# Contributed Talks

### Contributed Talks (15 min. + 5 min. Q&A)

Nate Cira, Stanford University, USA
Title: Dancing Droplets
Abstract: Inspired by the observation of intricate and beautifully dynamic patterns generated by food coloring on corona treated glass slides, I have investigated the behavior of two component droplets on high energy surfaces. These droplets exhibit a range of interesting behaviors including: forming a stable droplet on a high energy surface, long distance attraction or repulsion, and chasing/fleeing upon contact. I present explanations for each of these behaviors, and propose detailed models for the long distance interactions based on vapor facilitated coupling and the stable droplet formation based on evaporation. Finally I use my understanding to create several novel devices which: passively sort droplets by surface tension, spontaneously align droplets, drive droplets in circles, and cause droplets to bounce on a vertical surface. The simplicity of this system lends it particularly well to application as a toy model for physical systems with force fields and biological systems such as chemotaxis and motility.

Keyvan Piroird, Universitaire d' Orsay, France
Title: Capillary flow of oil in an aqueous foam
Abstract: When using appropriate surfactants, oil and aqueous foam can be intimately mixed without the foam being destroyed. We show that a foam, initially free of oil, can draw an oil drop under the action of capillary forces and stretch it through the aqueous network. We focus on the suction of oil by a single horizontal foam channel, known as a Plateau border. In such confined channels, imbibition dynamics are governed by a balance between capillarity and viscosity. Yet, the scaling law for our system differs from that of classical imbibition in porous media such as aqueous foam. This is due to the particular geometry of the liquid channels: Plateau borders filled with foaming solution are always concave whereas they can be convex or flat when filled with oil. Finally, the oil slug, confined in the Plateau border, fragments into droplets following a film breakup.

Penger Tong, Hong Kong University of Science & Technology, China
Title: Interfacial pinning, hysteresis and dynamics near a Moving Contact Line
Abstract: The study of the dynamics near a moving contact line between a liquid interface and a solid surface is of fundamental interest for our general understanding of a common class of problems involving elastic dynamics in random force fields and also has immense practical applications in tertiary oil recovery, drag reduction, advanced materials and microfluidics. In this talk I will present our recent experimental efforts in developing an atomic force microscope (AFM) based hanging fiber probe for the study of contact line dissipation and wetting dynamics [1,2]. With this new technique, AFM is used as a force sensor to measure the viscous dissipation of a moving contact line and capillary forces on a long vertical glass fiber of a few microns in diameter with one end glued onto a AFM cantilever and the other end in contact with a liquid-air interface. Applications of this technique to the study of contact line dissipation and wetting dynamics will be discussed.

[1] “Development of an atomic-force-microscope-based hanging-fiber rheometer for interfacial microrheology,” Xiaomin
Xiong, Shuo Guo, Zuli Xu, Ping Sheng, Penger Tong, Phys. Rev. E 80, 061604 (2009).
[2] “Direct measurement of friction of a fluctuating contact line,” S. Guo, M. Gao, X. Xiong, Y. J. Wang, X.-P. Wang, P.
Sheng and P. Tong, Phys. Rev. Lett. 111, 026101 (2013).
Work was supported by the Research Grants Council of Hong Kong SAR.

Robert Style, Yale University, USA
Title: Wetting of soft solids
Abstract: When we consider the wetting of solid surfaces, we generally ignore how stiff the solid is - the solid surface energies and roughness generally controls wetting behaviour. However, this is no longer true when the solid is sufficiently soft. Then, a whole range of new wetting phenomena arise caused by deformations of the substrate by the droplet. For example, I show that (i) Young's law for contact angles breaks down on soft surfaces, (ii) neighbouring droplets on soft surfaces experience attractive interactions, causing them to coalesce, (iii) droplets slide along gradients in stiffness - similarly to cellular durotaxis. This droplet behaviour can be used in novel patterning and droplet manipulation techniques. Finally, I will show how, even when embedded in soft surfaces, droplets exhibit unusual behaviour.

