In this paper we introduce an experimental protocol to reliably determine extensional relaxation times from capillary thinning experiments of weakly-elastic dilute polymer solutions. Relaxation times for polystyrene in diethyl phthalate solutions as low as 80 µs are reported: the lowest relaxation times in uniaxial extensional flows that have been assessed so far. These data are compared to the linear viscoelastic relaxation times that are obtained from fitting the Zimm spectrum to high frequency oscillatory squeeze flow data measured with a piezo-axial vibrator (PAV). This comparison demonstrates that the extensional relaxation time reduced by the Zimm time, ext / z , is not solely a function of the reduced concentration c/c* , as is commonly stated in the literature: an additional dependence on the molecular weight is observed.

We present new findings on how the presence of particles alters the pinch-off dynamics of a liquid bridge. For moderate concentrations, suspensions initially behave as a viscous liquid with dynamics determined by the bulk viscosity of the suspension.Close to breakup, however, the filament loses its homogeneous shape and localised accelerated breakup is observed. This paper focuses on quantifying these final thinning dynamics for different sized particles with radii between 3 µm and 20 µm in a Newtonian matrix with volume fractions ranging from 0.02 to 0.40. The dynamics of these capillary breakup experiments are very well described by a one-dimensional model that correlates changes in thinning dynamics with the particle distribution in the filament. For all samples, the accelerated dynamics are initiated by increasing particle-density fluctuations that generate locally-diluted zones. The onset of these concentration fluctuations is described by a transition radius, which scales with the particle radius and volume fraction. The thinning rate continues to increase and reaches a maximum when the interstitial fluid is thinning between two particle clusters. Contrary to previous experimental studies, we observe that the final thinning dynamics are dominated by a deceleration, where the interstitial fluid appears not to be disturbed by the presence of the particles. By rescaling the experimental filament profiles, it is shown that the pinching dynamics return to the self-similar scaling of a viscous Newtonian liquid bridge in the final moments preceding breakup. a) christian.clasen@cit.kuleuven.be

Steady-state extensional viscosity of a linear polymer solution using a differential pressure extensional rheometer on a chip

Capillary thinning experiments of semi‐dilute solutions of schizophyllan in water and DMSO are performed to determine the relaxation behavior in extensional flows as experienced, for example, in the flow through porous media in enhanced oil recovery. The extensional relaxation time λE is found to scale with concentration following a dissimilar power‐law dependency for the two solvents, λE ∝ c1.52 in water and λE ∝ c0.90 in DMSO. It is shown that the extensional flow fields are strong enough to break, and prevent the rebuilding of, intermolecular structures, due to hydrogen bonding that was observed to alter the viscoelastic response in shearing flows of aqueous schizophyllan solutions.

This paper deals with the thinning dynamics of a liquid bridge of dilute particulate suspensions in a Newtonian matrix. We describe the final stage before break-up of a colloidal suspension, which occurs in a liquid thread between two confining beads. The model, based on the one-dimensional mass and momentum conservation equations, shows quantitative agreement with the experimentally observed deceleration and final pinching behaviour of the medium between individually monitored particles in the suspension (Mathues et al., Phys. Fluids 27, 093301, 2015) and thus supports the hypothesis that an accelerated thinning of the Newtonian matrix is caused by an active stretching via independently moving particles.