Rheocoalescence: Relaxation time through coalescence of droplets

TL;DR

This paper investigates droplet coalescence dynamics in polymeric fluids, identifying three regimes based on concentration ratios, and introduces Rheocoalescence as a new method to measure fluid relaxation times.

Contribution

It uncovers three distinct coalescence regimes in polymeric fluids and proposes Rheocoalescence as a novel technique for measuring relaxation times.

Findings

Neck radius growth follows a power-law with a regime-dependent exponent.

Steady power-law exponent in inertio-elastic and viscoelastic regimes.

Deviation from universality in elasticity dominated regime.

Abstract

Dynamics of the pendant drop coalescing with a sessile drop to form a single daughter droplet is known to form a bridge. The bridge evolution begins with a point contact between the two drops leading to a liquid neck of size comparable to the diameter of the drops. To probe this phenomenon in polymeric fluids, we quantify the neck radius growth during coalescence using high speed imaging. In the current study, we unveil the existence of three regimes on basis of concentration ratio $c/c^*$ namely, inertio-elastic $c/c^*<c_e/c^*$, viscoelastic $c_e/c^*<c/c^*<20$ and elasticity dominated regimes $c/c^*>20$. Our results suggest that the neck radius growth with time (t) obeys a power-law behaviour $t^b$, such that the coefficient $b$ has a steady value in inertio-elastic and viscoelastic regimes, with a monotonic decrease in elasticity dominated regime. Based on this dependence of $b$ on concentration ratios, we propose a new measurement technique Rheocoalescence to obtain the relaxation time of the these fluids. We also show a deviation from universality proposed in literature for the elasticity dominated regime.