Two mechanisms of droplet splashing on a solid substrate

Abstract

We investigate droplet impact on a solid substrate in order to understand the influence of the gas in the splashing dynamics. We use numerical simulations where both the liquid and the gas phases are considered incompressible in order to focus on the gas inertial and viscous contributions. We first confirm that the dominant gas effect on the dynamics is due to its viscosity through the cushioning of the gas layer beneath the droplet. We then exhibit an additional inertial effect that is directly related to the gas density. The two different splashing mechanisms initially suggested theoretically are observed numerically, depending on whether a jet is created before or after the impacting droplet wets the substrate. Finally, we provide a phase diagram of the drop impact outputs as the gas viscosity and density vary, emphasizing the dominant effect of the gas viscosity with a small correction due to the gas density. Our results also suggest that gas inertia influences the splashing formation through a Kelvin-Helmoltz like instability of the surface of the impacting droplet, in agreement with former theoretical works.

Bouncing off the Walls: The Influence of Gas-Kinetic and van der Waals Effects in Drop Impact

“…Despite experimental advances, the precise mechanism by which ambient gas pressure changes can alter splashing remains a topic of debate. The physical effects proposed for the gas film's behavior include (1) gas kinetic effects (GKE; also known as rarefied gas effects) [8,[21][22][23], which become relevant for gas micro-and nanofilms, (2) inertia [24], and/or (3) compressibility [6,7], any of which can influence the stages of (i) precontact impact and (ii) postcontact wetting [25,26]. A full understanding of splashing would require all physics to be captured from the nano-to millimetric scale, with stage (i) providing an initial condition for (ii).…”

Role of the Dynamic Contact Angle on Splashing

“…Further studies have analyzed the role of the ambient gas on the lubrication force lifting the lamella [7,10,20,21] concluding that the surrounding gas viscosity is, arguably, the most influential parameter on splashing. Surprisingly, air at the impact point plays no significant role on splashing, but it is the air at the spreading edge that influences it [8,22,23].…”

Droplet Splashing on an Inclined Surface

“…Despite more than 140 years of study [5], there is a disagreement about the underlying mechanisms [1]. Theories based on the inertial dynamics [6][7][8][9][10][11], the Kelvin-Helmholtz instability [12][13][14][15], the air film dynamics [16][17][18][19], and the lamella aerodynamics [20][21][22][23] have been proposed. The inertial dynamics are unable to account for the effects of ambient pressure [21,24].…”