Bouncing-to-wetting transition for droplet impact on soft solids Surjyasish Mitra a , Quoc Vo b,∗ , and Tuan Tran b,∗∗ Soft surfaces impacted by liquid drops trap more air than their rigid counterpart. Due to the extended lifetime of this air film, the dynamics and eventual rupture of trapped air film depend on its interaction with the air cavity developed in the liquid during impact. In this work, we investigate the interaction between air cavity collapse and air film rupture for drops impacting on soft, hydrophobic surfaces using high-speed laser interferometry. We first reveal three different rupture dynamics of the trapped air film; the rupture may initiate either at the center of the dimple, at the edge of the dimple or at a random point in the dimple’s outer rim. We found that the transition between these rupture dynamics depends on both the impact velocity and the surface elasticity. Further, in the most special case of air-film rupture, the so-called dimple inversion, the rupture is directly caused by the collapsing air cavity in the droplet bulk. We further observe that in such cases, high-speed jets occur. We then provide a detailed characterization of the collapsing dynamics of the air cavity and subsequent jetting. 1 Introduction The air film separating an impacting liquid droplet with an im- pacted surface plays a crucial role in dictating impact outcomes. For high velocity impacts, reducing the pressure of this air film leads to splash suppression 1 , a crucial goal in printing technol- ogy 2 . For low velocity impacts, the presence of a sustained air film leads to droplet bouncing 3–7 , a phenomenon that forms the basis of self cleaning applications 8 . As a result, numerous inves- tigations have been focusing on impact phenomena involving the intervening air film right at the moment wetting occurs 1,3–7,9 . Typically, the prelude to the final touchdown between an ap- proaching droplet and a solid surface is the formation of a thin air film in which lubrication pressure is built up. The lubrica- tion pressure subsequently becomes sufficiently large that it de- forms the droplet’s bottom surface, creating a central dimple sur- rounded by an outer edge with one or two kinks, the regions where the air film thickness is minimum 5,6,10 . For impact on hy- drophilic surfaces, bouncing is ensured at low impact velocity. For higher impact velocity, the air film typically ruptures either at in- ner or outer kink leading to wetting initiation 5,6 . Most droplets impacting smooth hydrophilic surfaces like glass or mica either bounce or deposit due to random wetting initiation. Impact outcomes on hydrophobic surfaces are markedly differ- ent 11,12 . An impacting droplet, above a critical impact velocity, a School of Physical & Mathematical Sciences, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore b School of Mechanical & Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore. E-mail: ∗ xqvo@ntu.edu.sg, ∗∗ ttran@ntu.edu.sg may develop a pyramidal structure due to capillary waves in- duced upon impact. This in turn leads to formation of a cylin- drical air cavity that penetrates deeply into the droplet and even- tually collapses, shooting out a liquid jet 11 . The collapsing mech- anism of the air cavity during droplet impact on hydrophobic sur- faces was studied in details and it was found that the collaps- ing dynamics was dominated by inertia, similar to pinching-off of bubbles in liquid 11 . However, for larger impact velocity, inertia no longer dictates air cavity collapse that leads to very high jet velocity or bubble entrapment in the liquid bulk 11,12 . The rea- son for this transition remains elusive. Does the intervening air film have any role in dictating this transition? Most experimental studies involving droplet impact on hydrophobic surfaces utilized side-view imaging, which is incapable of resolving both the bulk cavity collapse and rupture of the air film 11–13 . Hence, interac- tions between the air film and the collapsing air cavity are largely unexplored. To probe such interactions, a necessary condition is the air films under impacting droplets must be sustained during air cavity for- mation in the bulk of the droplets. This condition is rarely met for impacts on rigid hydrophilic or hydrophobic surfaces due to random air film rupture 5,6,11,12 . Random rupture and subse- quent wetting causes premature collapse of the air cavity in the bulk. Recent experiments have shown droplets impacting soft, hydrophobic surfaces increases the lifetime of the air film 14 .A sustained air film trapped between the impacting droplet and the solid inhibits wetting initiation and can also facilitate higher bouncing probability 15 . At the same time, it enables us to probe the interactions between the air cavity in the bulk and the en- 1–7 |1 arXiv:2103.09473v1 [physics.flu-dyn] 17 Mar 2021