Sculpting 2D Crystals via Membrane Contractions before and during Solidification

TL;DR

This study investigates how membrane contraction and solidification influence the morphology of 2D phospholipid crystals in vesicles, revealing mechanisms that control crystal shape and stress relaxation during solidification.

Contribution

It introduces a combined mechanical and thermal model explaining the formation of various crystal morphologies in lipid membranes during solidification.

Findings

Crystal morphology depends on vesicle size and composition.

Formation of flower-shaped crystals is driven by membrane tension and elasticity.

Stress relaxation occurs over a time scale proportional to vesicle size squared.

Abstract

When phospholipids crystallize within the otherwise fluid membranes of giant unilamellar vesicles, the resulting molecularly-thin "2D" solids exhibit great variety in their morphology evolution. For instance within membranes containing moderate amounts of the crystallizing component, crystals grow with a fixed morphology depending on vesicle size. Conversely for membranes containing large amounts of the crystallizing species, we find small compact crystals on vesicles of all sizes. However on large vesicles, growing crystals sprout flower petals that lengthen progressively. These behaviors result from two combined mechanisms: First, like other 2D solids, the shear rigidity of phospholipid crystals renders them intolerant to morphologies with non-zero Gaussian curvature. As a result and especially at elevated membrane tension, the cost of bending elasticity is reduced, at the expense of line energy, by the formation of flowers as opposed to compact crystals. Second, the composition-dependent tension rise during cooling relaxes via water permeation of the membrane with a time constant scaling as $R^2$. The amount of crystal formed for a small decrease in temperature determines this composition-dependent increase in stress from thermal contractions versus solidification. Surface Evolver computations motivated using the predicted tension evolution to develop a processing space that maps to experimental observations for initial and growing crystal morphology. Important variable groups are identified, including a scaled ratio of bending to line energy, a vesicle size-independent group for membrane contractions, and a time constant for stress relaxation. Though processing stresses ultimately relax, the crystal morphology persists well beyond the processing window.