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Catlin Johnson, a master's degree candidate in forensic science at Buffalo State, will present the literature seminar "Science Underlying the Coffee-Ring" at 12:30 p.m. Thursday, December 4, in Classroom Building B119. Light refreshments will be served before the seminar. The Chemistry-Physics Seminar Series is supported by the Faculty-Student Association. This presentation was rescheduled from November 20.

Abstract
When a liquid drop containing dispersed solids dries on a solid surface, the particles are deposited leaving a ring-like stain. This phenomenon was known as the “coffee-ring” effect and successfully explained by Deegan et al. for the first time. They found that the coffee-ring is formed when particles are accumulated at the edge of a droplet by an outward capillary flow to replenish the loss of a solvent occurring at a liquid-air interface. Two important conditions should be met for the coffee-ring effect to occur during the solvent evaporation: (1) Marangoni flow, a flow created by a surface tension gradient, should be weak; (2) the contact line of a droplet on a surface should be pinned.

Since the coffee-ring effect causes problems when a formation of homogeneous deposits is necessary, recent research has been focused on suppressing and controlling the coffee-ring effect.2-5 These could be achieved in several different ways by loosening either one of the two conditions of the coffee-ring effect mentioned above. Adding a surface active agent to a droplet or evaporating an aqueous droplet in the presence of ethanol vapor effectively creates the Marangoni flow that circulates particles in a droplet, thus counteracting the formation of a coffee-ring. Others have recently reported that the coffee-ring effect can be suppressed by controlling the critical angle hysteresis (CAH) of a surface. They showed de-pinning the contact line of a droplet could eliminate the formation of a ring-shape deposit. The pinning and de-pinning behavior of a droplet was closely associated with the CAH of a surface. Additionally, it is reported that the shape of the suspended particles can control the coffee-ring effect. Ellipsoidal particles displayed uniform deposition after the solvent evaporation. Long-range interparticle interactions prevent dense packing of particles, which reduces the accumulation of particles at the edge of a droplet. Recent study further shows that the coffee-ring effect can be dynamically controlled by using a light-responsive surface-active agent.

The coffee-ring effect is a complicated phenomenon that involves surface, air-solvent interface, particles, and solvent. A better understanding of its science will provide useful insights to the applications that require the deposition of a colloid solution on a surface.