On the difference between stiff and soft membranes: Capillary Waves

Abstract

One problem of non-crystalline condensed matter (soft matter) is creating the right equilibrium between elasticity and viscosity, referred to as viscoelasticity. Manifestations of that can be found in everyday live, where the viscoelasticity in a tire needs to be balanced so it is still flexible and can dissipate shock-energy, yet hard enough for energy-saving operation. Similarly, the cartilage in joints needs to absorb shocks while operating at low- level friction with high elasticity. Two such examples with a biological applicability are stiff membranes, which allow for the sliding of joints and therefore maintain their function over the lifetime of the corresponding individual (decades) and the softening of cell membranes, for example for antimicrobial effects by dissolution in the case of bacteria (seconds). While the first should allow for low- friction operation at high elasticity, in the second scenario energy dissipated into the membrane eventually leads to membrane destruction. Here we address the intrinsic difference between these two types of membranes, differing in stiffness and displaying different relaxation behavior on the nanosecond time scale. The harder membranes show additional elastic modes, capillary waves, that indicate the high degree of elasticity necessary for instance in cartilage or red blood cells. The energy of these modes is in the order of 1 micro-eV. As model systems we chose a hard phospholipidmembrane of SoyPC lipids and a D2O/C10E4/decane microemulsion system representing soft surfactant membranes. Our results help to explain properties observed for many membranes in nature, where hard membranes lubricate joints or stay intact as red blood particles in tiniest capillaries, both with extremely long lifetimes. Contrarily, softened membranes can be destroyed easily under little shear stress within seconds.