Sculpting colloidal membranes through phase transformations

Abstract

Shaping macroscopic 2-D thin elastic sheets through differential strains in the system is a widely studied phenomenon applicable in diverse areas such as flexible electronics, origami and tissue growth. In contrast, at the molecular scales of lipid bilayers and cell membranes which are classic examples of thin fluid sheets, external agents such as proteins typically modulate their curvature and shape. This thesis work designs new strategies, at the intermediate colloidal length scales, to shape model fluid membranes using internal phase transitions based on intrinsic interactions and physical properties of the constituent rod-like colloidal particles. An isotropic mixture of highly monodispersed rod-like viruses spontaneously assemble into the membranes of aligned rods on the addition of non-adsorbing polymer through depletion attraction. The membranes are of one-rod-length thick and exhibit long wavelength thermal elastic fluctuations. The constituent virus rods are intrinsically chiral and thus trap a uniform chiral rod twist at the membrane edge. We utilize this intrinsic chiral interaction of the virus rods to shape the membranes into a three-dimensional globally buckled and locally wrinkled structures through crystallization. Moreover, we demonstrate that the surface roughness of a crystalline membrane can be tuned with the number of nucleation centers. The finite line tension of the membrane edge causes the membrane to have circular geometry. However, on the introduction of another rod having similar handedness in chirality but different aspect ratio shapes the membranes into an implausible geometry of cyclic polygon. We show that the origin of this anisotropic shape lies in the spreading phenomenon of a colloidal membrane of short-thick rods over another membrane composed of purely long rods in the presence of disorder. The pinning junctions that form the cornerstones of this membrane geometry are the location of compressed defective rods which are weakly cross-linked in nature instead of being topological defects. In another scenario, a binary mixture of two virus rods of similar length with opposite chirality and significantly different rigidity self-assembles into colloidal membranes containing finite sized solid anisotropic colloidal rafts in their bulk on the addition of non-adsorbing polymer. The nature and morphology of these anisotropic rafts are entirely dependent on the kinetics of phase separation and concentration of the depletant polymer. Remarkably, we demonstrate unique assembly conditions that enable access to different regimes of the phase separation in a single sample chamber. Lastly, we use the knowledge gathered from the core part of the thesis work to assemble colloidal membranes with rafts mimicking Pickering emulsions in appearance using a ternary mixture of virus rods. We discovered the presence of weak repulsive forces between these rafts, which switch to attractive in the absence of the rods that form its inner core. The results in this thesis work demonstrate various self-assembly conditions to sculpt the colloidal membranes into different morphologies through its associated phase transformations.