  The effect of energy absorption during the binary evolution of
Exotic-compact-objects (ECOs) is extensively studied. We review the underlying
mechanism that provides the energy dissipation in material objects - tidal
friction. We show that unlike typical astrophysical objects, where absorption
due to viscosity is negligible, in ECOs, absorption could potentially mimic the
analogous effect of black-holes (BHs) - tidal heating. We stand for their
differences and similarities in the context of energy dissipation during the
inspiral. Inspired by the membrane paradigm and recent studies, we demonstrate
how viscosity is a defining feature that quantifies how close is the ECO
absorption to that of a classical BH absorption. We show that for ECOs,
viscosity can induce significant modifications to the GW waveform, which in
some favorable scenarios of super-massive binaries of equal mass and spin,
enables the measurement of the ECO absorption in the future precision
gravitational-wave (GW) observations. Finally, we discuss the implications on
the ECO reflection coefficient and the relation to the universal viscosity to
volume entropy bound.
