Controlling Liquid Wetting of Textured Surfaces using Ultrasound

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Successful implementation of many common manufacturing processes rely on controlling when, where, and how fluids and liquids wet a solid. This includes processes like painting, bonding, cleaning, soldering,and brazing. This ability to control if and when a liquid wets a surface is also of importance in self-cleaning and stain resistant surfaces. Such surfaces can provide important technological and economic advantages in a variety of applications by, for example, reducing maintenance and cleaning costs of equipment and buildings such as naval ships, improving efficiency of solar cells by preventing haze and dirt buildup on their surfaces outdoors, and lowering the friction of objects moving through a liquid. This project examines how high frequency vibrations, i.e., ultrasound, can be used to change the wetting state of a liquid on a surface based on the relationship between the size scale of the surface texture and the vibration frequency. The combination of texture and applied vibration will enable new control over where and when a liquid wets a surface or de-wets from it. Potential applications include controlling bonding locations, facilitating separation of bonded components for recycling, and improving the wetting of powders by liquids. The research efforts will be integrated with the education of both graduate and undergraduate students. The project will also be used to enhance outreach programs with local K-12 schools through new educational activities on vibration and acoustics. Textured surfaces (ones with surface roughness) can be stable in multiple wetting states. The two extreme conditions occur when a fluid or liquid penetrates into the recesses of the texture (Wenzel state) and when the fluid rests on top of the peaks of the texture (Cassie state). The Wenzel state has much stronger adhesion due to the larger contact area. For each fluid/substrate combination, one state will typically have lower energy, but there is an energy barrier that must be overcome in order to transition to the lower energy wetting states. Prior work has demonstrated that transitions can be achieved by vibrating a droplet at its characteristic frequency. However, this is impractical in many applications because every droplet has different vibration frequencies. This project will use frequencies that are characteristic of the surface structure so that they can work on droplets of arbitrary sizes. Specifically, the project will identify the energy required at different frequencies, compare the effectiveness of different vibrational modes, and evaluate the impact of asymmetric vibrations on the fluid wetting. These objectives will be accomplished by both experimental measurements of the pressure to move the wetting front of a liquid on a patterned surface and numerical modeling of the contact lines subjected to vibration.