Water Transport Over and Through a Thin Permeable Medium with Wettability Contrast

The knowledge of interaction of a liquid drop with wettability-contrasted solid and permeable surfaces is an indispensable key to develop and improve many recent technological and biological applications, ranging from microfluidics to enhanced drainage of air-conditioning evaporators. The objective of the present work is to provide new insights in the field of liquid interaction with permeable and impermeable wettability-patterned surfaces. The work includes application of artificial intelligence (AI) tools for learning TiO2 coating data, patterning a permeable medium to achieve certain modes of precise transport of metered liquid microvolumes on and through such substrates, and lastly, developing and validating a mathematical model for the characterization of volume porosity and surface permeability. In the first task, a mathematical model is trained over a number of experiments and checked on others on being able to first classify the data based on specific features and second to predict the wettability of any surface coated with a specific TiO2 formulation and irradiated with ultraviolet (UV) light. Taking the TiO2 mass fraction and UV irradiation time as inputs, the water contact angle on a coating is produced as an output, and vice versa. In the second task, spatial wettability patterning is used to transport liquid over and through permeable media. The transport is demonstrated not only laterally on open surfaces of the permeable media, but also transversally through their thickness. Multiple strategic designs of wettability patterns are implemented to attain different schemes (modes) of three-dimensional transport in a high-density paper towel (HDPT). All schemes facilitate precise transport of metered liquid microvolumes (dispensed as droplets) on the surface and through the substrate. The third task demonstrates the use of a mathematical model integrated with an experimental setup to characterize a thin permeable medium. The setup consists of horizontally-laid permeable substrate coated with certain designs combining wettable and non-wettable domains suitable for the required measurements. The experimental setup features two inclined mirrors that help capture synchronized images of water droplets as they infuse through the permeable medium and emerge on its underside. This dissertation provides a wettability-patterning methodology for precise handling of liquid microvolumes on porous samples, and offers an experimental procedure to quantify the volumetric porosity of such materials. Thus, the present results could be useful for various applications ranging from microfluidics, to low-cost medical diagnostic devices and filtration products.