Investigation of mechanisms governing electrowetting and hydrodynamic interactions in the presence of draining channels

Modulation of solid-liquid interfacial interactions via electric field (or electrowetting) is an effective method to deform and move liquid drops on solid surfaces in micro-/nanoscale systems. The deformation and motion of a liquid drop on a solid surface in response to an external driving force is hindered by pinning of triple contact line. A better understanding of the mechanisms and limitations of electrowetting is warranted for design and optimization of active micro-/nanoscale systems. Modulation of hydrodynamic interactions via surface structures may contribute to the adhesion and locomotion mechanisms employed by tree frogs under flooded conditions: a better understanding of which will facilitate design of biomimetic systems inspired by the same. This thesis summarizes the results of investigation of the mechanisms and limitations of electrowetting (electrowetting on dielectric and potential-induced molecular reorganization) and the hydrodynamic interactions in the presence of draining channels.;The mechanism at play during electrowetting on dielectric is probed via capillary condensation inside surface force apparatus. Height of a nanometer-sized annular water meniscus is measured and observed to be independent of the applied potential. These nanoscale electrowetting measurements unequivocally demonstrate that spreading of a liquid conductive drop on a charged dielectric is driven by electromechanics and not by a change in solid-liquid interfacial energy. Macroscopic electrowetting response of substrates with a range of contact angle hysteresis is characterized to quantify the relationship between contact angle hysteresis, threshold potential for liquid actuation, and electrowetting hysteresis. These results are interpreted within the electromechanical framework corrected for pinning of the moving triple contact line and demonstrate that the electrowetting hysteresis and the contact angle hysteresis are equal in magnitude. Alternatively, potential-induced variation in contact angle hysteresis due to molecular reorganization at the solid-liquid interface (or change in the solid-liquid interfacial energy with applied potential) is demonstrated as a means to control macroscopic shape and motion of a drop on a homogeneous surface. In situ and reversible modulation of contact angle hysteresis or pinning force with repeated switching between positive and negative potentials results in drop stretching and contraction, such that the motion of a drop subjected to a constant driving force (gravity) mimics the motion of an inchworm.;The hydrodynamic interactions in the presence of draining channels are investigated using the surface force apparatus as a means to understand the adhesion and locomotion mechanisms employed by tree frogs under flooded conditions. The hydrodynamic interactions measured during drainage of fluid from a gap between a structured surface and a smooth surface agree with Reynolds’ theory at large separations. Deviations from theory, characterized by a reduction in the hydrodynamic repulsion, are observed below some critical separation, which depends on dimensions of the structural features. These results are analyzed within a scaling analysis to establish a separation of length scales that corresponds to the transition from the fluid being radially squeezed out of the nominal contact area to being squeezed out through the network of interconnected channels. Reduced hydrodynamic repulsion in the presence of draining channels would enable the tree frog toe pad to come into a more intimate contact with flooded surfaces.