Nanoscale mechanical probing of supported lipid bilayers with atomic force microscopy

We present theory and experiments for the force-distance curve F(z(0)) of an atomic force microscope (AFM) tip (radius R) indenting a supported fluid bilayer (thickness 2d). For realistic conditions the force is dominated by the area compressibility modulus kappa(A) of the bilayer and, to an excellent approximation, given by F = pi kappa(A)Rz(0)(2)/(2d-z(0))(2) . The experimental AFM force curves from coexisting liquid ordered and liquid disordered domains in three-component lipid bilayers are well described by our model, which provides kappa(A) in agreement with literature values. The liquid ordered phase has a yieldlike response that we model as due to the breaking of hydrogen bonds.