INSTABILITY AND FRONT PROPAGATION IN LASER-TWEEZED LIPID BILAYER TUBULES

We study the mechanism of the ''pearling'' instability seen recently i n experi ments on lipid tubules under a local applied laser intensity. We argue that the correct boundary conditions are fixed chemical pote ntials, or surface tensions Sigma, at the laser spot and the reservoir in contact with the tubule. We support this with a microscopic pictur e which includes the intensity profile of the laser beam, and shows ho w this leads to a steady-state flow of lipid along the surface and gra dients in the local lipid concentration and surface tension (or chemic al potential). This leads to a natural explanation for front propagati on and makes several predictions based on the tubule length. While mos t of the qualitative conclusions of previous studies remain the same, the ''ramped'' control parameter (surface tension) implies several new qualitative results. We also explore some of the consequences of fron t propagation into a noisy (due to pre-existing thermal fluctuations) unstable medium.