Structure, dynamics and phase behavior of short rod inclusions dissolved in a colloidal membrane

Abstract

Inclusions dissolved in an anisotropic quasi-2D membrane acquire new types of interactions that can drive assembly of complex structures and patterns. We study colloidal membranes composed of a binary mixture of long and short rods, such that the length ratio of the long to short rods is approximately two. At very low volume fractions, short rods dissolve in the membrane of long rods by strongly anchoring to the membrane polymer interface. At higher fractions, the dissolved short rods phase separate from the background membrane, creating a composite structure comprised of bilayer droplets enriched in short rods that coexist with the background monolayer membrane. These results demonstrate that colloidal membranes serve as a versatile platform for assembly of soft materials, while simultaneously providing new insight into universal membrane-mediated interactions.Introduction: Colloids, proteins and nanoparticles dissolved in bulk isotropic fluids interact by well-studied intermolecular forces that include steric exclusions, electrostatic repulsions, the hydrophobic effect, and van der Waals interactions 1 . In comparison, particles dissolved in anisotropic environments or confined on surfaces or interfaces can acquire more complex interactions and thus exhibit very different behaviors. For example, experiments have revealed exceedingly complex interactions and assembly pathways of colloids or nano-particles dissolved in anisotropic liquid crystals 2-4 or confined on oil-water interfaces 5-9 . Lipid bilayers provide an even more complex environment for self-assembly. Particles dissolved in a lipid bilayer simultaneously experience a liquid crystalline environment due to ordering of the hydrophobic lipid chains 10 , and are confined to a deformable quasi-2D plane, similar to particle-laden interfaces.Consequently, membrane-mediated interactions can drive assembly of exceedingly complex structures [11][12][13][14][15] . However, the nanometer length scale of conventional lipid bilayers makes studies of lipid bilayers challenging. Consequently, our knowledge of membrane-mediated interactions

Speeding up Dynamics by Tuning the Noncommensurate Size of Rodlike Particles in a Smectic Phase

“…3b and e). This was re-cently experimentally observed for short rods dispersed in colloidal monolayer of host rod-shaped particles with a length ratio r ∼ 0.5 [37]: the short rods were found to strongly anchor to the membrane interfaces, and only occasionally hop to the opposite interface. Our results confirm this anchoring behavior and extend it to particles even larger than the lamellar spacing.…”

Nematics that splay, twist, and bend:

“…More intriguingly perhaps is the regime II with r ∈ [n + 0.3, n + 0.6], where the minima z min 1 and z min 2 correspond to the centre-ofmass positions at which one of the ends of the guest particles touches one of the boundaries of the smectic layers (Figure 7.5(b) and (e)). This was recently experimentally observed for short rods dispersed in colloidal monolayer of host rod-shaped particles with a length ratio r ∼ 0.5 [264]: the short rods were found to strongly anchor to the membrane interfaces, and only occasionally hop to the opposite interface. Our results confirm this anchoring behaviour and extend it to particles even larger than the lamellar spacing.…”

Controlling the shape and topology of two-component colloidal membranes

“…We first changed the number fraction of short rods, n short , within the membrane while keeping the Dextran concentration fixed. The common chirality of the rods increased the miscibility of the two species, when compared to rods of opposite chirality [31,37]. At low number fractions of short M13-wt rods (n short < 0.15) we observed assembly of flat colloidal membranes.…”

“…In principle, short rods could preferentially reside next to the inner or outer membrane surface, or close to the membrane midplane [37]. By labelling both rods types, we determined the center of mass of both short rod and long rods (SI Appendix, Fig.…”