Curvature-driven migration of colloids on lipid bilayers
  Colloids and proteins alike can bind to lipid bilayers and move laterally in these two-dimensional fluids. Inspired by proteins that generate membrane curvature, sense the underlying membrane geometry, and migrate to high curvature sites, we explore the question: Can colloids, adhered to lipid bilayers, also sense and respond to membrane geometry? We report the curvature migration of Janus microparticles adhered to giant unilamellar vesicles elongated to present well defined curvature fields. However, unlike proteins, which migrate to minimize membrane bending energy, colloids migrate by an entirely different mechanism. By determining the energy dissipated along a trajectory, the energy field mediating these interactions is inferred to be linear in the local deviatoric curvature, as reported previously for colloids trapped at curved interfaces between immiscible fluids. In this latter system, however, the energy gradients are far larger, so particles move deterministically, whereas the colloids on vesicles move with significant fluctuations in regions of weak curvature gradient. By addressing the role of Brownian motion, we show that the observed curvature migration of colloids on bilayers is indeed a case of curvature capillary migration, with membrane tension playing the role of interfacial tension. Furthermore, since this motion is mediated by membrane tension and shape, it can be modulated, or even turned "on" and "off", by simply varying these parameters. While particle-particle interactions on lipid membranes have been considered in many contributions, we report here an exciting and previously unexplored modality to actively direct the migration of colloids to desired locations on lipid bilayers. 
Ningwei Li, Nima Sharifi-Mood, Fuquan Tu, Daeyeon Lee, Ravi   Radhakrishnan, Tobias Baumgart, and Kathleen J. Stebe
cond-mat.soft physics.bio-ph