Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99774

99774 - Length, Time, and Rate-Dependent Indentation Adhesion of Hydrogels 

Hydrogels are both important natural components and engineering materials. In both natural and engineering applications, hydrogels often involve adhesion interaction with other materials. Understanding the adhesion mechanism and quantifying the adhesion properties of the hydrogel are important. To measure the adhesion behavior, we applied the indentation method on hydrogels because of its convenience to use and its ability to be applied across a wide range of length and time scales. In this work, we probe the polyacrylamide hydrogel surface using the polystyrene spherical indenter. The indenter is first loaded and pressed into the hydrogel, then held in a certain depth of indentation for an amount of contact time, and finally retracted from the surface until full separation. By changing the indentation depth, contact time, and retraction speed, the length, time, and rate-dependence of the hydrogel adhesion are systematically measured. Using Atomic Force Microscope (AFM) and Microindenter, we first perform indentation measurements across a wide range of length scales with contact radii from 1.9 μm to 75.5 μm. During the loading period of the indentation, no long-range interaction is observed, and the force-displacement curve follows the Hertzian solution. The effect of adhesion only occurs during the retraction process. Additionally, the pull-off force increases with contact radius and then reaches a plateau value. These phenomena cannot be modeled by the conventional adhesion theories, such as the JKR theory and the Maugis-Dugdale theory. To model the adhesion hysteresis and the length-dependence and to extract intrinsic adhesion properties of the interface, we develop an analytical model. In this model, as the indenter is pulled up, a cohesive zone is developed. As the indenter is pulled further, the cohesive zone size increases, but the apparent contact radius is kept constant until the mechanical energy reaches the adhesion energy. Based on this picture, the analytical solution is developed, the adhesion hysteresis and length-dependence are predicted by the analytical solution, and the theoretical results fit well with experimental data. We then perform indentation measurements across a wide range of contact times from 0.1 s to 800 s. During the holding period, the force on the indenter decreases and then reaches a plateau. The pull-off force is observed to increase over contact time, and the time scale is independent of the relaxation time scale of the hydrogel. We finally perform indentation measurements across a wide range of retraction velocities from 1 nm/s to 50 μm/s. The pull-off force first decreases and then increases with the retraction velocity. The transition phenomenon has not been observed or modeled in the literature before. Here, this phenomenon is assumed to be due to the competition of two time scales, one is the time for the adhesion to build up and the other is the rate-dependent pulling force. In the indentation test, for a larger pull-off velocity, the amount of adhesion sites that have been built is less as the total contact time is shorter, but the pull-off stress of the adhesion site is greater due to the faster velocity of pulling. These two competing factors provide transition behavior in the pull-off force. To quantify this picture, we develop a kinetic model by applying the stochastic bonding theory to describe the time-dependent binding of the adhesive site and rate-dependent adhesive stress inside the contact area and in the cohesive zone. The indentation adhesion problem is solved analytically, and the theoretical results can predict both the time-dependence and the transition phenomenon. Additionally, it is found that the adhesion hysteresis and length-dependence are the special cases of the kinetic model at the high-speed limit. To further verify the kinetic adhesion model, we use a confocal microscope to measure the contact radius during the indentation test, and the theoretical results also fit well with the measured contact radius results. This systematic study provides more complete and precise descriptions of the adhesion behavior of hydrogels and promotes new understandings of the hydrogel adhesion mechanism.

Presenting Author: Dongjing He Georgia Institute of Technology

Presenting Author Biography: Mr. Dongjing He is a Ph.D. candidate in the George W. Woodruff School of Mechanical Engineering at Georgia Institute of Technology. He received his B.S. degree in Theoretical and Applied Mechanics at University of Science and Technology of China in 2017. He joined Prof. Yuhang Hu’s research group and started his graduate study in Mechanical Science and Engineering at University of Illinois at Urbana Champaign in 2017. He moved to Georgia Tech to pursue his Ph.D. degree with Prof. Yuhang Hu in 2018. Mr. He’s research focuses on the mechanics of soft materials, including the time-dependent bulk properties and the interfacial adhesion properties.

Authors:

Dongjing He Georgia Institute of Technology
Yang Lai Georgia Institute of Technology
Yuhang Hu Georgia Institute of Technology