Friction and Adhesion of Microparticle Suspensions on Repellent Surfaces.

With the recent advancements in the development and application of repellent surfaces, both in air and under liquid medium, accurate characterization of repellence behavior is critical in understanding the mechanism behind many observed phenomena and to exploit them for novel applications. Conventionally, the repellence behavior of a surface is characterized by the optical measurement of the dynamic contact angle of the target (to be repelled) liquid on the test surface. However, as already established in the literature, optical measurements are prone to appreciable error, especially for repellent surfaces with high contact angles. Here, we present an alternative, more accurate force-based characterization method of both friction and adhesion forces of microparticle-laden aqueous droplets over various repellent surfaces, where the force signature is captured by probing the surface with a droplet of the test liquid mounted at the tip of a flexible cantilever and then tracking the deflection of the tip of the cantilever as the probe droplet interacts with the surface. A systematic investigation of the response of repellent surfaces toward droplets with different microparticle concentrations reveals the dependency and sensitivity of measured adhesion and friction signature toward particle concentration. A comparison with the theoretical estimate from optical goniometry highlights the deviation of the theoretical data from experimentally measured values and further substantiates the need for such a force-based approach for accurate characterization of repellence behavior.