We seek to harness interfacial phenomena to achieve external, reversible, and local control of wetting and adhesion properties. The large surface to volume ratios provided when devices are shrunk to the micro- and nanoscale create particularly exciting opportunities for exerting control via tunable surface interactions.
To achieve this goal, we explore two separate avenues for the control of surface and interfacial properties: control of electrostatic interactions and design of surface structure. The importance of electrostaticsis approached by studying the nanoscale limits of electrowetting on dielectric, the design of responsive films that can be employed to move drops, and the use of surface charge as a means to control the assembly of nanoparticles at the oil-water interface. Our efforts in the control of surface structure have been focused on the understanding of the mechanisms for the adhesion of tree frogs under flooded condition, and on the importance of partial contact line pinning on the morphology of capillary bridges
Rolling spheres on bio-inspired microstructured surfaces
Authors: Brian K. Ryu, Charles Dhong, and Joelle Frechette
Microstructured surfaces, such as those inspired by nature, mediate surface interactions and are actively sought after to control wetting, adhesion, and friction. In particular, the rolling motion of spheres on microstructured surfaces in fluid environments is important for the transport of particles in microfluidic devices or in tribology. Here we characterize the motion of smooth silicon nitride spheres (diameters 3-5mm) as they roll down incline planes decorated with hexagonal arrays of microwells and micropillars. For both types of patterned surfaces, we vary the area fraction of the micropatterned features from 0.04-0.96. We measure directly and independently the rotational and translational velocity of the spheres as they roll down planes with inclination angles that vary between 5-30 degrees. For a given area fraction we find that spheres have a higher translational and rotational velocity on surface with microwells than on micropillars. We rely on the model of Smart and Leighton [Phys. Fluids A 5, 13 (1993)] to obtain an effective gap width and coefficient of friction for all the microstructured surfaces investigated. We find that the coefficient of friction is significantly higher for a surface with micropillars than one with microwells, likely due to the presence of interconnected drainage channels that provide additional paths for fluid flow and favor solid-solid contact on the surface with micropillars. We find that while the effective gap width at very low solid fraction is equal to the height of the patterned features, the effective separation decreases exponentially as the surface coverage of microstructures increases, with little measured differences between the two geometries. Superposition of resistance functions is employed to relate the rapid decrease in the effective gap height with increase in the surface coverage observed in experiments.