Soft Multifunctional Materials: From Dielectric Elastomers to Biological Cells



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Soft materials such as dielectric elastomers and biological vesicles are fascinating due to their unique propensity for large deformation, multifunctional characteristics, display of coupled physical effects and applications that range from robotics, energy harvesting, actuators, to drug delivery. In this dissertation, we seek to understand and explore novel modalities for creating soft materials that respond to external stimuli and enhancing their large deformations capabilities. Specifically, in this dissertation, we address the following problems in soft matter science and biological physics: (i) Liquid inclusions in soft materials: capillary effect, mechanical stiffening, and enhanced electromechanical response: Recent experiments and modeling appear to suggest that “small” liquid inclusions may significantly stiffen soft solids. In this work we develop a theoretical framework and construct a simple homogenization model that unambiguously explains this phenomenon. We hypothesize that the interplay of capillary effect, electrostatics, soft solids and ionic liquid inclusions may offer prospects for designing soft materials that display an unusually large electromechanical coupling and enhanced energy conversion capability.. (ii) Capillarity, liquid inclusions, soft material and the magnetoelectric effect: Taking inspiration from the analogous development described in the preceding paragraph for the electrostatic case, we discuss the prospects of designing the magnetoelectric effect for soft materials. (iii) The biological cell as a soft magnetoelectric material: Sharks, birds, bats, turtles and many other animals can detect magnetic fields. How do these animals detect magnetic fields? In this work we quantitatively outline the conditions under which a biological cell may detect a magnetic field and convert it into electrical signals detectable by biological cells. Our proposed mechanism appears to explain most of the experimental observations related to the physical basis of magnetoreception. (iv) Why can’t biological cells stay spherical? Biological cells are almost never really spherical even though there is nothing overtly present to break the symmetry of the cell. Using rather simple energy considerations, we show that even though all biological cells are subject to a completely radial electric field along the cell membrane, the spherical shape is unstable under most practical situations. This simple result appears to have been overlooked in the literature.



Soft multifunctional materials, Electromechanical coupling, Maxwell stress, Magnetoreception, Magnetoelectricity, Electrostriction, Magnetostriction, Homogenization, Capillary effect


Portions of this document appear in: Krichen, S., Liping Liu, and P. Sharma. "Biological cell as a soft magnetoelectric material: Elucidating the physical mechanisms underpinning the detection of magnetic fields by animals." Physical Review E 96, no. 4 (2017): 042404.