  Near a critical point, the time scale of thermally-induced fluctuations
diverges in a manner determined by the dynamic universality class. Experiments
have verified predicted 3D dynamic critical exponents in many systems, but
similar experiments in 2D have been lacking for the case of conserved order
parameter. Here we analyze time-dependent correlation functions of a quasi-2D
lipid bilayer in water to show that its critical dynamics agree with a recently
predicted universality class. In particular, the effective dynamic exponent
$z_{\text{eff}}$ crosses over from $\sim 2$ to $\sim 3$ as the correlation
length of fluctuations exceeds a hydrodynamic length set by the membrane and
bulk viscosities.
