Stimulus-Responsive Hydrogels

Figure 1: A collapsed gel specimen.

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.

Figure 2: Evolution of sharp phase interface in a hydrogel.

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.

Much of this work has stemmed from a collaboration with Stefan Zauscher at Duke University and Eliot Fried at Washington University in St. Louis. The work has been supported through generous grants from NSF and Sandia National Laboratories.

Investigating the Surface Response of SRHs

Figure 3: Click to view movie

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

  1. Chang D, Dolbow J, Zauscher S Switchable Friction of Stimulus-Responsive Hydrogels LANGMUIR 23(1): 250-257 2007
  2. Ji HD, Mourad H, Fried E, et al. Kinetics of thermally induced swelling of hydrogels INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES 43 (7-8): 1878-1907 APR 2006
  3. 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 194 (42-44): 4447-4480 2005
  4. 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 DEC 14 2004
  5. Dolbow J, Fried E, Jia HD Chemically induced swelling of hydrogels JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 52 (1): 51-84 JAN 2004