  We study the Einstein gravity equations projected on a timelike surface,
which represents the time evolution of what we call a gravitational screen. We
show that such a screen behaves like a viscous bubble with a surface tension
and an internal energy, and that the Einstein equations take the same forms as
non-equilibrium thermodynamic equations for a viscous bubble. We provide a
consistent dictionary between gravitational and thermodynamical variables. In
the non-viscous cases there are three thermodynamic equations which
characterize a bubble dynamics: These are the first law, the Marangoni flow
equation and the Young-Laplace equation. In all three equations the surface
tension plays a central role: In the first law it appears as a work term per
unit area, in the Marangoni flow its gradient drives a force, and in the
Young-Laplace equation it contributes to a pressure proportional to the surface
curvature. The gravity equations appear as a natural generalization of these
bubble equations when the bubble itself is viscous and dynamical. In
particular, it shows that the mechanism of entropy production for the viscous
bubble is mapped onto the production of gravitational waves. We also review the
relationship between surface tension and temperature, and discuss black-hole
thermodynamics.
