Buoyancy of a Thin Plate Pressing a Floating Oil Film on Water Xiang-Ying Ji † and Xi-Qiao Feng* ,†,‡ † Institute of Biomechanics and Medical Engineering, AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China ‡ Center for Nano and Micro Mechanics, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China ABSTRACT: Because of the superhydrophobicity of their legs, water striders and many other aquatic creatures can stand and walk effortlessly on water. Because of pollution, an oil film may exist on water in some practical situations. To date, however, it remains unclear how the presence of an oil film would affect the wetting behavior of an object floating on water. In this work, we investigated, both theoretically and experimentally, the buoyancy of a thin plate pressing the surface of a bilayered liquid system. In particular, the effect of the oil layer on the buoyancy force was examined. The critical depth and the corresponding buoyancy at the penetration of the plate into the liquids were obtained analytically. For a plate vertically pressing the liquid surface, the force-displacement loop during a complete advancing-receding cycle was analyzed. Experiments were also performed to verify the theoretical results. 1. INTRODUCTION Many aquatic creatures, such as water striders, water spiders, and mosquitos, possess an outstanding ability to live on water, which is mainly attributed to the hydrophobicity of their legs and the surface tension of water. 1-3 In recent years, the superior living ability of these little creatures on water has attracted the curiosity of many scientists. Because of their hierarchical surface structure, for example, water strider legs are superhydrophobic, with a contact angle up to 150°. A single leg of a water strider can bear a force ∼15 times the total weight of the insect’s body without piercing the surface of water. 3 For small objects such as these aquatic bugs with a characteristic size comparable to or smaller than the capillary length of a liquid, the wetting behavior exhibits a distinct size effect and surface tension plays a significant role in their buoyancy. In many practical situations, a thin film of oil or other liquids may exist on water because of pollution. However, it remains unclear how such a floating oil film affects the water repellency of these aquatic creatures and whether it would threaten their lives. The buoyancy of a microsized floating body on a homogeneous liquid such as water has been systematically investigated. 4-6 To uncover the mysteries of nature, the correlation between the superior water expelling ability and the surface microstructures of water strider legs has attracted considerable attention in recent years. Feng et al. analyzed the maximal depth and buoyancy of a superhydrophobic cylinder pressing a water surface before penetration. 7 The effects of the cross-sectional shape and flexibility of the leg on the buoyancy were also examined. 8-13 Despite the understanding of the interaction between a solid object and a homogeneous liquid, the buoyancy and motion of small objects interacting with heterogeneous bilayer liquids are poorly understood. Recently, considerable effort has been directed toward studying small objects remaining at an oil- water interface. 9,14,15 Some basic relations have also been obtained for oil-water-solid triphase interactions. 16-18 Pre- sented work is aimed at understanding the influence of an oil film on the buoyancy of small objects, e.g., aquatic creatures. The whole process of a rigid tilted plate pressing the surface of pure water or an oil layer floating on water is analyzed. The critical depth and buoyancy at the penetration of the thin plate into the water surface or the oil-water interface are obtained. Furthermore, the pressing-separating process of a hydrophobic and oleophilic plate vertically contacting the oil-water system is investigated, both theoretically and experimentally. 2. THEORY 2.1. Critical Buoyancy. 2.1.1. Pure Liquid. We first consider a thin rigid plate pressing a flat liquid surface, say water, as shown in Figure 1A. The pressing process and critical penetration of such a tilted plate on water were analyzed recently by Park and Kim, 11 but the issue of a plate pressing a bilayered liquid system has not been analyzed to date. Refer to Received: January 25, 2013 Revised: May 2, 2013 Published: May 9, 2013 Figure 1. (A) Model for a plate obliquely pressing a water surface. (B) Model for a plate obliquely pressing an oil-water bilayer system. Article pubs.acs.org/Langmuir © 2013 American Chemical Society 6562 dx.doi.org/10.1021/la400341q | Langmuir 2013, 29, 6562-6572