Su Ji Park, Pohang University of Science & Technology, South Korea
Title: Dynamic wetting on soft substrates studied by x-ray imaging
Abstract: When a droplet sits on a soft surface, its surface tension deforms the underlying material, creating a wetting ridge. Wetting ridge formation also affects besides static wetting, dynamic wetting behaviors such as spreading retardation (i.e. viscoelastic braking) and contact line pinning (i.e. stick/slip or stick/break behaviors). However, the underlying mechanisms are still largely unexplored mostly due to limitations in observation. Here, we directly visualize wetting ridges in real-time during spreading using x-ray microscopy with high spatial and temporal resolutions. We clearly show that ridge-growth dynamics is closely linked to the spreading regimes. Finally, we discuss spreading mechanisms on soft surfaces based on ridge-growth dynamics that determines ridge-geometry. We believe that our results would shed light on understanding of dynamic wetting behaviors on soft solids (e.g. contact angle hysteresis or evaporation) and also be potentially important to interpret complex biological processes on or in soft tissues (e.g. cell-substrate interactions).

Michael Benzaquen, ESPCI, France
Title: From adhesion to wetting of a soft particle
Abstract: Since the seminal works of Hertz, Johnson, Kendall, and Roberts (JKR), and Derjaguin, Muller, and Toporov (DMT), the contact of adhesive elastic solids has been widely studied. This area of research is of tremendous importance: the range of application now spreads from biology to engineering, as shown by the recent developments on latex particles, biological cells or micro-patterned substrates, to name a few. Using a thermodynamical approach [1], we calculate the adhesion-induced deformation of a spherical elastic particle placed on a rigid substrate, under zero external load, and including an ingredient of importance in soft matter : the interfacial tension of the cap. First, we limit the study to small deformation. In contrast with previous works, we obtain an expression for the free energy that precisely contains the JKR and Young-Dupré asymptotic regimes, and which establishes a continuous bridge between them. Then, we consider the large deformation case, which is relevant for future comparison with numerical simulations and experiments on very soft materials. Using a fruitful analogy with fracture mechanics, we derive the free energy of the problem and thus obtain the equilibrium state for any given choice of physical parameters.
[1] T. Salez, M. Benzaquen, E. Raphael, Soft Matter 9 (2013) 10699 - [Journal Cover]

Ling Chao, National Taiwan University, Taiwan
Title: Tunable Nucleation Time of Functional Enzyme-Lipid Features Studied by Membrane Array Statistic Tool
Abstract: Aggregation or assembly of lipids and proteins could significantly change the proteins’ function. A peripheral membrane enzyme, sphingomyelinase (SMase), has been reported to be able to assemble to a functional feature with its lipid substrate, sphingomyelin (SM), and its lipid product, ceramide (Cer). SMase seems to processes its substrate more effectively in this feature. Here, we report that the functional feature has a tunable formation time. The peculiar behavior is that the feature formation has a time lag depending on the membrane composition. We hypothesized that the time lag is due to the significant nucleation energy barrier when the feature phase forms in its metastable parent phase in the 2-D lipid membrane. To study the stochastic nucleation of the feature, we built a corralled lipid membrane platform with numerous isolated membrane systems in parallel to capture the nucleation statistics. Using the high-throughput approach and the appropriate experimental design to circumvent the interplay of the complicated phase segregation in membranes induced by SMase, we found that the nucleation rate of the feature can be tuned by the supersaturation of the enzyme, the lipid substrate and the lipid product, in the fluid phase of the membrane. The correlation between the supersaturation and the nucleation rate can be well described by the classical nucleation theory equation, suggesting that the feature formation follows the nucleation process with certain component ratio specified in the equation. The certain relative component ratio suggests that the feature may have certain organization instead of being random aggregation. In addition, our finding suggests that nucleation could serve as a time lag control mechanism in this enzymatic system, and ways to reduce nucleation energy barrier could be used to shorten the aggregation time lag and vice versa.

