Droplet transition from non-axisymmetric to axisymmetric shape: Dynamic role of lubrication film in a rectangular microfluidic channel

In the past 20 years, droplet microfluidics is burgeoning in many chemical and biological applications due to the unique capability of droplets to act as confined containers. Confinement is ensured even in the case of squeezed droplets within microchannels much smaller than droplet volumes due to the presence of a lubrication thin film that prevents contact between droplets and the channel walls. The thickness of the lubrication film depends on the dynamics of the entire microfluidic system, affecting the actual droplet's shape and velocity. Therefore, this film is extensively studied to obtain insight into the dynamics of flowing droplets, especially when confined in small channels. Circular cross section channels are the most studied for their axial symmetry, but practical applications present most likely non-axisymmetric channels, as a result of fabrication processes, such as soft lithographic rectangular channels. The latter showed unique transitional morphological behavior of droplets, which assumes an axisymmetric or non-axisymmetric shape during their flow inside a non-axisymmetric channel, depending on the lubrication film. This work gives a comprehensive experimental characterization of the dynamics of the lubrication film during the droplet shape transition. We settled on a novel approach based on the optical diffraction of a localized light beam provided by two-facing optical waveguides integrated with the microfluidics circuit. The technique allows for studying the dynamics of flowing droplets and their relationship with the lubrication film thickness. Additionally, this experimental system enables a precise definition of two regimes of lubrication film, and the critical capillary number at which the transition occurs.