Liquid marble and water droplet interactions and stability

Abstract

The interactions between two individual water droplets were investigated in air using a combination of coalescence rig and high speed video camera. This combination allows the visualization of droplet coalescence dynamics with millisecond resolution which provides information on droplet stability. Bare water droplets coalesced rapidly upon contact, while droplet stability was achieved by coating the droplets with polystyrene particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] hairs (PDEA-PS particles) to form liquid marbles. The asymmetric interaction of a water droplet (pH 3 or 10) armoured with the PDEA-PS particles (liquid marble) with a bare droplet at pH 3 exhibited intermediate stability with coalescence observed following an induction time. The induction time was longer for the pH 10 liquid marble, where the PDEA-PS particles have a hydrophobic surface, than in the case of a pH 3 liquid marble, where the PDEA-PS particles have a hydrophilic surface. Furthermore, film formation of PDEA-PS particles on the liquid marble surface with toluene vapour confirmed capsule formation which prevented coalescence with the neighbouring water droplet instead wetting the capsule upon contact within 3 milliseconds. This study illuminates the stability of individual particle-stabilized droplets and has potential impact on processes and formulations which involve their interaction.