Invited Talks

Invited Talks (45 min. + 15 min. Q&A)

Rachel Levy, Associate Professor, Harvey Mudd College, USA
Title: The Spreading of Surfactants on Thin Liquid Films 
Abstract: Recent experiments have enabled us to observe the spreading of surfactants on thin liquid films and compare the results to mathematical models developed in the 1990s.  We will discuss outward surfactant spreading from a disk-shaped region as well as inward surfactant spreading to close a disk-shaped surfactant-free hole.   In both cases the goal is to form a uniform surfactant layer on the surface of the film and to model the dynamics as the system reaches equilibrium.
Anne Juel, Associate Professor, Manchester University, UK
Title: Interfacial instabilities on the pore scale
Abstract: What links a baby’s first breath to adhesive debonding, enhanced oil recovery, or even drop-on-demand devices? All these processes involve moving or expanding bubbles displacing fluid in a confined space, bounded by either rigid or elastic walls. In this talk, we show how spatial confinement may either induce or suppress interfacial instabilities and pattern formation in such displacement flows.
We demonstrate that a simple change in pore geometry can radically alter the behaviour of a fluid-displacing air finger both in rigid and elastic vessels. A rich array of propagation modes, including symmetric, asymmetric, localised fingers, is uncovered when air displaces oil from axially uniform tubes that have local variations in flow resistance within their cross-sections. The most surprising propagation mode exhibits spatial oscillations formed by periodic sideways motion of the interface at a fixed distance behind the moving finger tip. This rich behaviour is in contrast to the single, symmetric mode observed in tubes of regular cross-section, e.g. circular, elliptical, rectangular and polygonal. Moreover, we show that the experimentally observed states are all captured by a two-dimensional depth-averaged model for bubble propagation through wide channels with a smooth occlusion, which is similar to that describing Saffman-Taylor fingering, but with a spatially varying channel height.
David Quéré, Professor, ESPCI, France
Title: Symmetry breaking in air flow around a drop, and its consequences.
Abstract: We discuss how breaking the symmetry of a flow of air around a drop can be exploited to generate a force to propel the liquid. We consider two situations: 1) Leidenfrost drops, which produce vapor and make it flow, due to their weight. Placing anisotropic textures on the underlying solid can rectify the vapor flow, and produce drop entrainment. Different devices producing such an effect are presented and discussed. 2) Drops on fibers on which air is blown. Air flows perpendicular to the fiber a priori are symmetric, but there is a range in Reynolds number where vortices behind the obstacle (the drop) become asymmetric, which is found to generate a Magnus force able to propel the liquid. In addition, it produces a repulsive interaction between neighboring drops, which yields remarkable dynamics.
Contributors to this talk: Dan Soto, Hélène de Maleprade, Guillaume Dupeux, Philippe Bourrianne, Pierre-Brice Bintein, Hadrien Bense and Christophe Clanet
Eric Corwin, Assistant Professor, University of Oregon, USA
Title: Statistical Mechanics for Breakfast: Creating an Ideal Gas with your Cereal
Abstract: A few cheerios floating on milk makes a boring breakfast.  But if you shake your bowl rapidly up and down you can create a macroscopic statistical mechanics in which random surface waves play the role of quantum fluctuations. We have constructed an active, driven system of chaotic Faraday waves whose statistical mechanics mimic those of an ideal gas, the epitome of a microscopic thermal system.  Thermodynamics in thermal systems ultimately arises from microscopic quantum fluctuations while our system, with a macroscopic effective temperature, is the result of the random motion of the macroscopic Faraday excitations. We use real-time tracking of floating probes, energy equipartition, and the Stokes-Einstein relation to define and measure a pseudotemperature, diffusion constant, and coefficient of viscous friction for a test particle in this pseudothermal gas. Because of its simplicity this system serves as an ideal playground in which to explore non-equilibrium statistical mechanics, much as the ideal gas is the starting point for equilibrium statistical mechanics.  Further, we report on multiparticle interactions in this system using tunable surface-tension mediated interactions between 3D printed floating particles.
Hepeng Zhang, Assistant Professor, Shanghai Jiao Tong University, China
Title: Mechanics and Statistics of Bacterial Locomotion
Abstract: The first part of the talk focuses on the mechanical principle that a single bacterium uses to propel itself. We show that though widely-accepted resistive-force theory qualitatively describes the underlying principle of zero Reynolds number propulsion, it fails quantitatively in the biologically relevant regime due to the negligence of hydrodynamic interactions. In the second part, I will discuss a range of emerging phenomena observed in experimental systems consisting of many bacteria. These phenomena originate from interactions between self-propelled organisms; they include anomalous density fluctuation, scale-invariant correlation, turbulence-like flow pattern, and Jamming at high densities.
