Session J20: Interactions with Elastic Surfaces, Particles and Fibers

Capillary bridges between two partially wetting soft plates

Capillary bridges, i.e. liquid droplets in contact with two solid substrates, play an important role in phenomena such as adhesion of insects to a wall surface. When the plate separation is small relative to the contact radius of the bridge, the force is attractive (adhesion) for liquid contact angles smaller than 90 degrees due to negative Laplace pressure, and repulsive for liquid contact angles larger than 90 degrees. We investigate capillary bridges between two partially wetting soft-layer-coated plates. We demonstrate how the softness modifies the capillary force on two soft plates. In particular, we show that varying the softness can alter the liquid contact angle, and hence so does the magnitude of the force.   

Relaxation Dynamics of Capillary Folding of Thin Elastic Sheets with Pinned Contact Lines

Capillary folding is the process of folding planar objects into three-dimensional (3D) structures using capillary force. It has many important industrial applications, e.g. the fabrication of microelectromechanical systems. In this talk, we introduce a 3D model for the capillary folding of thin elastic sheets with pinned contact lines. The energy of the system consists of interfacial energies between the different phases, and the elastic energy given by the nonlinear Koiter's model which allows large deformations of the sheet. We derive the governing equations for the static system using a variational approach and develop a numerical method to find equilibrium solutions via a relaxation dynamics. Simulation results are in good agreement with the physical experiments. We present bifurcation diagrams for the folding of a triangular sheet, which exhibit rich and fully 3D behaviors not captured by previous 2D models. Our results provide new insights into the nonlinear process of capillary folding and the design of microfabrication techniques.   

Reversibility, Path-Dependence, and Memory of a Creeping Triple-Phase Contact Line.

The contact line around a water drop on a horizontal surface has an irregular shape that does not relax to equilibrium, revealing the disorder of the solid substrate beneath. We show that the contact line has a detailed memory of its history of motion on the surface. To form a memory, we start with an initial volume of water and “train” the contact line with slow imbibition and drainage cycles at constant volume amplitude until its motion becomes periodic. Reducing the volume amplitude drastically changes the shape of the contact line when it returns to its starting volume, but when driven again with only one cycle at the training amplitude, it transitions back to its steady state. Driving above the training amplitude erases the memory making the steady state inaccessible. This behavior is reminiscent of return-point memory, a phenomenon best known in ferromagnets. Return-point memory, and the evolution to steady state, can give insight on how contact line hysteresis and the memory of its motion arise, offer a framework to study its reversible-irreversible transitions, and provide a comparison of this system to others that are far from equilibrium.    

Accelerated Transport of Highly Viscous Liquid through Microchannels by Multiphase Flow over Superhydrophobic Surface

It is well-known that the flow resistance of a fluid moving through a cylindrical pipe increases rapidly with decreasing pipe diameter, i.e., inversely proportional to the diameter to the fourth power, which is determined by the Hagen-Poiseuille law. Consequently, it becomes a big challenge to transport liquid efficiently in small pipes, and it is more difficult for highly viscous fluids such as molten materials widely used in 3D printing. However, as a channel decreases in size, the surface area-to-volume ratio becomes large, wherein the surface forces dominate the fluid transport. By taking the advantage of interfacial forces in drop-based transport of liquid, we investigate the transport of highly viscous droplets moving through microchannels with superhydrophobic surface using many-body dissipative particle dynamics simulations. Two liquid droplets with similar surface tension but significant differences in viscosity are considered. When the same pressure gradient is applied to drive the two different droplets, a faster motion of the higher viscous droplet than the lower viscous droplet is observed by a comparison of their center of mass velocities. This observation is opposite to traditional continuous fluid flow through microchannels but is consistent with recent experiments on viscosity-enhanced droplet motion. We quantify how viscosity, surface wettability, skin friction, and air-liquid surface tension affect the motion of viscous droplets as well as the internal flow field induced inside the moving droplets to improve our understanding on the mechanism of this anomalous flow phenomenon.   

Probing the dynamic surface deformation of droplet deposition on soft substrates using an interferometric nanostrain sensor

Sessile droplets on soft substrate produce a distinct surface deformation consisting of a dimple due to Laplace pressure and a wetting ridge formed by surface tension. While many discoveries have been made on static soft wetting behavior, non-equilibrium soft wetting is much less investigated, especially the wetting process when the droplet first contacts the substrate. In this study, an interferometric nanostrain sensor is used to measure the dynamic surface deformation of droplets impinging on hydrophobic soft substrate. Three different stages are investigated: (A) initial droplet impingement onto the surface with a pipette tip, (B) formation, and (C) breaking of the liquid bridge. In Stage A, we observed a repeating ‘stick-growth, slip-shrink’ phenomenon whereby both the dimple and ridge height increased during contact line sticking and decreased during contact line slipping. In Stage B, a steady decrease for both the dimple and ridge height is observed when the liquid bridge is formed. Lastly, bridge breaking induced a large oscillation in the surface deformation that lasted for about 40 ms, indicating possible capillary wave interactions with the soft substrate. Comparative observations of drop wetting over a hydrophilic soft substrate will also be reported.   

Time evolution of coiled fibers inside evaporating sessile drops

When a hydrophilic elastic fiber with a length of around 10 cm is introduced into a millimeter-sized water drop, it can form a multiply coiled structure completely immersed in liquid. We have studied the time evolution of fiber-drop systems that evaporate on a superhydrophobic surface. After introduction, the fiber is either in an ordered or a disordered state. Ordered initial states are preferably found for short fibers, while long fibers exhibit disorder. Upon evaporation, drop shapes largely deviating from a spherical cap are observed. The time evolution of the fibers is mostly smooth, interrupted by a few sudden reconfiguration events. Generally, as the drop evaporates, the fibers transition from a less ordered to an ordered state. Specifically, while the initial state is often a three-dimensional morphology, the final configuration deposited on the surface has a two-dimensional character. We present a morphology diagram of the fiber deposits on the surface, with the non-dimensional fiber length and elastocapillary length as independent variables. Only three different morphologies of the fiber deposits are found: circles, ellipses, and 8-shaped morphologies. The deposit morphology is mainly determined by the fiber length, while the elastocapillary length only plays a subordinate role.   

Wind-driven mixing of drops on fibers

Drops on fibers in crossflows have complex dynamics. At an intermediate Reynolds number, the imposed crossflow leads to a wake asymmetry which induces translation along the fiber. As the drops move, they deposit thin films on the fiber. These thin films can be picked up by other drops. Using fluorescent and brightfield imaging, we study the mixing of these films inside two adjacent drops. We generalize our results to multiple drops on a fiber and rationalize the mixing with analytical and numerical models for the films deposited on the fibers and the corresponding internal flows in droplets.