Guest Lecture by Prof. Sanjay Govindjee

Prof. Sanjay Govindjee from the University of California (Berkeley, CA, USA), Department of Civil and Environment Engineering was a guest at the Institute of Structural Analysis at TU Dresden.

The topic of his presentation was “Physics and Models for Liquid Crystal Elastomers”

Abstract:
Liquid crystal elastomers (LCEs) are a fascinating class of materials that display soft and semi-soft elastic behavior as well as a somewhat less explored viscoelastic behavior. These materials are composed of liquid crystal molecules and polymerizing agents to form a solid that behaves as an elastomer but also like a liquid crystal. The interaction of these two features provides for a wide and complex range of macroscopically observed phenomena, including, for example, optical actuation, extreme softness, pattern formation, and high damping, to name a few.

In this talk, I will present a general introduction to the physics of these unique materials at both microscopic and macroscopic scales. This will lead to insights into the long-standing, well-established models that are used to understand their elastic behavior. Consideration will also be given to models that can account for time-dependent (i.e., viscoelastic) behavior—an aspect that remains underexplored in the literature—as well as computational considerations.

The focus will be on mono-domain liquid crystal elastomers that exhibit both a viscous director response and a viscoelastic elastomeric network response. The modeling framework will be built from the continuum scale, employing the formal principles of invariance of expended power to develop governing balance laws that account for loads capable of imposing twist directly at the continuum level. This approach is combined with free-energy dissipation arguments to constrain the constitutive relations based on hypothesized functional dependencies of the free-energy function. The resulting mathematical framework yields natural and intuitive evolution laws for viscous kinematics. The utility of the model will be demonstrated through comparisons with experimental data, highlighting both its strengths and the cautionary insights relevant to engineering applications of LCEs.

We thank Prof. Govindjee and all the participants!