Self-propulsion of floating objects

Abstract

Interfacial phenomena are of growing interest due to the burgeoning field of microfluidics enabled by recent advances in microfabrication techniques. One particular area of interest in the realm of interfacial effects is the generation of self-propulsion of floating bodies. We examine means of interfacial propulsion employed by floating bodies. Nature utilizes dynamic and quasistatic phenomena to induce propulsion. However, we show that quasi-static propulsion due to manipulation of the shape of the surface, as used by some insects in regions with background curvature, is impossible on a flat surface. We move from consideration of quasi-static phenomena to investigate the use of vertical vibration of a fluid bath as a mechanism to incite self-propulsion of objects at the interface. We build on the work of Pucci et al. (2013), who first noted that fluid lenses floating on an interface driven above the Faraday threshold may be induced to self-propel. Here, we advance the study of Faraday lenses by characterizing the shapes assumed by these lenses as a function of the size of the lens and the driving acceleration, yielding a rich array of dynamics at different driving forces. Seeking to describe the instability and transient behavior of the lenses we develop a model for the elongation of the films as they undergo the Faraday instability, predicting a t14 scaling that is well supported by experimental results. We also test the dependence of the radiation pressure on the bath acceleration by using the radiation pressure to drive motion. Particular attention is given to a Faraday boat and gear, which consist of rigid bodies with unstable fluid lenses pinned inside. The wave-field of the lens is rendered asymmetric due to the asymmetry of the body. We find that we can drive propulsion of these rigid bodies via radiation pressure.