Assessing micro- and nanoscale adhesion via liquid metal-based contact angle measurements in vacuum

Abstract

Understanding wetting phenomena is of critical importance for various fields in theoretical and applied surface sciences as well as for the functionality of micro- and nanoelectromechanical systems. Contact angle measurement is one of the well-established methodologies for the wettability assessment of a surface. However, it faces major challenges when applied to micro- and nanoscale structures. Here, we exploit the superior properties of liquid metal alloys to contact angle measurement thus creating a methodology that allows for reliable examination of wetting phenomena on small-scale surface sections within the single-digit micrometer range and below. The technique applies electromigration to prepare oxide-free liquid metal droplets with diameters of less than ten micrometers in a vacuum environment, enabling a scanning electron microscope to be used for contact angle measurement. Static and dynamic contact angle measurements can be realized via the targeted manipulation of the droplets. The characterization of microscale surface sections with different nanoscale surface texture is demonstrated. Following this approach, unique characterization of wetting properties on the small scale becomes feasible. The methodology presented is therefore of significant importance for various emerging research fields such as micro- and nanorobotics as well as studies of few asperity contact mechanics on the micro- and nanoscale.