Droplets on liquids and their long way into equilibrium

TL;DR

This study investigates the morphological evolution of liquid droplets on liquid substrates during dewetting, revealing deep penetration of droplets and multiple intermediate shapes before reaching equilibrium, supported by experiments and thin-film modeling.

Contribution

It combines experimental imaging and theoretical modeling to elucidate the shape evolution and dynamics of droplets on liquid substrates during dewetting.

Findings

Droplets penetrate deeply into the substrate layer.

Intermediate shapes are shape-independent but volume-dependent in timescale.

Experimental results agree with thin-film model predictions.

Abstract

The morphological paths towards equilibrium droplets during the late stages of the dewetting process of a liquid film from a liquid substrate is investigated experimentally and theoretically. As liquids, short chained polystyrene (PS) and polymethyl-methacrylate (PMMA) are used, which can be considered as Newontian liquids well above their glass transition temperatures. Careful imaging of the PS/air interface of the droplets during equilibration by \emph{in situ} scanning force microscopy and the PS/PMMA interface after removal of the PS droplets reveal a surprisingly deep penetration of the PS droplets into the PMMA layer. Droplets of sufficiently small volumes develop the typical lens shape and were used to extract the ratio of the PS/air and PS/PMMA surface tensions and the contact angles by comparison to theoretical exact equilibrium solutions of the liquid/liquid system. Using these results in our dynamical thin-film model we find that before the droplets reach their equilibrium they undergo several intermediate stages each with a well-defined signature in shape. Moreover, the intermediate droplet shapes are independent of the details of the initial configuration, while the time scale they are reached depend strongly on the droplet volume. This is shown by the numerical solutions of the thin-film model and demonstrated by quantitative comparison to experimental results.