This work demonstrates electrowetting-induced droplet detachment in air from coplanar electrodes using a single voltage pulse. It also presents two models to predict when this detachment will occur. Previous works approximated the minimum energy for detachment based on (i) adhesion work at the solid–liquid interface and (ii) interfacial energy changes along all three interfaces in the system. This investigation updates those models to include changes in gravitational potential energy during detachment and provides validation by testing predicted detachment thresholds against experimental observations. Droplets of varying volume were ejected from electrowetting devices with (i) radially symmetric four-part coplanar electrodes and (ii) single electrodes with a ground wire inserted directly into the droplet. All experiments were performed in air. Incorporation of gravitational potential energy improves predictions for critical electrowetting number and captures the observed increase in applied voltage required with increased droplet volume. These new models will be of particular benefit in three-dimensional digital microfluidics applications that manipulate droplets in air.

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This investigation experimentally examines the role of interface capture on the transport and deposition of colloidal material in evaporating droplets. It finds that deposition patterns cannot be characterized by the ratio of interface velocity to particle diffusion rate alone when the two effects are of the same order. Instead, the ratio of radial velocity to particle diffusion rate should also be considered. Ring depositions are formed when the ratio of radial velocity to the particle diffusion rate is greater than the ratio of interface velocity to diffusion. Conversely, uniform depositions occur when the ratio of radial velocity to diffusion is smaller than the ratio of interface velocity to diffusion. Transitional depositions with a ring structure and nonuniform central deposition are observed when these ratios are similar in magnitude. Since both ratios are scaled by diffusion rate, it is possible to characterize the depositions here using a ratio of interface velocity to radial velocity. Uniform patterns form when interface velocity is greater than radial velocity and ring patterns form when radial velocity is larger. However, Marangoni effects are small and Derjaguin, Landau, Verwey, and Overbeek (DLVO) forces repel particles from the surface in these cases. Further research is required to determine if these conclusions can be extended or modified to describe deposition patterns when particles are subjected to appreciable Marangoni recirculation and attractive DLVO forces.

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.

Lab-on-a-chip (LOAC) devices are emerging technologies that aim to perform all of the laboratory functions of traditional diagnostic tests on single microchips. Microarrays are one promising type of LOAC device that consist of an array of droplets for testing tens to thousands of samples simultaneously. Microarrays are commonly used in gene sequencing, pathogen detection, determining microbial resistances, and conducting enzyme-linked immunosorbent assays (ELISAs). As droplets in these arrays dry, the majority of material within the droplet is deposited around the periphery. This phenomenon is referred to as the coffee stain effect. The non-uniform depositions left by this effect can result in variation of fluorescence intensity measurements in automated vision systems. A means of producing more uniform particle depositions for the microscopy analysis would allow for more accurate test results. One promising method for suppression of the coffee stain effect involves the use of electrowetting on dielectric (EWOD). EWOD devices apply an electrokinetic force at the three-phase contact line to manipulate the shape of a droplet interface. The Mugele group has already begun investigating EWOD’s effects on the coffee stain effect and found that an AC voltage applied to droplets on EWOD devices can suppress the coffee stain effect and produce smaller, more uniform droplet deposition patterns. This work presents (i) a method to characterize the deposition pattern left by a desiccated droplet as a function of radial position and (ii) a discussion of the microfabrication technique used to create devices to perform EWOD assisted desiccation for both AC and DC voltages.