Beyond contact angle measurements: probing liquid-solid interfaces using a water meniscus
How intense is the interaction between a solid and liquid? The answer to this fundamental question is critical to almost every familiar system, such as chemical reactions, cellular dynamics, crop growth, petroleum exploration, and steam condensation.
Before interfacial behavior can be optimized for better process or device performance, a measurement technique is necessary to quantify the solid-liquid interaction force. Conceptually speaking, to reflect how strong a force is, it is usually convenient to measure a deformation caused by the force. However, the capillary force of liquids is often too weak to cause any observable solid deformations, making the measurement extremely challenging.
Recent MechSE graduate and current postdoctoral scholar at the University of Chicago Jingcheng Ma, Professor Nenad Miljkovic, and mechanical engineering graduate student Ishrat Zarin have proposed a new measurement technique that characterizes the solid-liquid interfacial tension, and have published their work in a recent paper in Physical Review Letters.
In their paper, the authors demonstrate an amplification of the interfacial deformation by placing a very thin solid sheet on a liquid meniscus. The curvature of the meniscus is determined by the interfacial forces, allowing for direct surface force measurement. The solid film is designed to be ultra-thin, so that it is compliant to the liquid capillary force, and its elasticity plays a negligible role in the meniscus shape.
“This method can provide rich information that is difficult to obtain otherwise,” said first author Ma. “The contact angle method has been the go-to method in surface science for the last six decades. You can place a droplet on a solid surface, then measure the droplet’s contact angle to tell how much the water likes the solid. By what if water completely spreads and wets the solid? In fact, water only shows a finite contact angle on certain non-polar polymers, so characterization of high-energy surfaces like metals, ceramics, or semiconductors, which are important in many industries, is currently challenging.”
The researchers said their technique can be used for a variety of solids, from non-polar, soft polymers to highly wetting and rigid metals.
“Initially, we intended to implement capillary peeling to make free-floating thin metal films on water, which would be interesting from the perspective of thin film processing. However, we came up with a system to study surface properties of metals using polymer sacrificial films often used in microfabrication. I think it is an uncomplicated but effective way of measuring such a crucial thermodynamic property that was unexplored for a long time,” said Zarin.
The Office of Naval Research and the International Institute for Carbon Neutral Energy Research (I2CNER) supported this research.