The Role of Air Entrainment in Drop Impact Dynamics on Lubricated Surfaces
Authors:   , Pack Min Young , Sun Ying 
Published: July 2021

Drop impact is fundamental to various natural and industrial processes such as rain-induced soil erosion and spray coating technologies. The recent discovery of the role of the air entrainment between the droplet and impacting surface has produced numerous works uncovering the unique physics that correlates the micro-scale and nano-scale spatio-temporal dynamics to the macro-scale impact outcomes such as droplets bouncing on thin air films no thicker than a few microns. In this work, the macroscopic droplet morphologies under impacting conditions are juxtaposed with the microscopic behavior of the air film underneath the droplet where an interplay of various forces - inertial, viscous, capillary and intermolecular forces - coexist and dictate the final outcomes of the impacting drop. Lubricant infused surfaces (LIS) have recently been studied due to their potential benefit in various applications such as pressure-stable, self-healing, ice- and liquid-repellent materials. 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 lubricated substrates 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 [mu]m 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) 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. While the pre-contact air entrainment effects have been well studied, the post-air entrainment effects and the subsequent wetting instability at the contact line is not well understood - particularly for Weber (We) numbers well below the splash threshold. We uncover the post-air entrainment dynamics of impacting droplets on lubricated surfaces by changing both the droplet impact velocities (0.9 - 1.4 m/s) and viscosities (10 - 100 mPa s) under ambient conditions to elucidate the growth of instabilities at the wetting front on atomically smooth, viscous silicone oil films of varying viscosities at constant thickness of ~ 3 [mu]m. For We number greater than 10 at which the disjoining pressure overcomes the capillary pressure, the air film fails at the lowest air film thickness and the liquid-liquid contact line grows radially outward. What is the criterion for the interface to be unstable? We suspect that it is the air rim that moves with the wetting front that is unstable due to the air pushing against the liquid. Overall, this work focused on developing a novel dual TIRM-RIM technique enabling the facile in situ measurements of the underlying air film under various drop impact scenarios. Future work will focus on the droplet jetting dynamics on LIS.