Microfluidic elucidation of the effects of interfacial rheology on droplet deformation

Abstract

Proteins and peptides, adsorbed at a fluid–fluid interface, can form a mechanically strong cohesive interfacial network, which significantly affects the behavior of foams and emulsions. Droplet deformation in the presence of interfacial protein and peptide networks was systematically investigated in defined hydrodynamic extensional flows using a specially designed microfluidic chip. A correlation between dynamic droplet deformation and the Capillary number was developed for pure systems (oil in water and the buffer solution) and for the low molecular weight surfactant sodium dodecyl sulfate (SDS), which can be used to estimate interfacial tension following observation of deformation. Surprisingly, we found that droplet deformation in the presence of proteins (β-lactoglobulin, lysozyme and β-casein) was described by this correlation, which indicated that proteins did not significantly affect the droplet deformation behavior in the systems we examined. However, the peptide AFD4, which forms rapidly a mechanically strong cohesive film, reduced droplet deformation. Further investigation revealed that the kinetics of protein network formation was slower than the kinetics associated with interfacial tension reduction; the short residence time of droplets in the microfluidic channel (10 s) did not provide sufficient time for protein network formation. Conversely, peptide AFD4 formed the interfacial cohesive network rapidly after the formation of droplets because of its rapid interfacial adsorption due to its small size (2345 Da). The correlation obtained based on interfacial tension was found to be also valid for droplet deformation in the presence of a strong interfacial cohesive network, provided interfacial elasticity replaces interfacial tension in the Capillary number. Combining different techniques including Profile Analysis Tensiometer (PAT), CIT, and microfluidics, this study provides direct insight into the behavior of proteins and a peptide surfactant (AFD4) at the fluid–fluid interface in confined flows and reveals a similarity in effects for interfacial elasticity and tension in controlling droplet deformation.