Modulating Contact Angle Hysteresis to Direct Fluid Droplets Along a Homogenous Surface

A change in surface properties driven by an applied external stimulus is a powerful method to move fluid or drops on a surface. Techniques available to date generally involve the creation of a gradient in surface properties (chemistry, topography, or temperature) or patterning multiple electrodes on a surface that can be turned on/off. Moving (and stopping) a drop with an external stimulus on a homogeneous surface, however, is more challenging. Here we demonstrate that external control of contact angle hysteresis using low-density self-assembled monolayers lead to directed drop motion on a homogeneous surface. We show that an applied electrical potential on a liquid drop to initiate and stop motion on a homogeneous surface without any patterned electrodes. Characterization of the surfaces via impedance spectroscopy and infrared spectroscopy suggests that the change in contact angle hysteresis is linked to potential-dependent conformational changes of chains on the surface.