Researchers in the DCML are actively developing models and simulations of stimulus-responsive hydrogels (SRHs). These materials exhibit dramatic volume changes in response to small changes in external stimuli, such as temperature, solvent concentration, and light. They are biologically inspired materials, and are of interest for use in a number of micro-scale devices.
The actuation of an SRH proceeds via motion of a phase interface separating swelled and collapsed regions. Our computational efforts are focused on developing numerical methods to robustly simulate the evolution of this interface. There are several challenges to this work. First, it is clear that the phase transition is driven by a coupling between stress and diffusion in the system. Second, we must consider the large, finite strains that occur in the gels.
As we are also interested in arbitrary gel shapes and their interaction with local media (i.e. fluid), traditional methods for sharp interface simulation are not readily applicable. Instead, we have developed the hybrid eXtended Finite Element / Level Set Method for this application. With the XFE/LSM, the sharp phase interfaces are embedded within a finite element approximation and represented independently of the underlying mesh. As such, the phase transition can be simulated without remeshing.
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Investigating the Surface Response of SRHs
We are actively performing numerical and experimental investigations into the unique surface characteristics of SRHs. The movie on the right shows a simulation of two soft gel specimens in frictional contact. This type of experiment is often used to characterize the frictional response of gels. Unfortunately, researchers have developed expressions to extract the coefficient of friction based on this experiment that employs a number of assumptions that are not very accurate. In this case, accurate simulations of contact are of tremendous benefit.
We have observed dramatic changes in surface response with phase state in these materials. An order-of-magnitude change in the apparent coefficient of friction is routinely observed. We have also observed an increase in the coefficient of friction with increasing sliding velocity, and are attempting to capture this behavior with simulation.
References
- Chang D, Dolbow J, Zauscher S. Switchable Friction of Stimulus-Responsive Hydrogels, Langmuir 23 (1): 250-257, 2007
- Ji H, Mourad H, Fried E, et al. Kinetics of thermally induced swelling of hydrogels, International Journal of Solids and Structures 43 (7-8): 1878-1907, 2006
- Dolbow J, Fried E, Ji H. A numerical strategy for investigating the kinetic response of stimulus-responsive hydrogels, Computer Methods in Applied Mechanics and Engineering (42-44): 4447-4480, 2005
- Ji H, Dolbow JE. On strategies for enforcing interfacial constraints and evaluating jump conditions with the extended finite element method, International Journal for Numerical Methods in Engineering 61 (14): 2508-2535, 2004
- Dolbow J, Fried E, Ji H. Chemically induced swelling of hydrogels, Journal of the Mechanics and Physics of Solids 52 (1): 51-84, 2004