Elasticity-Mediated Assembly and Transitions of Colloids in an 2D Fluid*

With interest in cell trafficking and signaling, or with completely different motivation to engineer pliable contoured coatings with integrated functionality, there has been a drive to identify the principles which govern the assembly of small rigid objects in elastic 2D fluids. Examples of the latter can include the cell membrane or copolymer lamellae. Here, we exploit phospholipid vesicle bilayers containing rigid Brownian plate-shaped domains as platform to explore principles of elasticity-dominated colloidal assembly in a dynamic 2D contour. The plate-shaped colloids, though made from solidified membrane lipids, do not coalesce and instead maintain a fixed shape throughout the experiment, demonstrating a minimal influence of any line tension. Here we demonstrate three classes of domain configurations, controlled by the availability of membrane bending, which is manipulated osmotically. When vesicles are deflated from a spherical shape by as much as 25% volume colloidal plates form a vesicle-encompassing pseudo-hexagonal lattice that maximizes the plate-plate distances. When the vesicles are inflated to within about 2% of their spherical volume, the colloidal plates associate closely but do not touch, assembling into chains. These two classes of assemblies are relatively persistent with long lived structures that suppress Brownian motion. A sharp boundary, near 2% deflation, distinguishes the two classes of assemblies with the domain distances switching sharply at these conditions suggesting cooperative behavior such as phase transition. In the final state, when vesicles are deflated by as much as 5% from a perfect sphere, domains are disordered and dynamic, with the positions changing on the timescale of minutes. This behavior was demonstrated for vesicles whose colloids occupied ~17% of the surface area and were in a size range of 10-40 um and with 4-100 colloids per vesicles, establishing the broad character of this behavior and the extreme utility of bending interactions.