Splashing of a very viscous liquid drop impacting onto a solid wall wetted by another liquid

Abstract

In this experimental work the main focus is on the impact of a single drop of a very viscous liquid onto a thin, horizontal wall film of different liquid. Splashing resulting from drop impact onto a wetted wall occurs in many natural and engineering applications like in internal combustion engines and spray cooling. While the splashing threshold for low viscosity liquid drops has been extensively examined, impact of a very viscous drop is much less studied. The viscosities of drop and wall film liquids are varied up to kinematic viscosities of 100,000 mm²/s. The liquids used in the experiments are miscible. The impact outcome is determined by the impact parameters and fluid properties. The effect of very viscous liquids used as drop fluid and as wall film liquid on the kinematic of the corona expansion is investigated in the experiments. The results of drop impact onto solid walls are compared to obtain the limiting asymptotic values for the splashing threshold. Finally, a semi-empirical model for the splashing threshold, for the maximum spreading radius Dmax and for the maximum spreading times tmax are developed for extremely viscous liquids. Keywords Drop impact, high viscous liquids, wetted wall IntroductionSplashing as a result of a drop impact onto a thin liquid film [1][2][3] or onto a dry substrate [4-6] occurs in many natural and engineering applications like spray coating, spray cooling, paint spraying, ink-jet printing and in internal combustion engines. Impact of a supercooled drop onto a surface of an aircraft can lead to ice accretion, while collision with a heated substrate can lead to drop evaporation or intensive boiling [7]. The drop/wall interaction is affected by the fact that the drop and the liquid film are different liquids and may exhibit different degrees of miscibility and viscosity. A comprehensive review on the phenomena, modeling and application of drop impact can be found in [8]. Drop impact onto a wetted wall can lead to various different outcomes. In the case of splashing

Miscibility and wettability: how interfacial tension influences droplet impact onto thin wall films

“…For droplets with a much higher viscosity than the wall film, a two-stage crown was observed with the wall-film liquid at the upper part of the crown. Similar crown morphologies with high viscosity ratios have been observed for immiscible droplet/wall-film systems by Kittel et al (2018a), and can be explained by the strong differences in the time scale of the ejecta sheets coming from the droplet and the wall film (Marcotte et al 2019). By influencing the liquid repartition inside the crown, the viscosity ratio also influences the occurrence of the holes in the crown wall.…”

“…The droplet liquid is assumed to completely cover the wall-film liquid on the inside of the crown, as represented in figure 2. This full coverage by the droplet liquid on the inner crown wall has been observed numerically for aqueous droplet/wall-film systems (Zhang et al 2019), and experimentally for dyed water droplet impacting onto silicone oil films (Shaikh et al 2018), for (Kittel et al 2018a). In the current work, the maximum value of Re f is 196.…”

“…However, these numbers are not representative of the full binary droplet/wall-film systems since the liquid properties of droplet and wall film differ. A proper way of taking both droplet and wall-film liquid properties into account in these non-dimensional numbers is still under discussion in the literature (Geppert et al 2014;Kittel, Roisman & Tropea 2017). Recently, averaged liquid properties (subscript avg) such as ρ…”

“…Originally studied in the pioneering work of Worthington & Cole (1897) with water and milk, most of the studies on binary systems are with miscible liquid pairs (Thoroddsen, Etoh & Takehara 2006;Geppert et al 2016;Bernard et al 2017;Geppert et al 2017;Kittel, Roisman & Tropea 2018b;Geppert 2019) and focus on splashing limit, crown dynamics or on the viscosities. In contrast, binary systems with exclusively immiscible droplet/wall-film systems have been investigated mainly with respect to the characteristics of the secondary droplets (Shaikh et al 2018) and to the repartition of the droplet and wall-film liquids in the crown depending on the viscosity ratio (Kittel, Roisman & Tropea 2018a).…”

Optical investigation on the interaction between a fuel-spray and an oil wetted wall with the focus on secondary droplets

“…The interaction of droplets on surfaces and fluid films are well described in the literature. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] The droplet impact and rebound as well as the surface wetting of dry walls is described from Richter. 6 The interaction between a droplet and a fluid film is highly influenced by the droplet size to film thickness ratio.…”