Abstract:

The electrostatic self-assembly of charged Brownian objects typically occurs in cases of short-range interactions. The objects form Coulombic materials that are close-packed and have long-range order. Here, we present a system in which two kinds of non-Brownian millimeter-sized beads tribocharge differently, experience long-range electrostatic interactions, and still form ordered two-dimensional structures. We provide a complete characterization of the kinetics of formation of these materials, as the total number of beads is held constant and the relative number of beads that tribocharge negatively or positively is modified. We agitate the beads by shaking the dish in which they are contained. We show that the beads commonly adopt a transient structure that we call a rosette. A rosette consists of a central bead surrounded by six close-packed neighbors of a different kind. The symmetry of the final structure depends on the relative number of negatively and positively charged beads, and it is not necessarily the same as that of the transient structure. Our results bear important implications in the de novo design of Coulombic materials given our ability to isolate transient structures, identify the moment of their appearance, and quantify the impact of agitation, tribocharging, and Coulombic energy minimization on their persistence.

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