Forces between oil drops in polymer-surfactant systems: Linking direct force measurements to microfluidic...

Linking atomic force microscopy and microfluidics opens up the possibility of probing adhesive interactions between drops in a high-throughput context. A microfluidic device designed to form, and subsequently break-up, chains of drops, where the drop break-up is sensitive to the underlying surface forces between drops, not hydrodynamic drainage forces, would play a key role in developing this link.

Both techniques have been used to quantify the forces between oil drops in the presence of complexes formed with anionic surfactant, sodium dodecylsulphate, and neutral, water soluble polymer, poly(vinylpyrrolidone). Measurement and modelling of the interaction forces between both rigid and deformable surfaces demonstrated that the attraction between the drops is due to depletion forces, whereas the repulsive force is a combination of electrical double layer and steric forces, indicating complexes exist both in the bulk and at the drop interface.

The interaction behaviour between the force measurements and the microfluidic observations showed a strong correlation, where the observed adhesion between drops in the microfluidics is sensitive to the drop deformation and Laplace pressure. Correlation between the two techniques provides insight into the surface forces between drops in flowing systems and has potential utility in the formulation of emulsions.