Hydrodynamics of particles embedded in a flat surfactant layer overlying a subphase of finite depth

The motion of membrane-bound objects is important in many aspects of biology and physical chemistry. A hydrodynamic model for this Fconfiguration was proposed by Saffman & Delbruck (1975) and here it is extended to study the translation of a disk-shaped object in a viscous surface film overlying a fluid of finite depth H. A solution to the flow problem is obtained in the form of a system of dual integral equations that are solved numerically. Results for the friction coefficient of the object are given for a complete range of the two dimensionless parameters that describe the system: the ratio of the sublayer (eta) to membrane (eta(m)) viscosities, Lambda = eta R/eta(m)h (where R and h are the object radius and thickness of the surface film, respectively), and the sublayer thickness ratio, H/R. Scaling arguments are presented that predict the variation of the friction coefficient based upon a comparison of the different length scales that appear in the problem: the geometric length scales H and R, the naturally occurring length scale l(m) = eta(m)h/eta, and an intermediate length scale l(H) = (eta(m)hH/eta)(1/2). Eight distinct asymptotic regimes are identified based upon the different possible orderings of these length scales for each of the two limits Lambda much less than 1 and Lambda much greater than 1. Moreover, the domains of validity of available approximations are established. Finally, some representative surface velocity fields are given and the implication of these results for the characterization of hydrodynamic interactions among membrane-bound proteins adjacent to a finite-depth sublayer is discussed briefly.