Droplet impact near a millimetre-size hole: closed pit versus open-ended pore Rianne de Jong * , Oscar R. Enr´ ıquez, and Devaraj van der Meer Physics of Fluids Group, MESA+ Institute for Nanotechnology, and J.M. Burgers Centre for Fluid Dynamics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received Tuesday 11 th March, 2014 Abstract We investigate drop impact dynamics near both closed pits and open- ended pores experimentally. The resulting impact phenomena differ greatly for a pit or a pore. For the first, we observe three phenomena: a splash, a jet and an air bubble, whose appearance depends on the distance between impact location and pit. Furthermore, we found that splash velocities can reach up to seven times the impact velocity. Drop impact near a pore, however, results solely in splashing. Surprisingly, two distinct and disconnected splashing regimes occur, with a region of plain spreading in-between. For pores, splashes are less pronounced than in the pit case. We state that, for the pit case, the presence of air inside the pit plays a crucial role: it promotes splashing and allows for air bubbles to appear. 1 Introduction Droplet impact can be observed regularly in daily life, for example, when rain- drops hit the ground or while washing the dishes. Additionally, in industrial processes such as spray coating, spray cooling and ink-jet printing, drop impact is of major importance. The topic has been studied for more than a century, starting with Worthington, who in 1876 listed the huge variety in shapes that the drop can take after impact on a solid substrate (Worthington, 1876). It has been found that the liquid properties, the droplet size and velocity, the wetta- bility and roughness of the substrate, as well as the surrounding gas pressure, all have an influence on the behaviour of the drop after impact (Yarin, 2006; Rioboo et al., 2001; Richard et al., 2002; Clanet et al., 2004; Xu et al., 2005; Tsai et al., 2009; Bouwhuis et al., 2012). In spite of the fact that many real surfaces are likely to be rough and inho- mogeneous, droplet impact research has mostly concentrated on homogeneous isotropic solid surfaces. However, some authors looked at smooth solid sub- strates containing a single obstacle (Delbos et al., 2010; Ding & Theofanous, 2012; Lorenceau & Qu´ er´ e, 2003; Roisman et al., 2010; Josserand et al., 2005), * Email address for correspondence: jong.riannede@gmail.com 1 arXiv:1403.3574v1 [physics.flu-dyn] 14 Mar 2014