Abstract

As a droplet containing colloidal material dries on a surface, the suspended particles are deposited onto it. Aqueous colloidal droplets often leave the majority of particles at the periphery of the deposition area. This phenomenon is known as the coffee ring effect, and it arises when a radially outward evaporative flows dominates within a droplet as it dries. This deposition non-uniformity can impact a diverse range of applications. In medical diagnostic testing, it can introduce uncertainty into microarray test results, while in inkjet printing it can produce finer printed circuitry. Controlling colloidal deposition could allow for higher-quality medical diagnostics and printing techniques.

Application of electric fields to evaporating droplets is a promising method for controlling the coffee ring effect. Applying a voltage across a droplet can produce electrowetting forces at the contact line and electrophoretic forces on suspended particles. These forces have the potential to influence contact line pinning, internal flows, and migration of particles within the droplet. Electric fields have previously been used to manipulate colloidal transport in droplets laden with a variety of particles. The present work experimentally characterizes the effects of the particle type, size, and actuation polarity on the contact line dynamics and resultant depositions of evaporating particle-laden droplets.

Deionized water droplets were seeded with a variety of particles and evaporated on an SU-8 photoresist substrates. Particle size was found to play a role in the final deposition patterns produced by carboxylate-modified polystyrene-laden droplets. Droplets with smaller particles repinned in the final stages of evaporation and produced ring depositions, while droplets with larger particles remained mobile and produced more uniform depositions. A comparison of the evaporation times to particle settling times, showed that gravitational effects may influence the deposition behavior of larger particles. Particle material also plays a role in the resultant deposition pattern. Titanium oxide-laden droplets tended to pin longer and exhibited smaller receding contact angles as compared to polystyrene-laden droplets. Titanium oxide-laden droplets also tended to leave less distinct rings and large uniform regions in the final depositions. Actuation polarity was found to influence contact line mobility during the evaporation of particle-free and polystyrene-laden droplets. Polystyrene-laden droplets exposed to negative DC electric fields receded in a slip-stick pattern, while those exposed to positive DC electric fields receded more uniformly. No evidence of electrophoretic migration was observed, as both field polarities produced similar deposition patterns. Work is ongoing to quantify the electrophoretic effect.

Library of Congress Subject Headings

Colloids--Mechanical properties; Drops; Evaporation

Publication Date

7-29-2016

Document Type

Thesis

Student Type

Graduate

Degree Name

Mechanical Engineering (MS)

Department, Program, or Center

Mechanical Engineering (KGCOE)

Advisor

Michael Schertzer

Advisor/Committee Member

Kara Maki

Advisor/Committee Member

Michael Schrlau

Comments

Physical copy available from RIT's Wallace Library at QD549 .B87 2016

Campus

RIT – Main Campus

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