Ho-Young Kim, Associate Professor, Seoul National University, South Korea
Title: Liquid imbibition in cellulose sponge.
Abstract: A cellulose sponge, which is a representative example of a porous hydrophilic structure, can absorb and hold a significant amount of liquid within its pores. The strong wettability of its intertwined fibers drives the absorption against the resistance caused by gravity and viscosity. In this talk, we will present the results of experimental and theoretical investigation of the dynamics of the capillary imbibition of various liquids in the porous material. We classify the wicking processes into two categories based on whether the pores are completely filled or not. We find that the capillary rise dynamics in the sponge is different from that predicted by the classical theory of Washburn when the pores are partially filled with liquid. Namely, the dynamics no longer obeys the diffusive rule but follows a power law in time with an exponent different from 1/2 in a so-called partial-filling regime. We provide a theoretical model to explain this novel wicking dynamics. In addition to capillary rise, we touch upon the chemical attraction and consequent swelling of wet sponges.
Linda Cummings, Associate Professor, New Jersey Institute of Technology, USA
Title: Free surface dynamics of nematic liquid crystal
Abstract: Spreading thin films of nematic liquid crystal (NLC) are known to behave very differently to those of isotropic fluids. The polar interactions of the rod-like molecules with each other, and the interactions with the underlying substrate, can lead to intricate patterns and instabilities that are not yet fully understood. The physics of a system even as simple as a film of NLC spreading slowly over a surface (inclined or horizontal) are remarkably complex: the outcome depends strongly on the details of the NLC's behavior at both the substrate and the free surface (so-called "anchoring" effects). We will present a dynamic flow model that takes careful account of such nematic-substrate and nematic-free surface interactions. We will present model simulations for several different flow scenarios that indicate the variety of behavior that can emerge. Spreading over a horizontal substrate may exhibit a range of unstable behavior. Flow down an incline also exhibits intriguing instabilities: in addition to the usual transverse fingering, instabilities can be manifested behind the flowing front in a manner reminiscent of Newtonian flow down an inverted substrate. Finally, if time permits, we will also consider the influence of van der Waals' interactions, which may be relevant for extremely thin films.
This is joint work with Lou Kondic (NJIT), Michael Lam (NJIT) and Te-Sheng Lin (Loughborough). Funding from NSF DMS 1211713 is gratefully acknowledged.
Masao Doi, Professor, Beihang University, China
Title: Contact line dynamics in polymeric materials
Abstract: Contact line is the edge line of the interface between two materials (fluid, rubber, solid substrate etc).  When contact lines move, large deformation is created in the material. The energy dissipation associated with this deformation is the key factor in many practical problems, such as adhesion, de-adhesion, friction etc. Here I discuss some aspects of contact line dynamics for soft rubbery materials (soft rubber, pressure sensitive adhesives). It is shown that meso-scale structures (cavities and fibrils) created around the contact line is important in the energy dissipation.   
Shankar Ghosh, Associate Professor (G), Tata Institute of Fundamental Research, India
Title: Super-spreading of charged granular matter
Abstract: A Granular system is often assumed to have weak cohesive and adhesive interaction.  However, the commonplace observation of food grains sticking to the plastic bags in which they are sold tells us that such an assumption is not generic in nature. What we have done  is to take a large number of mono disperse glass particles (replacement for the food grains) in a plastic vial (replacement for the plastic bag)  and subject the system to a constant mechanical agitation. Because of triboelectric charging the glass particles stick to the wall of the vial and form a monolayer. The height of this is determined by the strength of the mechanical agitation. This observation is expected, what follows is not. The initial rapid growth is followed by a long period of dormancy. The monolayer then makes an abrupt transition into a super-spreading regime where the height overshoots in the form of a few-particles-wide but many-particles-long streaks which eventually merge to form a film ordered in horizontal layers. This abrupt transition is preceded by a precursor appearance of transverse periodic modulation of particle density, the high density regions of which form the seeds for the particle-jets that initiate the super-spreading regime. This work is the first report on spreading and superspreading of granular matter on vertical walls, a phenomenon common to liquids (capillary rise as an example of spreading and tears/legs of wine as an example of superspreading). The study opens up new ways to generate interfacial mass transport in granular systems, a problem of practical importance (e.g., printing processes) and  provides a model macroscopic system to investigate (with constituent level resolution) certain aspects of liquid spreading especially with regard to the formation of a precursor film and the response of such a film to stresses. 
