Capillary migration, a well-known phenomenon, is the directed transport of a droplet when the continuous phase presents a gradient in interfacial tension (IFT).The vast majority of prior literature has focused on thermocapillary migration, where the IFT gradient is due to a temperature profile. Due to the relatively weak coupling between temperature and IFT, the resulting forces and migration velocities are limited. The objective of this project is to investigate the capillary migration of droplets in gradients of surfactant concentration. This approach exploits microfluidic techniques to generate precise two-dimensional surfactant gradients in microchannels, orthogonal to the direction of flow, which are then used to control the capillary migration of droplets. Due to the strong coupling between surfactant concentration and IFT, this can achieve greater than two orders of magnitude capillary force and hence substantial migration velocities without heating the droplet. A novel feature is that the migration velocity of the drop is inversely related to its interfacial properties, which in turn depends strongly on its chemical composition. As a result, tensiophoresis has the unique ability to passively sort droplets by their chemical contents, without additional chemical labels or actuators. This project, primarily experimental in focus, will build a fundamental understanding of the phenomenon and demonstrate its application through the following specific aims: 1) Design microfluidic devices which generate linear and logarithmic gradients in surfactant concentration; 2) Verify the scaling of droplet migration over several operational regimes determined by surfactant concentration, geometric length scales, and viscosity; 3) Investigate the impact of surfactant properties and dynamics on transport regimes; and 4) Use tensiophoresis for label-free droplet sorting based on protein concentration.
The life sciences industry is moving towards progressively smaller reaction volumes in order to reduce the costs and environmental footprint of high throughput screening. Performing such assays in microdroplets can reduce reaction volumes by 3-6 orders of magnitude compared to conventional technology. This dramatically reduces reagent consumption, improves assay throughput, and enables novel types of biological assays (for example, single cells) not possible with conventional tools. This research will contribute the first label-free method for sorting droplet reactors based on their chemical composition. Given the critical importance of high throughput screening in modern biological research, this technology will ultimately benefit medical diagnostics, environmental analysis, and basic science. On the fundamental level, this project contributes to two scientific areas. In the context of multiphase processes, it will discover novel ways to control and apply the phenomenon of capillary migration, and build a unique experimental platform for studying non-equilibrium physicochemical hydrodynamics at liquid-liquid interfaces. In the context of separation science, tensiophoresis can be considered a new category of phoretictransport phenomena which enables the sorting of liquid droplets on the basis of their interfacial properties. The interdisciplinary nature of this work will provide a valuable training environment for graduate students, including women and underrepresented minorities recruited through Wayne State SURA program.
