Suppression of fragmentation of drop chains in confined geometries

Droplet chains in confined suspension flows exhibit complex collective dynamics that can range from density-wave propagation to diffusive relaxation, to chain fragmentation. For an externally imposed Poiseuille flow, a unidirectional density wave propagation occurs because the particle interactions inherit the vector-like nature of the incident flow. In contrast, in a Couette flow the leading-order particle interactions are symmetric, which leads to either a stable diffusive behavior or an unstable one when the diffusion constant is negative. The instability that causes the fragmentation of low-density chains is driven by the symmetric quadrupolar interparticle hydrodynamic interactions. Using a simple simulation model with dipolar, quadrupolar, and repulsive near-field hydrodynamic interactions, we show that in some cases low-density chains initially undergo fragmentation but are subsequently stabilized by a spontaneously formed train of short solitary waves. A propagating soliton, consisting of a group of closely spaced particles, absorbs the disordered particles in front of it and leaves equally spaced drops in its wake, thus stabilizing the drop-chain structure. Such a stabilization mechanism may be useful in microfluidic manipulation of drop chains.