Flow focusing of nanofibrils is controlled by effective interfacial tension

Flow focusing of a cellulose nanofibrils (CNF) suspension is studied numerically and experimentally. The geometry is flat and consists of 1x1 mm² channels that meet in the form of a plus (+). Three of the four branches are inlets and one is the outlet. A central flow consisting of a suspension of CNF in water is focused by pure water flowing from two channels that are orthogonal to the central flow, whereafter all fluid is ejected through the outlet. The flow case has been chosen since it has been demonstrated that it can be used to assemble very strong cellulose filaments (ACS Nano 12, 7, 6378-6388). Optical Coherence Tomography (OCT) is used to determine the shape and velocity of the central flow, and the experimental data are matched to multiphase computational fluid dynamics simulations. The behaviour of the colloidal suspension in the central flow is seen to be controlled by the effective interfacial tension between the pure water and the water with CNF. It turns out that a weak surface tension must be applied in the simulations in order to get a good agreement with the experimental data. When properly tuned, the agreement between simulations and experiments is excellent.