Flocculation and its tuning are of utmost importance in the optimization of several industrial protocols in areas such as purification of waste water and civil engineering. Herein, we studied the polyelectrolyte‐induced flocculation of clay colloids on a model system consisting of purified clay colloids of well‐defined size fractions and ionene polyelectrolytes presenting regular and tunable chain charge density. To characterize ionene‐induced clay flocculation, we turned to the combination of light absorbance (turbidity) and ζ‐potential measurements, as well as adsorption isotherms. Our model system allowed us to identify the exact ratio of positive and negative charges in clay–ionene mixtures, the (c+/c−) ratio. For all samples studied, the onset of efficient flocculation occurred consistently at c+/c− ratios significantly below 1, which indicated the formation of highly ionene‐deficient aggregates. At the same time, the ζ‐potential measurements indicated an apparent zero charge on such aggregates. Thus, the ζ‐potential values could not provide the stoichiometry inside the clay–ionene aggregates. The early onset of flocculation in clay–ionene mixtures is reminiscent of the behavior of multivalent salts and contrasts that of monovalent salts, for which a large excess amount of ions is necessary to achieve flocculation. Clear differences in the flocculation behavior are visible as a function of the ionene charge density, which governs the conformation of the ionene chains on the clay surface.

We may distinguish two regimes as a function of clay and ionene polymer charge densities (ρ c and ρ p respectively). The first regime applies to ρ p > ρ c and ρ p ∼ ρ c , i.e. for highly and "matching" charged chains. Under these conditions the intercalated chains lie in a flat conformation within the tactoids, irrespective of the ionic strength (within the range studied, i.e. up to 0.05M NaBr). For weakly charged chains, ρ p < ρ c , undulation of the ionene chains within the tactoid is seen. The degree of undulation increases with ionic strength, due to the decreasing persistence length of the ionene chains. The extent of stacking (5-10 platelets per tactoid) is a general feature of all the systems and its origin remains unknown. The system corresponding to the closest match in charge separations on the clay surface and on the polymer chain (ρ p ∼ ρ c) features the highest abundance of tactoids. This coincides with the highest macroscopic density as deduced from simple visual inspection of sediment volumes. This leads to the open question regarding the link between the density at the nanoscale and the macroscopic density and sedimentation behaviour of the aggregate.

Aluminium salts such as aluminium chlorohydrate (ACH) are the active ingredients of antiperspirant products. Their mechanism of action involves a temporary and superficial plugging of eccrine sweat pores at the skin surface. We developed a microfluidic system that allows the real time observation of the interactions between sweat and ACH in conditions mimicking physiological sweat flow and pore dimensions. Using artificial sweat containing bovine serum albumin as a model protein, we performed experiments under flowing conditions to demonstrate that pore clogging results from the aggregation of proteins by aluminium polycations at specific location in the sweat pore. Combining microfluidic experiments, confocal microscopy and numerical models helps to better understand the physical chemistry and mechanisms involved in pore plugging. The results show that plugging starts from the walls of sweat pores before expanding into the centre of the channel. The simulations aid in explaining the influence of ACH concentration as well as the impact of flow conditions on the localization of the plug. Altogether, these results outline the potential of both microfluidic confocal observations and numerical simulations at the single sweat pore level to understand why aluminium polycations are so efficient for sweat channel plugging.

Are aluminium ions unavoidable in antiperspirants? To further the question, we present confocal microscopy images of dendritic plug appearing in sweat flowing across a microfluidic channel in presence of aluminium...