Shape Evolution of Droplets Growing on Linear Microgrooves

Resonance Modes of Water Drops Pinned to a Vibrating Rectangular Post

We studied the effects of vertical vibrations on a water drop that was pinned to the sharp edges of a rectangular post. By varying the frequency and amplitude of the vertical displacement, distinct resonance peaks were observed using a simple optical technique. The vibrational spectra of the first two modes exhibited two closely spaced peaks, which corresponded to standing waves that exist along the major and minor contour lengths of the drops. The values of the resonance frequencies can be explained rather well by a simple model, which was originally proposed for axially symmetric drops.

Directional Liquid Wicking in Regular Arrays of Triangular Posts

Wicking of wetting liquids into micropatterns of posts with homogeneous triangular cross section is studied in experiments and by numerical energy minimizations. To test for directional wicking, we fabricated regular arrays of posts with various combinations of line fractions and aspect ratios using standard photolithography processes. In agreement with numerical energy minimizations of the liquid film morphology, we find spontaneous wicking in the experiments only for line fractions and aspect ratios where the homogeneous liquid film represents the state of lowest interfacial free energy and where no local energy minimum could be detected in our numerical energy minimizations. The numerical results further demonstrate that the stability of a certain morphology of the terminal meniscus controls the direction of wicking relative to the orientation of the triangular posts. The observed selectivity of spontaneous wicking with respect to the meniscus orientation can be exploited to build a microfluidic rectifier for partially wetting liquids.

Polymer Spreading on Unidirectionally Nanotextured Substrates Using Molecular Dynamics

A unidirectional nanotexture alters the wettability of a substrate and can be used to create patterned polymer films, tailored polymer coverage/reflow, or aligned polymer molecules. However, the physical mechanisms underlying polymer spreading on nanoscale textures are not well-understood, and competing theories exist to explain how texture peaks and grooves alter the wettability of a substrate. We use molecular dynamics to simulate polymer spreading on substrates with unidirectional nanoscale textures as a function of texture shape and size and compare to polymer spreading on a flat substrate. We show that the texture groove shape is the primary factor that modifies polymer spreading on unidirectionally nanotextured substrates because the texture groove shape determines the minimum potential energy of a substrate. At the texture groove, the energy potentials of several surfaces combine, which increases polymer attraction and drives spreading along the texture groove. A texture groove also acts as a sink that inhibits polymer spreading perpendicular to the texture. Texture peaks create energy barriers that inhibit polymer spreading perpendicular to the texture, but this is a secondary mechanism that does not significantly affect anisotropic spreading. This research unifies competing theories of anisotropic liquid spreading documented in the literature and aims to aid in the design of nanoscale textures and ultrathin liquid film systems.

Investigation of Equilibrium Droplet Shapes on Chemically Striped Patterned Surfaces Using Phase-Field Method

We systematically investigate the equilibrium shapes of droplets deposited on a set of chemically striped patterned surfaces by using an Allen?Cahn-type phase-field model. Varying the widths of the stripes d, the volume V, as well as the initial positions of the droplets, we release the droplets on the top of the surfaces and observe the final droplet shapes. It is found that there are either one or two equilibrium shapes for a fixed ratio of d/ V^1/3 and each equilibrium shape corresponds to an energy minimum state. The aspect ratio of the droplets ? shows a periodic oscillation behavior with a decreasing amplitude as d/ V^1/3 decreases, similar to the stick?slip?jump movement of a slowly condensing droplet on a chemically striped patterned surface. Additionally, by comparing the movements of slowly evaporating and condensing droplets, we have observed a hysteresis phenomenon, which reveals that the final shapes of droplets also rely on the moving paths. Through modifying the dynamic contact angle boundary condition, the contact line movements of droplets under condensation and evaporation, which are far from equilibrium, are addressed.