Collective Growth in a Small Cell Network
10:15 - 11:15
Jasmin Imran Alsous
Chemical & Biological Engineering, Princeton University, USA
Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
We force objects of different nature through constrictions all the time: sand in an hourglass, particles in a fluid through a porous medium, blood through a narrowed vessel or people leaving a room in panic. In all these cases it is important to make sure that the system keeps continuously flowing, sometimes even lives are at risk. The case of particles in a fluid affects porous mediums, filters and membranes, which become unusable when clogged. We use microfluidic devices with a bottleneck of squared cross-section through which we force dilute polystyrene particle solutions with diameters comparable to the bottleneck size and down to one tenth its size. In low friction conditions, we show experimental evidence of a strong transition at a critical particle-to-neck ratio, just as it occurs in dry granular systems (Zuriguel et al., Phys. Rev. E, 2003). We describe analytically such a transition by modelling the arch formation as a purely stochastic process, which yields a good agreement with the experimental data.
If the constriction is removed particles flow without trouble. However, their dynamics are far from trivial even in diluted conditions. At critical interparticle distances, particles tend to interlace their trajectories, only bonded by hydrodynamic interactions. While classical studies on non-Brownian self-diffusivity report average particle displacements of fractions of the particle diameter, the trajectories observed in our system show displacements of several particle diameters. Furthermore, entangled particles seem to "synchronize" their motion with others located at several particle diameters. Particle trajectory statistics are obtained from the experiments for different shear rates and particle sizes showing the same results in wide range of parameters. The results are then compared with particle dynamics simulations and analyzed to elucidate the nature of the hydrodynamic interactions entering into play. The reported phenomenon could be applied to promote advective mixing in micro-channels or particle/droplet self-assembly.