Observation of Contact Line Dynamics in Evaporating Droplets Under the Influence of Electric Fields

Abstract

Deposition of colloidal material in evaporating droplets is important in many applications including DNA sequencing and medical diagnostic testing. When colloidal droplets evaporate, the majority of material is often deposited at the periphery of the resultant deposition in a coffee-ring pattern. Formation of this pattern is the result of contact line pinning and the interplay between evaporative and surface tension effects in the droplet. When the contact line is pinned and the evaporative flux in the droplet is highest at the periphery, a radially outward flow is generated to conserve mass that deposits particles in the fluid at the contact line. Evaporation at the contact line can also create a temperature gradient across the droplet. This gradient gives rise to a surface tension driven flow that can resuspend particles in the droplet. When the evaporative flow dominates, particles are deposited at the contact line in a coffee-ring pattern. The presence of the coffee-ring pattern is undesirable in many printing and medical diagnostic processes. Suppression of the coffee-ring effect has been achieved by addition of surfactant, enhancement of surface tension flow, surface modification, alteration of particle shape, and application of an electric field. Manipulation of the coffee-ring effect has been achieved through the application of both AC and DC electric fields. One result of the presence of this field is the electrowetting force at the contact line which acts to reduce the contact angle and increase contact area. Since this force acts at the contact line, it may disrupt typical contact line dynamics, including evaporative dynamics, which are responsible for the formation of the coffee-ring effect. This work will experimentally examine contact line dynamics of evaporating droplets in the presence of DC electric fields. Droplets of water will be desiccated on a device with a photolithographically patterned electrode covered with a thin layer of SU8-3005. Experimental cases with applied DC fields will be compared with unactuated control cases to examine changes in transient interface shape and contact diameter.