William Irvine, Assistant Professor, University of Chicago, USA
Title: Unraveling knotted fields
Abstract: To tie a shoelace into a knot is a relatively simple affair. Tying a knot in a field is a different story, because the whole of space must be filled in a way that matches the knot being tied at the core. The possibility of such localized knottedness in a space-filling field has fascinated physicists and mathematicians ever since Kelvin’s 'vortex atom' hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types. Perhaps the most intriguing physical manifestation of the interplay between knots and fields is the existence of knotted dynamical excitations. I will discuss some remarkably intricate and stable topological structures that can exist in light fields whose hydrodynamic-like evolution is governed entirely by the geometric structure of the field. I will then turn to experimental hydrodynamics: how to make knotted vortex loop configurations in fluids and how they evolve once made.
Michael Dickey, Associate Professor, North Carolina State University, USA
Title: A Micromoldable Liquid Metal that Supports a Thin Oxide Skin
Abstract: This talk will describe efforts in our research group to study, manipulate, and utilize the thin oxide film that forms spontaneously on gallium-based liquid metal alloys.  The oxide layer provides unique opportunities to pattern liquids into non-equilibrium shapes as well as engineer the interfacial  properties of the metal.  Metals that are liquid at or near room temperature are interesting because they are soft and can flow readily in response to stress.  Thus, liquid metals can form conductive components that are conformal, deformable, stretchable, injectable, and shape-reconfigurable.  Mercury, gallium, and several of its alloys are liquid metals at or near room temperature.  Gallium is attractive because it has low toxicity, essentially no vapor pressure, and a low viscosity. The oxide that forms on gallium, which has conventionally been considered a nuisance, provides many new opportunities.  We will discuss the properties of the metal and its oxide within this context, as well as methods to control and reconfigure the shape of the metal by manipulating the oxide layer. Applications of this work include compliant electrodes, 3D printing, microfluidic components, soft memory devices, microscale droplets, ultra-stretchable antennas / wires / interconnects, self-healing circuits, shape-reconfigurable systems, and optical components.
Hans Jürgen Butt, Professor, Max Planck Institute for Polymers, Germany
Title: Fluorescence correlation spectroscopy to study dynamics at interfaces
Abstract: Two example of how fluorescence correlation spectroscopy can help to analyse the dynamics at interfaces will be discussed.
(1) An important step towards an understanding of hydrodynamics is determining the correct boundary conditions. A new method for direct studies of flows in the close proximity of a solid surface has been developed. It is based on total internal reflection fluorescence cross-cor­relation spectroscopy (TIR-FCCS). The effect of TIR is used to create an evanescent optical field that excites fluorescent tracer particles flowing with the liquid. Using this method we measured slip of water on hydrophilic and hydrophobic surfaces. Within an accuracy of at least 10 nm no slip was found on either surface.
(2) Fluorescence correlation spectroscopy was applied to analyse the two-dimensional diffusion of nanoparticles, dendrimers, and rhodamine 6G at water-alkane interfaces. These tracers served to analyse the nanorheological properties of the interfaces at different length scales. A slowdown of the nanoparticle diffusion at the liquid-liquid interfaces was observed. The effect was most evident when the viscosities of both liquid phases were similar, i.e. at decane-water interfaces. In contrast rhodamine diffuses faster at the alkane-water interfaces than in bulk. This fast diffusion may be explained by the existence of an interfacial layer with thickness comparable to the tracer size that has a different structure and dynamics than the bulk water.
1.         Wang, D., S. Yordanov, H. Mohan Paroor, A. Mukhopadhyay, C.Y. Li, H.-J. Butt & K. Koynov: Probing diffusion of single nanoparticles at water-oil interfaces. Small 20117, 3502-3507.
2.         Wang, D., L. Pevzner, C. Li, K. Peneva, C.Y. Li, D.Y.C. Chan K. Müllen, M. Mezger, K. Koynov & H.-J. Butt: Layer with reduced Vis­cosity at water-oil interfaces probed by fluorescence correlation spectroscopy. Phys. Rev. E 201387, 012403.
3.         Schaeffel, D., S. Yordanov, M. Schmelzeisen, T. Yamamoto, M. Kappl, K. Koynov, B. Dünweg & H.-J. Butt: Hydrodynamic boundary condition of water on hydrophilic and hydrophobic surfaces. Phys. Rev. E 201387, 051001.
Kathleen Stebe, Professor, University of Pennsylvania, USA
Title: Energy stored in deformation fields: Opportunities for directed assembly in soft matter
Abstract: Colloidal particles are often directed to assemble by use of applied external fields – e.g. by exploiting particle charge or ferromagnetism, and by applying electro-magnetic fields to induce interactions and to steer the particles into well-defined structures at given locations. Here, we exploit fields that arise spontaneously when microparticles are placed in contact with deformable matter. In particular, we have been exploring energy stored in deformation fields around microparticles as a means of directing colloidal assembly.
