Impact of droplets on immiscible liquid films

Abstract

The impact of droplets on liquid films is a ubiquitous phenomenon not only in nature but also in many industrial applications. Compared to the widely-studied impact of droplets on films of identical fluids, the impact of droplets on immiscible films has received far less attention. In the present work, we show using high-speed imaging that immiscibility has a profound effect on the impact dynamics. The impact of a water droplet on an oil film leads to the formation of a compound crown followed by a central jet, whereas that of an oil droplet on a water film results in rapid spreading on the film surface driven by a large, positive spreading factor. In the former scenario, the central jet occurs due to the severe stretching of the droplet during the formation of the crown and then the retraction of the droplet by capillarity, which leads to the collision of fluid at the impact point. A model for the elongation dynamics of the central jet is proposed based on energy conservation. The effects of key parameters controlling the impact process are analysed, including the droplet Ohnesorge and Weber numbers, the viscosity ratio, and the dimensionless film thickness. Different impact outcomes are discussed, such as bouncing, deposition, and oscillation of the impact droplet, the formation and collapse of the compound crown, and the formation and tip-pinching of the central jet. This study not only provides physical insights into the impact dynamics, but could also facilitate the control and optimisation of the droplet impact process in a number of applications as highlighted herein.

Deep pool water-impacts of viscous oil droplets

“…The splash crown consists of both drop and pool liquids, and depending on the drop's impact velocity, viscosity and interfacial surface tension with pool liquid, it might spread over the crater surface to a varying extent. The presence of drop liquid in the crown was recently also reported in the works by Che & Matar 19 and Shaikh et al, 20 both of which studied water drop impacts on thin films of oil. Such a crown which is made of both the liquid components was termed a 'compound crown' 19 -its outer surface is composed of target pool liquid (in present study, water) and the inner surface is covered with the drop liquid (here, silicone oil).…”

“…The presence of drop liquid in the crown was recently also reported in the works by Che & Matar 19 and Shaikh et al, 20 both of which studied water drop impacts on thin films of oil. Such a crown which is made of both the liquid components was termed a 'compound crown' 19 -its outer surface is composed of target pool liquid (in present study, water) and the inner surface is covered with the drop liquid (here, silicone oil). After ejection of tiny-drops by corona, the rim begins to stabilise as the sheet retracts back into the pool.…”

“…Such finger formation in the crown has been reported during water drop impacting on thin oil film experiments. 19,20 We report that the same phenomenon occurs when an immiscible oil drop is impacts on a deep pool of water. A detailed understanding of this may give insights into fragmentation processes found in nature and industry.…”

“…The latter focuses on more traditional quantities of interest such as maximum diameter of drop spreading, and depths of the craters formed. More recent works by Che & Matar 19 and Shaikh et al 20 have studied immiscible drop impacts, but on thin films.…”

The impact of an oil droplet on an oil layer on water

“…The impingement of a liquid droplet on another liquid has been studied extensively (Pumphrey & Elmore 1990;Wang & Chen 2000;Cossali et al 2004;Bisighini et al 2010;Castillo-Orozco et al 2015;Che & Matar 2018;Shaikh et al 2018). When a droplet impinges on a deep pool of the same liquid, e.g.…”