title: Time-Dependent Wetting Behavior of Hydrogels
Creators: name: Kashani, Amir
Publisher: name: University of Nevada, Las Vegas
publisherIdentifier: https://ror.org/0406gha72
publisherIdentifierScheme: ROR
Year: 2023

description: Hydrogels are 3-D polymers with hygroscopic properties that enable them to absorb and retain large amounts of water. In many applications such as biomedical applications, including drug delivery, wound healing, tissue engineering, biosensors, and contact lenses, agriculture, food and beverage, cosmetics, and environmental, hydrogels also need to maintain a high degree of wetting in order to ensure effective functionality and interaction with surrounding fluids or tissues in desired applications. The combination of wetting and absorptive behavior of hydrogels that are often encountered in biomedical, biological, industrial and the mentioned applications presents a challenging physical problem that is not completely understood. Similar to other surfaces, the wettability of hydrogels can be studied by measuring contact angles, which are further classified into advancing and receding modes. While absorption and wetting behavior are typically thought of as independent behaviors, some previous studies suggest these behaviors are linked. To elucidate this swelling-absorption relationship, I investigated the changes in contact angles of water droplets on hydrogels of varying swelling behaviors. To do so, I helped develop an experimental setup that includes the ability to capture images, perform image processing, make contact angle and droplet size measurements, as well as control droplet growth. I observed that the hydrogel surface had a time-dependent response to droplets being placed on it, and that this response depended on the advancing speed of the droplet, the thickness of the surface, and the diffusion of water into the gel. In certain cases, with very fast advancing speeds, droplets would collapse into a lower contact angle state. To characterize this collapse behavior, I planned a set of experiments where I varied hydrogel thickness and droplet advancing speed as two independent variables. The resulting contact angle and generally observed wetting behavior were dependent variables. I created samples of three thicknesses of polyacrylamide (PAAm) hydrogels and applied including eight different advancing speeds. Recognizing that there is a time scale of diffusion of water into the gel and a time scale of droplet advancing speed, I hypothesized that the ratio of these two time scales, which is a type of Peclet number, determines whether this collapse occurs. If the time scale of advancing is much shorter than the time scale of diffusion (high Peclet number), then the droplet would be expected to collapse. After performing 75 experiments, I confirmed my hypothesis and found a threshold Peclet number of around 40 described the collapse criterion accurately. This study provides valuable insights into the wetting behavior of hydrogels through the characterization of their wettability based on surface thickness and spreading speed of a droplet on its surface. This information can be used to define a selective region of the Peclet number for a desired field of application. The results suggest a way to classify the performance and response of hydrogels based on their thickness with respect to the rate of water diffusion and the spreading speed of liquid on their surface. The desired response of hydrogels may vary in different applications, and defining a parameter for selecting an appropriate hydrogel surface can have a significant impact on the desired performance. The presented Peclet number in the results of this study could serve as an important parameter for selecting hydrogel surface for specific applications.