Esther W. Gomez, Chemical Engineering, University of California, Berkeley, 106 Lewis Hall, Berkeley, CA 94720, Nathan G. Clack, Biophysics, University of California, Berkeley, 109 Lewis Hall, Berkeley, CA 94720, and Jay T. Groves, Chemistry, University of California, Berkeley, 105 Lewis Hall, Berkeley, CA 94720.
Two dimensional dispersions of like-charged particles assemble into a variety of structures ranging from dispersed to condensed phases. Here, the order within electrostatically levitated colloidal monolayers is studied as a function of surface potential and ionic strength. Particle and substrate surfaces are coated with lipid membranes, which enables the surface composition and charge to be tuned continuously over a wide range. Using a combination of optical imaging techniques we are able to monitor both the colloid structure and the surface potential. We observe that the colloidal ordering is electrostatically dominated and that interactions can be tuned from repulsive to long-range attractive by changing the particle surface composition. The collective behavior is asymmetric with respect to the sign of the surface potential and independent of the choice of soluble counterions.