Fig. 3.

Quantitative viscoelastic mapping of DPPC:Chol mixtures. The E_storage and E_loss maps for (A) the pure DPPC (scan size, 2.7 µm) and (B) DPPC:Chol (15%) (scan size, 4.6 µm) appeared darker than the substrate, suggesting that the material is more dissipative than the background. Their corresponding loss tangent images (tanδ) show essentially the same congruent, but inverted, pattern compared with the corresponding E_storage and E_loss images. In the tanδ map, the DPPC and DPPC:Chol (15%) appears lighter than the substrate, suggesting that the material dissipates greater energy than which it stores. At higher Chol concentrations of (C) 33 mol % (scan size, 5.4 µm) and (D) 60 mol % (scan size, 4.9 µm), the E_storage and E_loss maps of the system appear homogenous; however, the membrane appears to stiffen, becoming more viscous. In addition, the corresponding tanδ maps also demonstrate the homogeneity of the model system at these increased concentrations (33% and 60%) with lighter inclusions in the monolayer, suggesting that they may be more viscous than the surrounding SLBs. [Scale bar: 500 nm (left-hand corner).]