A nematic coffee ring effect

An evaporating water-alcohol droplet on a surface will experience a gradient in surface tension that drives a Marangoni flow within the droplet. Particulate matter within the droplet can follow this flow and become adsorbed at the edge of the droplet, pinning the droplet as it evaporates and building up a layer of material in what is known as the coffee ring effect. In this work we investigate evaporating ethanol-water droplets containing a small fraction of the liquid crystal 5CB. As microliter-sized droplets undergo a liquid-gas transition, the changing concentrations at the droplet edges induce a demixing transiton, resulting in arrays of phase-separated droplets. These droplets spontaneously undergo an isotropic-nematic transition, a phase transition caused by a phase transition caused by a phase transition. This visually-striking phenomenon was investigated as part of a course-based undergraduate research experience (CURE) in which students explored aspects of the parameter space, tuning features such as the chemical composition of the droplets and the affinity of the surfaces. IN this talk I will report on our findings from the collected class data and some open questions.