Hydrodynamics of domain relaxation in a polymer monolayer

The line tension between two phases within a monolayer can be determined from the characteristic relaxation time of deformed domains, if the hydrodynamics of that relaxation, in particular the relative roles of surface and bulk viscosity, can be established. This is accomplished here for a polymer monolayer by varying the viscosity of the bulk substrate. A Poly(dimethyl)siloxane monolayer segregates into dense and dilute polymer domains on aqueous glycerol and glucose solutions (of viscosity 1.2<η/ ${\mathrm{\eta }}_{\mathrm{water}}$<75) as well as on pure water. The surface pressures of these polymer films are, for moderate surface pressures and within experimental precision, independent of the glycerol and glucose content of the substrate solutions. Isolated polymer domains relax toward the circular form, linearly for the early ‘‘bola’’ form and exponentially for moderate deformations. Relaxation times ${\mathit{T}}_{\mathit{c}}$ are measured for domains of size 10 μm<R<80 μm and 0.2 sec<${\mathit{T}}_{\mathit{c}}$<60 sec. Relaxation of monolayer domains in the two limits, in which surface or bulk viscosity dominates, is discussed. All data are consistent with dissipation dominated by viscosity in the substrate. The deduced line tension is λ=(1.1±0.3)×${10}^{\mathrm{-}12}$ N.