Comparison of splashing in high- and low-viscosity liquids

Abstract

We explore the evolution of a splash when a liquid drop impacts a smooth, dry surface. There are two splashing regimes that occur when the liquid viscosity is varied, as is evidenced by its dependence on ambient gas pressure. A high-viscosity drop splashes by emitting a thin sheet of liquid from a spreading liquid lamella long after the drop has first contacted the solid. Likewise, we find that there is also a delay in the ejection of a thin sheet when a low-viscosity drop splashes. We show how the ejection time of the thin sheet depends on liquid viscosity and ambient gas pressure.PACS numbers: 47.20.Gv,47.55.Ca,The discovery by Xu et al. [1], that the splash of a liquid drop hitting a smooth dry surface is suppressed by lowering the ambient air pressure, has galvanized research on gas-liquid interactions during impact. However, despite numerous experimental [2][3][4][5][6][7][8], theoretical [9,10], and numerical [3,11,12] efforts, the mechanism by which air causes a drop to splash remains unresolved.The situation is made more complicated, by the influence of liquid viscosity µ on the interplay of gas and liquid. At low viscosities, a beautiful crown-shaped corona emerges almost immediately after impact as shown in Fig. 1(a) [1,2]. However, a small increase in viscosity reveals a splash with a strikingly different appearance, that evolves much more slowly ( Fig. 1(b)). This higher-µ drop first contacts the surface and then spreads smoothly as a thick lamellar sheet. From this lamella, a thinner sheet of liquid is subsequently ejected almost parallel to the substrate. It is the thin sheet that eventually breaks apart to form the splash [4]. The existence of two distinct splashing regimes is made manifest in the non-monotonic dependence of the threshold pressure, P T , which is the ambient gas pressure above which splashing occurs, on the viscosity [2]. As shown in Fig. 2, P T decreases with increasing viscosity at low-µ, while the trend is reversed at higher µ.These differences have been taken to suggest that distinct mechanisms might underlie the two types of splash. Indeed, theories for low-µ splashes have been proposed that do not take into account any spreading of a liquid film on the substrate before the onset of the splash [9,10]. On the other hand, the fact that, regardless of viscosity, splashes are invariably suppressed when the ambient pressure is sufficiently low suggests that there may be a common mechanism for both the violent corona and the slowly evolving thin sheet. It is therefore imperative that one investigate whether the splash mechanisms in these two cases have common features even though the timescales for corona (or thin-sheet) ejection and the overall shape of the splashing drops differ dramatically. This paper studies the onset of thin-sheet and corona ejection in the two cases. As previously noted, at high-µ, thin-sheet ejection is delayed when the pressure is lowered [4]. The major conclusion from the present work is that this is also true in the low-viscosity regime. Corona ejection ...

Drop splashing is independent of substrate wetting

“…1(a) shows that a drop does not splash immediately. Instead the liquid spreads radially outward in the form of a lamella 23,24 . The simultaneous interference image shows that the lamella fully wets the substrate.…”

“…However, the focus of this work is on the capillary number at the contact line/edge of the droplet, which, as is shown below, is of the same order between simulations and experiments. In addition, as the splashing threshold has been shown to scale across a wide range of parameters 5,24,39 , comparisons with experiments should not be compromised.…”

Double Contact During Drop Impact on a Solid Under Reduced Air Pressure

“…Indeed, Driscoll and Nagel [7] verified with interferometry that outside of the central air disc the liquid is in contact with the solid. It is now accepted that the above suppression of splashing is related to the dynamics at the tip of the expanding lamella moving along the substrate, not the central air disc [8,9]. Gliding on the air layer is only observed at very low impact velocities [10,11], or for highly viscous drops, which do not splash [12].…”

Simulations of splashing high and low viscosity droplets

“…Unfortunately, the computational cost of these simulations make it unfeasible to perform simulations for a larger number of different viscosities: a simulation of a low viscosity ethanol splash takes several weeks and the simulation of a high viscosity silicone oil splash takes several months to complete. Therefore it was not possible to investigate the origins of the different trends which are observed when plotting the splashing threshold pressure as function of viscosity 2,12,13 . However, our observations concerning the thin gas film and lamella behavior are made for both low viscosity and high viscosity liquids.…”

“…While, none of the above models considers a different high and low viscosity splashing regime, there is experimental evidence that two different regimes exist 2,12,13 . When plotting the ambient threshold pressure, above which splashing can be observed, as function of the viscosity of the liquid two distinct regimes can be observed; for low viscosities the threshold pressure decreases with increasing viscosity while this trend is reversed for higher viscosities 2,12,13 . In this work computer simulations are presented for ethanol and silicone oil droplets impacting a dry solid surface at both atmospheric and reduced ambient pressure.…”