Effect of particle shape and addition of oppositely charged particles with different wettability on droplet bridging

Abstract

This work demonstrates the effect of a heterogeneous mixture of oppositely charged particles with different shapes and wettability on the stability of droplet bridging. For this purpose, micron-sized hydrophilic hematite particles with ellipsoidal, spherocylindrical, cubic, and dumbbell shapes are employed as emulsifiers. To prepare the emulsions, these particles are modified in situ with oleic acid in an immiscible water–decane system. The results confirm that droplet bridging is controlled by tuning the particle wettability and adhesive force alone while the required oleic acid concentration range for droplet bridging and the resulting two-dimensional (2D) particle arrangement at the bridge depend on the particle shape. Ellipsoids and spherocylinders form short chains with random orientation in a side-by-side arrangement, whereas cubes and dumbbells arrange into long-range 2D crystal structures with square and triangular lattices, respectively. Moreover, the effect of adding oppositely charged particles with different wettability (hydrophobic polystyrene and hydrophilic silica particles) to the hematite particles on droplet bridging and particle ordering is also investigated. When the amount of hydrophobic particles added to the hematite system exceeds a critical ratio, the system transitions from bridged emulsions to individual droplets. Increasing the number of hydrophobic particles destabilizes bridging by causing the bridged interfaces to merge whereas adding hydrophilic particles promotes stable bridged emulsions by remaining within the hematite bridge without merging the interfaces. This approach enables the design of 2D ordered crystals of binary particles at the droplet bridge. Notably, this study demonstrates that oppositely charged particles can be used to tune and stabilize bridged emulsions under optimal conditions.