Effective interfacial tension in flow-focusing of colloidal dispersions: 3-D numerical simulations and experiments

Abstract

An interface between two miscible fluids is transient, existing as a non-equilibrium state before complete molecular mixing is reached. However, during the existence of such an interface, which typically be at short timescales, composition gradients at the boundary between the two liquids cause stresses effectively mimicking an interfacial tension. Here, we combine numerical modelling and experiments to study the influence of an effective interfacial tension between a colloidal fibre dispersion and its own solvent on the flow in a microfluidic system. In a flow-focusing channel, the dispersion is injected as core flow that is hydrodynamically focused by its solvent as sheath flows. This leads to the formation of a long fluid thread, which is characterised in 3D using Optical Coherence Tomography and simulated using a volume of fluid method. The simulated flow and thread geometries very closely reproduce the experimental results in terms of thread topology and velocity flow fields. By varying the effective interfacial tension numerically, we show that it clearly influences threading dynamics and that it can be described by an effective capillary number. Furthermore, we demonstrate that the applied methodology provide means to measure the ultra-low but dynamically highly significant effective interfacial tension. † Email address for correspondence: fredrik@mech.kth.se arXiv:1901.08939v1 [physics.flu-dyn]

Flow fields control nanostructural organization in semiflexible networks

“…One possibility could be that the shortest fibrils cause an increase of the intrinsic solvent viscosity, where only a viscosity modification of the solvent of a factor six, would result in the expected values of L max and L min , whereas the shear viscosity of low-concentration CNF can be 100-1000 times higher than water depending on shear rate. 4 Another plausible reason could be that fibril-fibril interactions in the semi-dilute CNF causes the fibrils of all lengths to have slower rotary diffusion and thus appearing longer (compared to the dilute case). The recent work by Brouzet et al 5 along with the theory of semi-dilute rigid rods by Marrucci and Grizzuti 55,56 makes the use of a sample dependent interaction parameter b to adjust the flow-stop results to the physical length distribution from TEM.…”

“…Further comparisons between the geometries are given in the ESI. † It is important to note that the local concentration of CNF can safely be assumed to be constant in the FFC core as the translational diffusion is on the order of days over distances of millimeters 4 (and of course that incompressibility of the dispersion ensures a divergence free flow field everywhere).…”

“…Although translational diffusion also will affect spatial variations of orientation distributions, the timescales of rotational diffusion is usually much shorter. 4 Thus, the diffusive dynamics of nanofibrils is primarily described by their rotational motion.…”

Elucidating the Opportunities and Challenges for Nanocellulose Spinning

“…First, the rheology of the dispersion and effective interfacial tension between the dispersion and sheath fluids will determine the flow regime. [83,84] The desired detached core flow for filament spinning is usually referred to as a threading regime. Given the properties of sheath fluid and dispersion, the flow regime could also be dropping (core dispersion forms droplets) or tubing (core dispersion never detaches from the walls perpendicular to sheath flows).…”

Shear-free mixing to achieve accurate temporospatial nanoscale kinetics through scanning-SAXS: ion-induced phase transition of dispersed cellulose nanocrystals

“…Fortunately, given the right range of the effective interfacial tension and viscosity ratio between core and sheath, the core fluid can be detached from the walls and flowing in the center of the channel. 35,36 Once achieved, the system can provide an ideal continuous mixing situation with practically no shear and constant velocity of the main substance, and where all streamlines in the beam have the same residence time in the channel. This in turn allows a trivial conversion from spatial positions to time instances can be obtained for the study of time-resolved mixing kinetics.…”