FIG. 2.

(a) Lipid membrane tail coordinates in the xy-plane. Highlight shows an arbitrary bead selected from the tail plane, with neighbors within a 3.0-nm xy-plane cut-off distance. (b) Instantaneous interfacial beads are detected via nearest neighbor composition criterion (gray circles), binned (magenta stars), and fitted through the binned interface positions using cubic spline (magenta line). Each DPPC (DIPC) bead was considered to be at interface if $0.25<X_i^{\text{DIPC}}<0.55$ $\left(0.25<X_i^{\text{DPPC}}<0.55\right)$, and each CHOL bead was considered to be at the interface when $0.33<X_i^{\text{DPPC}}<0.66$. (c) Fluctuations of the interface h(x) from its mean position (straight line) and (d) their average power spectrum $⟨h^2\left(k\right)⟩.$