Dynamic forces between a moving particle and a deformable drop

The dynamic interaction forces between a solid silica particle and an immobilized tetradecane droplet have been measured using atomic force microscopy. The dominant colloidal forces are repulsive electrical double layer interactions due to the partitioning of added sodium dodecyl sulfate to the deformable oil/water (O/W) interface. Over a range of drive velocities up to 50 μm/s, these colloidal forces, plus hydrodynamic interactions due to fluid flow in the aqueous film (~22 nm thick) between the particle and O/W interface, as well as deformations of the interface, contribute to the overall dynamic force between the silica particle (radius ~12 μm) and tetradecane drop (radius of curvature ~55 μm). Within the tolerance of experimental parameters, excellent agreement is observed between the measured forces and those predicted by a theory that accounts for the above phenomena. The theory also furnishes details not directly measurable, such as the time-dependent deformations and velocities of the O/W interface and variations of the thickness of the aqueous film between the particle and the drop during the course of the experiment