Supervisor: Fabrice Thalmann

Phospholipids undoubtedly have a crucial role in the lubrication properties of biosystems. The most convincing example is the case of biological contacts such as human joints, which have a very low coefficient of friction. The comprehension of such biophysical systems implicates both fundamental as well as applied challenges, for instance in regard to osteoarthritis and the manufacture of joint prostheses. Using the molecular dynamics engine GROMACS, we simulate a lipid bilayer (DSPC) undergoing stress applied in different ways and observe the response of the membrane depending on the physical state (fluid or gel). The first method (CPF: constant pull force) consists in applying a pair of constant forces to two subsystems, and therefore induce a stationary relative displacement ofthese two subgroups. The second method (FKR: force kick relaxation) consists in preparing the two subsystems with an opposite finite relative velocity, so as to guarantee the immobility of the center of mass of the complete system, and to follow over time the mechanical relaxation of these systems towards equilibrium.

The results clearly show the presence of a viscoelastic regime that we attribute to the elasticity oflipids tilt. In the gel state, we observe a nonlinear regime, corresponding to a shear thinning. Theapparent friction coefficient b tends to decrease when the force increases. Therefore, the bilayer in the gel state is subject to a slow and more complex dynamics than in the fluid state. Moreover, our approach can be generalized to supported bilayers for which we obtain results on the diffusion and friction of the different layers.

Keyword: lipids, bilayer, friction, numerical simulation, GROMACS, MARTINI, supported membranes