Tuning Sedimentation Through Surface Charge and Particle Shape

Mud forms the foundation of many coastal and tidal environments. Clay suspensions carried downstream from rivers encounter saline waters, which encourages aggregation and sedimentation by reducing electrostatic repulsion among particles. We perform experiments to examine the effects of surface charge on both the rate and style of sedimentation, using kaolinite particles as a model mud suspension and silica spheres with equivalent hydrodynamic radius as a control. Classic hindered settling theory reasonably describes sedimentation rate for repulsive clay particles and silica spheres, which form a highly concentrated jamming front. The hindered settling description breaks down for attractive clay particles, which aggregate to form clay gels that consolidate like a soft solid. Water flow form fracture‐like channels in the bulk of the gel, which disappear as gel enters a creep regime. Results may help toward understanding the effect of surface charge and particle shape on the sedimentation and erodibility of natural mud. Plain Language Summary When suspended sediment is transported from land to the ocean by river, the water surrounding the sediment particles changes from fresh to salty. This change creates increased interparticle attraction, leading sediment to aggregate and deposit. In contrast to ocean salinity, artificial fertilizers may contain different salts that have the opposite effect on interparticle forces, creating repulsion that suppresses aggregation. These chemical effects, and the way particles sink, are modulated by the shape of the sediment too. Here we perform experiments to examine these effects on sedimentation, using kaolinite particles as a model mud suspension and glass beads as a control. We see that how the particles sink is sensitive to chemistry: when they are repulsive a classic “hindered settling” theory predicts their deposition well, and when attractive the particles link up in a network that behaves like a single structure that collapses under its own weight. The flow of water out of the structure—which we call a gel—as it collapses becomes localized into fracture‐like channels that disappear as the deformation slows down and the gel gets denser. Our observations improve understanding of mud sedimentation, which is essential to predict how estuaries and coastal environments change. Key Points Aggregation of clay particles enhances sedimentation and forms mud in nature Changing surface charge results in a phase transition from h