Researchers at Harvard and Princeton Universities are working together to understand the nature and mechanisms of formation of electrets and to design and fabricate new electret materials and structures. Project leaders are Prof. George M. Whitesides at Harvard and Prof. Ilhan A. Aksay at Princeton.
The conventional definition of an “electret” is probably too elastic, but it has come to mean almost any material that shows a permanent, fixed electrostatic surface charge or an oriented permanent dipole. Although electrets are often characterized as solid (dielectric) materials, a less restrictive view encompasses both solid and liquid systems. Rigid particles or macroscopic surfaces that retain permanent charge or oriented dipoles are rightly termed “solid electrets.” “Liquid electrets,” on the other hand, are formed by inserting charge in the form of electrons, ions, nanometer size micelles, or charged colloidal particles into a liquid or onto a liquid-gas or liquid-solid interface; charge relaxation is slow. The body (or bodies) can then be manipulated with external electrostatic fields. With some liquid electrets (e.g., a polymer above its glass transition temperature), unique interface morphologies can be “frozen in” by cooling. Charge diffusion, and dissipation of patterned charge by diffusion of charge or by diffusion of molecules bearing charge will be faster than in polymers, but there are many potential uses for systems in which liquids could be given a permanent, net charge.
Our work seeks to understand the nature of the charge and the mechanisms and limits of these materials, building upon new information to develop new electret materials. One example is using patterning polymers with trapped electrostatic charge. We are especially interested in the use of colloidal (adsorbed molecules and particles on surfaces) electrets in directed self-assembly. In these systems, crystalline morphologies can be controlled using electrohydrodynamic (EHD) manipulation of suspensions, controlling induced electrical charges on liquid and colloidal jets to pattern large area surfaces, and thereby fabricating microcellular structures amenable to localized reaction control, actuation, sensing, and self-repair. A significant part of our effort is directed to laboratory-scale demonstrations of prototype devices, to better transfer research to useful technology and as a demonstration of new phenomena.
In most cases the atomic/molecular-level nature of electrets is poorly understood (or, in fact, not understood at all). Physics-based examination of solid electrets has tended to stop at the observation and characterization of the permanent (or long-duration) electrostatic field. The understanding of liquid electrets is somewhat less opaque. However, due to their very nature, deformation and motion complicate the behavior of multiphase liquid electrets. The phenomenology of electrets is extensive, and they are, in fact, used in many important applications as well as potential hazards to electronic systems (for example, in the potential damage to microelectronic systems due to buildup of static charge).
Making the major step in understanding required to control and manipulate electret structures requires a fundamental approach including, but not limited to, the molecular/atomic level. The fundamental understanding of electrets requires a multiscale modeling approach, spanning atomic to macroscopic length scales. To this end, we have a significant effort in computational modeling led by Prof. Roberto Car (Princeton), first seeking to understand electret behavior at the atomistic scale and carrying the modeling effort through to the macroscopic. New models under development will serve to connect molecular scale behavior to experiment, relating structural properties to atomistic and molecular behavior.
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