Effects of viscoelasticity on droplet dynamics and break-up in microchannels: a Lattice Boltzmann study

<strong>Seminar of the School of Physical Sciences ----------------------------------------------</strong> Title: <strong>Effects of viscoelasticity on droplet dynamics and break-up in microchannels: a Lattice Boltzmann study</strong> Speaker:<strong> Anupam Gupta</strong> (University of Rome Tor Vergata, Rome) Date:<strong> August 7, 2015 (Friday)</strong> <strong>Abstract:</strong> The effects of viscoelasticity on the dynamics and break-up of liquid threads in microfluidic devices, i.e., T-junctions &amp; Cross-Junction, are investigated using numerical simulations of dilute polymeric solutions for a wide range of Capillary numbers (Ca) ,i.e., changing the balance between the viscous forces and the surface tension at the interface, up to Ca = 10^{-1}. A Navier-Stokes (NS) description of the solvent based on the lattice Boltzmann models (LBM) is here coupled to constitutive equations for finite extensible non-linear elastic dumbbells with the closure proposed by Peterlin (FENE-P model). We present the results of three-dimensional simulations in a range of Capillary numbers and flow-rate ratios which is broad enough to characterize all the three characteristic mechanisms of breakup in the confined T-junction, i.e., squeezing, dripping and jetting regimes and all the three characteristic mechanisms of breakup in the confined flow-focusing device, i.e., droplet formation at the cross-junction (DCJ) and droplet formation downstream of the cross-junction (DC). The various model parameters of the FENE-P constitutive equations, including the polymer relaxation time $\tau_P$ and the finite extensibility parameter $L^2$, are changed to provide quantitative details on how the dynamics and break-up properties are affected by viscoelasticity. We will present both the cases of Droplet Viscoelasticity (DV), where viscoelastic properties are confined in the dispersed (d) phase, as well as cases with Matrix Viscoelasticity (MV), where viscoelastic properties are confined in the continuous (c) phase. Moderate flow-rate ratios Q~ O(1) of the two phases are considered in the present study. Overall, we find that the effects are more pronounced in the case with MV, as the flow driving the break-up process upstream of the emerging thread can be largely perturbed by the polymeric stresses.