In one context, we use capillary interactions that occur between anisotropic microparticles at fluid interfaces. The microparticles have undulated contact lines owing to wetting boundary conditions; the fluid interface deforms, creating an area field around the particle that bears the signature of the particle shape and wetting. The product of this area and surface tension is an energy field, which we exploit to direct particles to migrate, orient and assemble. We focus on the role of particle shape in determining pair interactions. At planar interfaces, interactions in the far field obey a universal form. In closer proximity, particle aspect ratio impacts preferred alignment. Near contact, faceting, corners, and particle roughness can dominate the capillary energy landscape, dictating equilibrium configurations. On curved interfaces, particle deformation fields couple to interface curvature in analogy to charges migrating in applied electric fields. Particles orient and migrate in curvature gradient. Even planar particles, which would not interact on planar interfaces, migrate in curvature fields.
In another context, we exploit elastic energies and defect fields that arise in confined liquid crystals. For example, when a nematic liquid crystal is confined using surfaces with well-defined anchoring energies, the director field and associated defect fields can be molded to store elastic energy. This energy can be used to steer particles within the bulk or particles that are trapped at the nematic-air interface. We explore this theme using topographically patterned solid surfaces to define defect fields that steer particles trapped at fluid interfaces into assembles mimicking the defect texture. Related examples for particle migration in smectic films, with either free surfaces or on topographically complex surfaces are discussed.
Keng-Hui Lin, Associate Professor, Academia Sinica, Taiwan
Title: Investigating cellular behaviors in three dimensions by cellular solids of monodisperse foam
Abstract: Traditionally, cell biological investigations have mostly employed cells growing on flat, two-dimensional, hard substrates, which are of uestionable utility in mimicking microenvironments in vivo. We designed a novel caffold to achieve cell culture in the third dimension (3D), which offers control over substrate stiffness, surface conjugation, and pore sizes. The caffolds are made from monodisperse foam generated by the microfluidic device. I will discuss important parameters for successful fabrication and characterizing scaffolds. Next, I will show cellular studies of various cell types cultured on the scaffolds and show their resemblance with in vivo histology pictures. We particular focus on fibroblast cells to study its adhesions. I will show the rich dynamics of adhesions of a fibroblast grown in 3D. This novel foam scaffold provides a powerful assay to investigate cellular biology in 3D.
Satoshi Nakata, Professor, Hiroshima University, Japan
Title: Nonlinear phenomena of self-propelled motors at air-water interface
Abstract: Mode-switching of a self-propelled motor on water was investigated from the view point of nonlinearity, such as oscillation, bifurcation, synchronization, hysteresis, and so on. We have introduced the high nonlinearity into the self-propelled motor to enhance the features of motion. I would like to introduce self-propelled motors like water-walking insects.
Reciprocating motion depending on the chemical structure of the monolayer1
A camphor disk as a self-propelled motor was floated on a molecular layer of N_-stearyl-p_-nitroaniline (C18ANA) which gave a pressure (Πv_s_ _area (A_) isotherm with a local minimum and a local maximum. The nature of the camphor motion changed depending on the Π _v_s_ _A_ _isotherm, and in particular reciprocating motion was observed. The characteristic motion of a camphor disk is discussed in relation to the Π _v_s_ _A_ _isotherm.
Synchronized sailing and collective motion2
Camphor disks move around an annular water channel spontaneously and interact through the camphor layer that develops on water. Two collective motion modes, discrete and continuous density waves, are generated depending on the number of self-propelled objects. The two modes are characterized by examining the local and global dynamics, and the mechanism is discussed in relation to the distribution of camphor molecules.
Self-motion with memory3
A simple self-propelled object, driven by a difference in surface tension, was found to exhibit intermittent self-motion (alternately in motion and at rest) in an annular water channel, with resting positions and features of motion in subsequent cycles remaining almost the same as those previously visited; i.e., memories of the resting positions and features of motion were observed. The occurrence of the memory phenomenon was found to depend on the relationship between the resting time and the period for one lap of the annular channel.
[References] 1. S. Nakata, T. Miyaji, T. Ueda, T. Sato, Y. S. Ikura, S. Izumi, M. Nagayama, J. Phys. Chem. 117, 6346 (2013).
2. Y. S. Ikura, E. Heisler, A. Awazu, H. Nishimori, S. Nakata Phys. Rev. E. 88, 012911 (2013).
3. S. Nakata, M. Hata, Y. S. Ikura, E. Heisler, A. Awazu, H. Kitahata, H. Nishimori. J. Phys. Chem. 117, 24490 (2013).