem, the system under consideration provides a simple model for various systems in which biomembranes are bonded by elastic filaments or tubes, such as high density lipoproteins, biofilms, and the dorsal mesentery. A pair of dimensionless parameters are shown to control bifurcations from a flat, circular ground state. While one of these measures the strength of the areal free-energy of the soap film relative to the lineal bending-energy of the flexible loop, the other combines the ratio of the twisting to bending rigidities of the loop with a dimensionless measure of twist. For sufficiently small values of the latter parameter, two separate groups of bifurcation modes are identified. On the other hand, for values greater than the critical twist arising in Michell's problem of the bifurcation of a twisted annular ring, only one bifurcation mode exists. Bifurcation diagrams indicate that a loop with greater twisting rigidity shows more reististance to transverse buckling. However, a twisted and closed filament spanned by a surface endowed with uniform surface tension buckles at a twist less than the critical value for an elastic ring.
V. Sathish Akella, OIST Graduate University, Japan
Title: An emulsion based technique to measure protein crystal nucleation rates of lysozyme
Abstract: We developed a microfluidic based technique to measure the nucleation rates of protein crystals. The technique involves nucleating large number of emulsion drops containing supersaturated protein solution at constant temperature and counting the number of drops containing no crystals with time. Using the proposed method, we measured the nucleation rates of lysozyme crystals. At each nucleation rate measurement we observed two nucleation rates ‘slow’ and ‘fast’, both of which are due to the presence of two different types of heterogeneous nucleation sites. At least one of the two types of nucleation sites are lysozyme aggregates. We analyzed the measured nucleation rates according to the predictions of Classical Nucleation Theory (CNT) and extracted the barrier heights and the size of critical nuclei associated with each nucleation process. According to CNT, nucleation rates vary exponentially with supersaturation; with the exponent characterizing the activation barrier associated with the nucleation process and the pre-exponetial factor (kinetic pre-factor) characterizing the growth kinetics of the nucleus. A detailed analysis of the kinetic pre-factor suggests that the pre-factor varies 9 orders of magnitude within a temperature range of 5 C. Contrary to conventional wisdom, the kinetic pre-factor plays a greater role in nucleation kinetics than does the activation barrier.
Research performed with Aaron Mowitz, Michael Heymann and Seth Fraden Physics Department, Brandeis University.
David Kleiman, OIST Graduate University, Japan
Title: Bend, twist, and stretch: The many shapes of Möbius bands
Abstract: For centuries, the Möbius band has been of interest to scientists and artists alike, however, while the Möbius band's presence in pop-culture grows, its equilibrium shapes remain elusive. To determine these equilibria we model a Möbius band using discrete points related through linear and angular harmonic potentials. The angular harmonic potentials are associated with energy due to out-of-plane bending while the linear harmonic potentials are associated with energy due to in-plane stretching. Whereas previous investigations have focused on strictly developable bands, our model accounts for both bending and stretching. Further, an approximately developable band may be obtained as a limiting case by making parameter choices that penalize stretching. To yield the equilibrium shape of the band, we use a conjugate gradient algorithm in LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) to minimize the total deformation energy. When stretching is allowed, we observe new equilibrium shapes. More stretchable bands exhibit near-planar midlines and have more evenly distributed mean and Gaussian curvatures when compared to their developable counterparts, and some stretchable bands exhibit an expanded circular centerline. These bands correspond the maximum ratio of stretching energy to bending energy. For the smallest length to width ratio considered, this maximum corresponds to a band collapsed into a self-intersecting non-chiral shape. Our results afford insights regarding the mechanical properties of topological micro- and nano colloids and can be applied for future design and manufacturing of novel materials.
Research performed in collaboration with Denis Hinz and Eliot Fried.