  The existence of two types of internal friction wet and dry is revisited, and
a simple protocol is proposed for distinguishing between the two types and
extracting the appropriate internal friction coefficient. The scheme requires
repeatedly stretching a polymer molecule, and measuring the average work
dissipated in the process by applying the Jarzynski equality. The internal
friction coefficient is then estimated from the average dissipated work in the
extrapolated limit of zero solvent viscosity. The validity of the protocol is
established through analytical calculations on a one-dimensional free-draining
Hookean spring-dashpot model for a polymer, and Brownian dynamics simulations
of: (a) a single-mode nonlinear spring-dashpot model for a polymer, and (b) a
finitely extensible bead-spring chain with cohesive intra-chain interactions,
both of which incorporate fluctuating hydrodynamic interactions.
Well-established single-molecule manipulation techniques, such as optical
tweezer-based pulling, can be used to implement the suggested protocol
experimentally.
