Failure mechanisms of air entrainment in drop impact on lubricated surfaces

Abstract

Lubricated surfaces have recently been introduced and studied due to their potential benefit in various configurations and applications. Combining the techniques of total internal reflection microscopy and reflection interference microscopy, we examine the dynamics of an underlying air film upon drop impact on a lubricated substrate where the thin liquid film is immiscible to the drop. In contrast to drop impact on solid surfaces where even the smallest asperities cause random breakup of the entraining air film, we report two air film failure mechanisms on lubricated surfaces. In particular, using ≈5 μm thick liquid films of high viscosity, which should make the substrate nearly atomically smooth, we show that air film rupture shifts from asperity-driven to a controlled event. At low Weber numbers (We < 2, We = ρUR/σ, U the impact velocity, R the drop radius, and ρ the density and σ the surface tension of the droplet) the droplet bounces. At intermediate We (2 < We < 10), the air film fails at the center as the top surface of the drop crashes downward owing to impact-induced capillary waves; the resulting liquid-liquid contact time is found to be independent of We. In contrast, at high We (We > 10), the air film failure occurs much earlier in time at the first inflection point of the air film shape away from the drop center, where the liquid-liquid van der Waals interactions become important. The predictable failure modes of the air film upon drop impact sheds light on droplet deposition in applications such as lubricant-infused self-cleaning surfaces.

Regimes of wettability-dependent and wettability-independent bouncing of a drop on a solid surface

“…2015 c ; Pack et al. 2017) show that in some cases, gas film thickness can vary between and before contact ensues. For such low values of film thickness, intermolecular forces and non-continuum effects cannot be ignored during the final rupture process.…”

“…Pack et al. (2017) noted that in their intermediate cases (), the air film fails near the central dimple due to impact-induced capillary waves. The above discussion provides evidence for such a failure mechanism.At lower Weber numbers, such as with , surface tension restricts strong distortion to the drop shape as evident in figure 13( b , c ), but at higher Weber number, such as with , a large depression is created on the upper surface of the drop as seen in figure 13( d , e ).…”

“…To create atomically smooth surfaces, Pack et al. (2017) deposited a thin layer of high viscosity oil on the solid surface and studied the failure of air film during impact. They argue that the thin oil layer results in a more controlled rupture unlike the asperity-driven rupture on regular dry solid surfaces.…”

Interaction Forces between Water Droplets and Solid Surfaces across Air Films

“…This leads to a delay in the wetting/merging process. 10,22 During the later stage of the impact, the intervening air film collapses, 23,24 resulting in a spreading of liquid drops on solid surfaces, or a merging of two free surfaces.…”

Transitional stages of thin air film entrapment in drop-pool impact events

“…The drop was numerically found to 'skate' on a thin air film. Skating was later observed experimentally by Kolinski et al (2012); Kolinski, Mahadevan & Rubinstein (2014), de Ruiter, van den Ende & Mugele (2015) and Pack et al (2017). Other numerical studies including Mandre et al (2012) and Duchemin & Josserand (2011) furthered our understanding of drop skating on solid walls after the capillary transition.…”