When colloidal droplets evaporate, material is often deposited at the periphery in a coffee-ring pattern. An understanding of colloidal transport and deposition in evaporating droplets is critical for optimization of many medical diagnostic devices and printing processes. This phenomenon can also be used for nanoparticle self-assembly, containerless materials processing, and manufacturing of flexible electronics. The forces affecting colloidal transport can be categorized as (i) convective effects, (ii) particle diffusion, and (iii) boundary effects. Knowledge of fluid flow in an evaporating droplet is required to understand particle transport when convection effects dominate over particle diffusion. The evaporative radial flow identified by Deegan et al. [1] is the result of contact line pinning and high evaporation flux at the contact line. Hu and Larson later identified that dominance of thermal Marangoni recirculation within the droplet can suppress ring formations by convecting particles to the center of the deposition.

Bhardwaj et al. [2] demonstrated that van der Walls and electrostatic forces can dominate colloidal transport and capture particles on the substrate to form uniform depositions. These effects are represented by the Derjaguin, Verwey, and Overbeek (DLVO) force. This force can attract or repel particles from the surface or other particles when they are separated by less than the Debye length. Attractive DLVO forces result in uniform depositions when they dominate over the evaporative and Marangoni flows in evaporating droplets. Particle capture on the substrate has also been achieved by antibodies-antigen reactions, or by leveraging magnetic and electrophoretic forces.

Particles can also be captured on the interface of evaporating droplets. Li et al. [3] observed uniform colloidal monolayers when water droplets were evaporated at elevated temperatures. They argued that particles were being captured on the interface in these cases as the particle diffusion rate was smaller than the velocity of the collapsing interface.

This work will find a low cost method for electrowetting assisted deposition and examine how electric fields can disrupt the interplay between convection, diffusion, and interface trapping in evaporating colloidal droplets. Application of AC and DC electric fields has the potential to suppress the coffee-ring effect by independently controlling the shape of the fluid interface, the contact angle hysteresis, and the motion of particles suspended in the droplet. This investigation will compare the interfacial evolution and deposition patterns left by evaporating protein laden droplets under AC and DC fields to examine their suitability for medical diagnostic applications.

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