Whitesides Research

Whitesides Group Research

Electrets

Charging and discharging of a rolling sphere

Electrets are materials that have a quasi-permanent electric field at their surfaces due to either an imbalance of charge at the surface (space charge electrets) or to aligned dipoles through the bulk of the material (dipolar electrets). Space charge electrets are most easily generated by contact electrification. The electrostatic properties of electrets have been exploited for many applications including xerography, powder coating, and electrostatic precipitation. The separation of charge due to contact electrification often results in electric fields that exceed the dielectric strength of the medium in which charging occurs (e.g., ~3 MV/m in air) which can be sufficient to breakdown air in the proximity of the surface. These discharges can cause damage to electronics and other machinery, ignite flammable materials and induce explosions. We are interested in: i) understanding the mechanism of charge development and dissipation, and ii) using that understanding to develop strategies to either prevent the build-up of charge or to control where discharges occur. We developed the “Rolling Sphere Tool” (RST) (see Figure 1), first described in our lab by Grzybowski, to measure the kinetics of contact electrification between rolling stainless steel spheres and insulating surfaces. With this tool, we are able to investigate both charging and discharging within the same system. Using the RST, we have shown that charge separation involves the transfer of ions between two contacting materials.

Self-Assembly of two-dimensional electrostatic crystals

We use millimeter-sized polymer spheres, which charge through contact electrification, to model crystallization behavior. For example, two different types of polymer spheres at a one-to-one number ratio form square lattices. These types of assemblies can be performed in “neat” systems or in the presence of “solvent”. We can control the assembly behavior by changing the agitation conditions or the filling ratio of the container (see Figure 2).

Beads-on-a-string: a macroscopic physical model for long-chain molecules.

The beads-on-a-string model is a cornerstone of theoretical polymer science. We can take the advantage of mastering electrostatic interactions to model other kinds of interaction: for example the hydrogen bounds in biological polymers, as DNA or RNA. We proved that a beads-on-a-string system made of a well-designed alternation of oppositely charged beads could fold and lead to collapses that all have the same stable configuration. The system correctly predicts the folding of a sequence that was modeled after an RNA hairpin of 12 units in length. Our “analog computer” has several advantages, while exploring nonlinearities and other complexities omitted from the theoretical model (see Figure 3).

Assembly through contact electrification

We are using millimeter sized polymer spheres, which charge through contact electrification, to model crystallization behavior. For example we can use two different types of polymer spheres at a one to one number ratio to form square lattices. These types of assemblies can performed in neat systems or in the presence of solvent. We can control the assembly behavior by changing the agitation conditions or the filling ratio of the container (see Figure 4).

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