(541a) Probing Droplet Shape Relaxation and Its Dependence on Viscosity Using a Microfluidic Hydrodynamic Trap

Microfluidics has emerged as a powerful tool to probe the response of droplets to various hydrodynamic conditions. Deformation and oscillation of droplets in liquid-liquid systems can have a significant impact on the formation and stability of complex emulsions. In previous work, we have performed droplet deformation measurements under extensional flow in microfluidic devices, which was used to measure dynamic interfacial tension for systems containing surfactants [1]. Further, when trapped stationary droplets are perturbed, their damped oscillatory response depends strongly on the viscosities of the two phases. This principle has been applied by researchers to measure viscosity of droplets trapped in air with optical tweezers [2]. In this work, we extend this technique to liquid-liquid systems using a microfluidic hydrodynamic trap. We adapt a microfluidic trap based on the design by Shenoy et al. [3] for droplet generation and subsequent trapping by feedback control on a microfluidic device. Using this device, droplets of aqueous solutions containing glycerol are formed in various oils and trapped at a stagnation point in a four-channel cross slot. Once trapped, droplets are perturbed using a piezoelectric actuator, and their shape oscillation is imaged at high-speed. Droplet oscillation frequencies and damping constants are measured from image analysis of the high-speed videos. We study the effect of changing the dispersed phase viscosity using glycerol-water solutions over a large range of viscosities, as well as the impact of changing continuous phase viscosity on droplet oscillation frequency and damping constant. The observed behavior of droplets of various sizes and varying viscosities are compared to theoretical predictions of droplet shape decay in liquid-liquid systems [4,5].

References:

[1] S. Narayan, D. B. Moravec, B. G. Hauser, A. J. Dallas, and C. S. Dutcher, Energy Fuels (2018).

[2] J. P. Reid, A. K. Bertram, D. O. Topping, A. Laskin, S. T. Martin, M. D. Petters, F. D. Pope, and G. Rovelli, Nat. Commun. 9, 956 (2018).

[3] A. Shenoy, C. V. Rao, and C. M. Schroeder, Proc. Natl. Acad. Sci. 113, 3976 (2016).

[4] C. A. Miller and L. E. Scriven, J. Fluid Mech. 32, 417 (1968).

[5] Y. Bayazitoglu and P. V. R. Suryanarayana, Acta Mech. 95, 167 (1992).