Role of surrounding gas in the outcome of droplet splashing

Abstract

This study investigates the influence of the surrounding gas on a droplet impacting a smooth dry glass surface at high Weber and Reynolds numbers. It was performed using a flywheel experiment and different gases at ambient pressure. We analyzed the splashing outcome by measuring the size, velocity, and angle of the secondary droplets and by calculating the total volume ejected. We show that gas entrapment is not the mechanism responsible for splashing at high Weber and Reynolds numbers. We demonstrate that splashing is influenced by the density, followed by the viscosity, and last by the mean free path of the surrounding gas. Furthermore, the surrounding gas primarily affects the number of secondary droplets ejected and their ejection angle, whereas the droplet size and horizontal velocity are independent of the surrounding gas properties. We provide the first theoretical expression for the total volume ejected using the theory of Riboux and Gordillo [Phys. Rev. Lett. 113, 024507 (2014)], which attributes the secondary droplet generation to a lift force experienced by spreading lamella. The relationship between the ejected volume and the splashing parameter is described by a power function.

On the splashing of high-speed drops impacting a dry surface

“…However, recent experiments indicate that this value may be smaller when impacting blood drops (De Goede et al 2017) or Newtonian liquids on surfaces with large advancing contact angle (Quetzeri-Santiago et al 2019). A variation of this splashing threshold due to surrounding gas effects was also highlighted in Burzynski & Bansmer (2019), where the splashing threshold β for high-speed impacts fluctuates between 0.16 and 0.19 depending on the properties of the gas. Additionally, the data used by Riboux & Gordillo (2014) provides hints that this splashing threshold slightly increases with the impact velocity (see figure 7b in their supplemental material).…”

“…Additionally, we calculate the total ejected volume by extrapolating the detected secondary droplets in the focal plane around the impact centre. The basic algorithm of this method was presented by Burzynski & Bansmer (2019) and it is illustrated in figure 5(c-f ). This extrapolation method consists of the following five steps to estimate the ejected volume flux.…”

“…This instability has been speculatively assumed by Li et al (2018) after examining the structure and wavelengths of the unsteady azimuthal undulations present in the spreading lamella at the early stage of impact. Xu et al (2005) have shown that aerodynamic effects determine the splashing threshold; however, the properties of the surrounding gas do not affect the type of splash, corona, or prompt (Stevens et al 2014;Roisman et al 2015;Burzynski & Bansmer 2019). This is emphasised by analysing the gas Weber number of the lamella We l = ρU 2 l h µ /σ , which for the small thickness and low gas density results in a very small number.…”

“…They demonstrated than the arithmetic mean diameter decreases with increasing impact velocity, while the droplet velocities also increase. Using a similar set-up with even higher spatial resolution, Burzynski & Bansmer (2019) demonstrated the role of the surrounding gas on the ejection of secondary droplets in the prompt splash regime, which indicates that the size distribution of the ejected droplets is independent of the gas. They also estimated the total ejected volume and demonstrated that splashing is influenced primarily by the density, followed by viscosity, and lastly by the mean free path of the gas.…”

An Experimental Study on the Effects of Atomized Rain of a High Velocity Waterjet to Downstream Area in Low Ambient Pressure Environment

“…Depressurized chambers have been applied to mechanism research and practical applications in many related fields. In the study of water mist and water-droplets, Cai [32] discovered that the water mist tends to gather towards the conical surface under operating conditions of reduced ambient pressure (<P 0 ) and constant nozzle pressure; Wang et al [33] found that the mean diameters of water mist decrease as the ambient pressure decreases (<P 0 ) under conditions with the same working pressure of water mist system, while axial velocities of water mist droplets are affected by the ambient and system working pressure; Tsai et al [34] indicated that there is a dry area at the collision center that can trap an air mass after an ethanol drop (millimeter-sized) impacts a solid wall, and a lower ambient pressure leads to a smaller splash; Mitchell et al [35] presented droplet splash experiments using 3.5mm diameter ethanol droplets discretely impacting on metal surfaces at 4m/s, in which the impulse and peak force of collisions were slightly higher in low ambient pressure (23.3 kPa) than in stranded pressure (101 kPa) despite the lower ambient pressure significantly reducing the corona splash; Latka et al [36] analyzed the inhibiting effects of surface roughness and environmental pressure, such that the splash can be suppressed under low ambient pressure conditions with very rough surfaces and the splash can be produced under high ambient pressure conditions with relatively smooth surfaces; Burzynski et al [37] expounded that the angle and number of secondary droplets are affected by the surrounding gas, but the size distribution and horizontal velocity are not affected by the impacting course of a droplet and a smooth dry glass surface. Hence, low ambient pressure has a high potential to impact the movement of atomized rain droplets.…”