Near-field studies of anisotropic variations and temperature-induced structural changes in a supported single lipid bilayer.

Temperature-controlled polarization modulation near-field scanning optical microscopy measurements of a single supported L_{ß^{'}} 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayer are presented. The effective retardance (S=2π(n_{e}-n_{o})t/λ, where t is the thickness of the bilayer and λ is the wavelength of light used) and the direction of the projection of the acyl chains (θ) were measured simultaneously. We demonstrate how one is able to align the system over the sample and measure a relative retardance ΔS, a crucial step in performing temperature-controlled experiments. Maps of ΔS and θ, with a lateral resolution on the order of ∼100 nm are presented, highlighting variations deriving from changes in the average molecular orientation across a lipid membrane at room temperature. A discussion of how this information can be used to map the average three-dimensional orientation of the molecules is presented. From ΔS and the known thickness of the membrane t the birefringence (n_{e}-n_{o}) of the bilayer was determined. Temperature-controlled measurements yielded a change of ΔS∼(3.8±0.3) mrad at the main transition temperature (T_{m}∼41^{∘}C) of a single planar bilayer of DPPC, where the membrane transitioned between the gel L_{ß^{'}} to liquid disorder L_{α} state. The result agrees well with previous values of (n_{e}-n_{o}) in the L_{ß^{'}} phase and translates to an assumed average acyl chain orientation relative to the membrane normal (〈ϕ〉∼32^{∘}) when TT_{m}. Evidence of super heating and cooling are presented. A discussion on how the observed behavior as T_{m} is approached, could relate to the existence of varying microconfigurations within the lipid bilyer are presented. This conversation includes ideas from a Landau-Ginzburg picture of first-order phase transitions in nematic-to-isotropic systems.

Strong Casimir force reduction through metallic surface nanostructuring.

The Casimir force between bodies in vacuum can be understood as arising from their interaction with an infinite number of fluctuating electromagnetic quantum vacuum modes, resulting in a complex dependence on the shape and material of the interacting objects. Becoming dominant at small separations, the force has a significant role in nanomechanics and object manipulation at the nanoscale, leading to a considerable interest in identifying structures where the Casimir interaction behaves significantly different from the well-known attractive force between parallel plates. Here we experimentally demonstrate that by nanostructuring one of the interacting metal surfaces at scales below the plasma wavelength, an unexpected regime in the Casimir force can be observed. Replacing a flat surface with a deep metallic lamellar grating with sub-100 nm features strongly suppresses the Casimir force and for large inter-surfaces separations reduces it beyond what would be expected by any existing theoretical prediction.