Contact Angle Hysteresis on Curved Surfaces

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Contact between liquid droplets and solids arise in many everyday settings such as rain droplets on a car, condensation of dew on plant leaves, or insects walking on water ponds. To understand or control the behavior of these and related systems, we must first understand the shape of the fluid surfaces and the forces that they can apply. In all of these systems, there is an interface between two fluids (air and water, for example), which meets the surface of a solid object. The angle between the fluid interface and the solid surface (the "contact angle") is the key parameter that determines the interface shape and the forces applied by the fluids. Experiments show that the contact angle has a consistent value when a fluid interface advances across a dry surface, and a smaller value when the interface recedes (moves in the other direction). This variability of the contact angle, known as hysteresis, determines key properties such as whether droplets slide off a surface or remain trapped. There is no general theory for contact-angle hysteresis, but it is typically thought to be an inherent property of the materials being used. This project investigates a new finding that contact angle hysteresis also varies with the shape of the fluid interface. The project is important because understanding the role of geometry in contact-angle hysteresis will give broader insights into the phenomenon that we currently lack. Ultimately, the results of this project will contribute to improved technological processes such as coating of fibers with fluids, stabilizing oil droplets with particles in food or oil-recovery applications, and even assembling electronic components by floating them on a liquid surface.This project will provide measurements of the of the contact-line shape and the contact angle by direct observation under conditions of advancing and receding fluids. Chemical treatment of the solid surface will be varied and a range of fluids will be used. Experiments focus on millimeter-scale objects so that the interface can be directly visualized. From the known interface shapes, the surface energies will be calculated numerically and, where possible, analytically. These results will allow testing of proposed mechanisms of contact-angle hysteresis. In addition to generating new basic science, the proposed research will support educational projects for local middle-school students and career training for undergraduate and graduate students at the University of Massachusetts Amherst.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.