Molecular parameters of polymers affecting the stability of interdiffusing fluid layers

Abstract

The boundary disturbances that arise when a polymer solution is layered onto a solution of higher macroscopic density containing low-molecular-weight solute are shown to be highly regular interdigitated structured flows (the process is commonly termed droplet sedimentation). Similar flows have been observed by Preston et al. ( Nature (London) 287, 499 (1980)) in other ternary (polymer:polymer:solvent) solutions. The initial stages of the instability in droplet sedimentation have been monitored through the use of blue dye-labeled polymer or refractive index changes. It is found that the kinetics of structured flow formation are markedly altered when the polymer is at sufficient concentration to form a transient, statistical polymer network. This finding is closely allied to previous observations of the requirement of transient polymer network-forming systems to establish structured flows in ternary (polymer) systems. Other variables studied include varying the concentration of electrolyte in the underlay fluid and varying the molecular weight, concentration, and chemical composition of the polymer in the overlay fluid. The conditions for instability appear to be consistent with the formation of a density inversion at the boundary through strong-coupled diffusion between interdiffusing components.