Ultrasound control of oil-in-oil Pickering emulsions preparation

Particle-stabilised emulsions are of interest to many scientists in both academia and industry as they hold promise for numerous applications. There is a lot of research effort put into developing new methods for their fabrication. Often, different experimental techniques are used for monitoring the process of the emulsion formation. However, the control of the emulsion fabrication and its real-time characterisation is generally challenging. In this work, we propose a convenient method to control fabrication of Pickering emulsions using ultrasound. The benefit of acoustical measurements compared to other techniques is their ability to test the medium in a non-destructive way, without requiring special sample preparation (e.g. dilution like in the case of DLS or NMR) nor a usage of thin sample cells (e.g. in the case of optical microscopy). Here, ultrasonic measurements are able to follow droplets growth during the emulsion development using the limited coalescence approach. We found that ultrasonic attenuation increased with the droplet size within the time frame of droplet stabilisation. Following these changes in ultrasonic attenuation enabled the study of macroscopic behaviour-for example, estimation of a time when droplets achieve their final size and become fully covered by solid particles. These results are compared with the results obtained from the optical measurements. We also make an attempt to theoretically describe ultrasound propagation in particle-stabilised emulsions by comparing our experimental results with the scattering theory ECAH for emulsions.