Tak Shing Chan, University of Saarland, Germany
Title: Air entrainment by advancing contact lines
Abstract: We study the entrainment of air by advancing contact lines by plunging a solid plate into a very viscous liquid. Above a threshold velocity, we observe the formation of an extended air ﬁlm, typically 10 microns thick, which subsequently decays into air bubbles. Exploring a large range of viscous liquids, we ﬁnd an unexpectedly weak dependence of entrainment speed on liquid viscosity, pointing towards a crucial role of the ﬂow inside the air ﬁlm. This induces a striking asymmetry between wetting and dewetting: while the breakup of the air ﬁlm strongly resembles the dewetting of a liquid ﬁlm, the wetting speeds are larger by orders of magnitude.

J. F. Hernandez-Sanchez, University of Twente, Netherlands
Title: Marangoni spreading due to a localized alcohol supply on a thin water film
Abstract: Bringing the interfaces of two miscible fluids into contact naturally generates strong gradients in surface tension. Here we investigate such a Marangoni-driven flow by continuously supplying isopropyl alcohol (IPA) on a film of water, using micron-sized droplets of IPA-water mixtures. These droplets create a localized depression in surface tension that leads to the opening of a circular and thin region in the water film. At the edge of the thin region, there is a rim growing and collecting the water of the film. We find that the spreading radius scales as $r \sim t^{1/2}$. This result can be explained from a balance between Marangoni and viscous stresses, assuming that the gradients in surface tension are smoothened out over the entire size of the circular opening. We derive a scaling law that accurately predicts the influence of the IPA flux as well as the thickness of the thin film at the interior of the spreading front.

Priya Subramanian, Max Planck Institute for Dynamics & Self Organization, Germany
Title: Emergent Dynamics in Active Elastic Systems
Abstract: Active systems such as bacterial suspensions and ciliary beds are intrinsically out of equilibrium and show rich, self-organized dynamics. We are interested in a subset of these systems which exhibit elasticity as well as activity, such as systems consisting of connected self-propelled (active) colloids. Geometric constraints along with the polarity of the colloids results in an active compressive force that tends to buckle the filament. We find that depending on the type of constraint at the fixed end (clamped,pivoted or torsional spring), we obtain oscillatory or divergent instability for an initially straight filament. We also observe a flutter boundary for a torsionally clamped end below which oscillatory instabilities are supported. Above the flutter limit, the filament displays divergent instability similar to a pivoted end. Building on these studies, we are currently exploring the use of linked active colloids to form synthetic mimics of cilia and to understand the dynamics of the interactions between two or more filaments.

Alban Sauret, Princeton University, USA
Title: Wetting and drying of liquid on crossed fibers
Abstract: ibrous media are common in various natural systems (feathers, adhesive pads) but also in engineered systems (filters, textile industry). These materials can often be described as an array of random fibers in which a wetting liquid tends to accumulate at the nodes. Here we investigate the wetting properties of the simplest element of an array of random fibers: two rigid fibers crossing with an inclination angle $\delta$, separated by a minimum inter-fiber distance distance $d_0$ and wetted by a volume $V$ of a perfectly wetting liquid. The liquid can adopt different morphologies depending on the volume of the deposited drop, the tilting angle or the inter-fiber distance, as well as the fiber radius. We report morphologies ranging from a column shape to a mixed morphology state in which the drop lies at the end of a column, and a compact drop centered at the node. An analytical model is provided that predicts the transition between the morphologies, the wetting length as well as the presence of a non-symmetric state in the mixed morphology regime. Because of these distinct morphologies, the liquid exhibits different drying kinetics, which affects the overall drying time. Our study suggests tuning the morphology of the liquid could have important implications for drying processes.