Speaker Name: Prof. Aloke Kumar (IISC Bangalore)

Date: 03-09-2025 (Wednesday)

Time: 2:30 PM

Venue: LC002

Abstract: Coalescence of liquid drops is an energy minimization phenomenon in which two drops merge to attain a thermodynamically stable coalesced state. This process is widely observed across a diverse range of natural and industrial processes and in the lab can be studied in three configurations namely sessile-sessile, sessile-pendant and pendant-pendant. Coalescence in Newtonian fluids has been well studied and it has been shown that the neck growth with time (t) follows a power law behaviour t^b, such that b can have universal values (or not) depending on the fluid viscosity and coalescence geometry. We show that coalescence in macromolecular fluids like polymer solutions is affected by the presence of macromolecular relaxations. Thus, in complex fluids, the coalescence process deviates from the Newtonian universality. Based on our experimental observations and literature, we generalize the sluggish merging of droplets to a new regime, namely the sub-Newtonian regime with arrested coalescence being the limiting case. Coalescence in complex fluids has all the promise to become an area of research unto itself. As an application, we will discuss how such understandings can help with in-space manufacturing. We will also present results from an Indo-French experiment on board parabolic flights.