The control of droplet formation and size using microfluidic devices is a critical operation for both laboratory and industrial applications, e.g. in micro-dosage. Surfactants can be added to improve the stability and control the size of the droplets by modifying their interfacial properties. In this study, a large-scale data set of droplet size was obtained from high-speed imaging experiments conducted on a flow-focusing microchannel where aqueous surfactant-laden droplets were generated in silicone oil. Three types of surfactants were used including anionic, cationic and non-ionic at concentrations below and above the critical micelle concentration (CMC). To predict the final droplet size as a function of flow rates, surfactant type and concentration of surfactant, two data-driven models were built. Using a Bayesian regularised artificial neural network and XGBoost, these models were initially based on four inputs (flow rates of the two phases, interfacial tension at equilibrium and the normalised surfactant concentration). The mean absolute percentage errors (MAPE) show that data-driven models are more accurate (MAPE = 3.9%) compared to semi-empirical models (MAPE = 11.4%). To overcome experimental difficulties in acquiring accurate interfacial tension values under some conditions, both models were also trained with reduced inputs by removing the interfacial tension. The results show again a very good prediction of the droplet diameter. Finally, over 10 000 synthetic data were generated, based on the initial data set, with a Variational Autoencoder (VAE). The high-fidelity of the extended synthetic data set highlights that this method can be a quick and low-cost alternative to study microdroplet formation in future lab on a chip applications, where experimental data may not be readily available.

The effect of surfactants on the flow characteristics during rapid drop formation in a microchannel is investigated using high-speed imaging, micro-particle image velocimetry and numerical simulations; the latter are performed using a three- dimensional multiphase solver that accounts for the transport of soluble surfactants in the bulk and at the interface. Drops are generated in a flow-focusing microchannel, using silicone oil ( $4.6$ mPa s) as the continuous phase and a 52 % w/w glycerol solution as the dispersed phase. A non-ionic surfactant (Triton X-100) is dissolved in the dispersed phase at concentrations below and above the critical micelle concentration. Good agreement is found between experimental and numerical data for the drop size, drop formation time and circulation patterns. The results reveal strong circulation patterns in the forming drop in the absence of surfactants, whose intensity decreases with increasing surfactant concentration. The surfactant concentration profiles in the bulk and at the interface are shown for all stages of drop formation. The surfactant interfacial concentration is large at the front and the back of the forming drop, while the neck region is almost surfactant free. Marangoni stresses develop away from the neck, contributing to changes in the velocity profile inside the drop.

This Letter explores the influence of colloids at liquid-liquid interfaces on droplet pinch-off dynamics in microfluidic devices, examining hard polystyrene particles and soft pNIPAM microgels. We uncover a significant deviation in droplet formation time compared to pure systems, similarly to surfactant-laden systems, yet notably, colloids exert minimal impact on droplet size, indicating potential nonlinear effects. The dynamics of neck thinning without colloids agree with the classic pendant drop scaling laws, while particle presence replaces traditional viscous and inertial-viscous regimes with a single power law, suggesting an elastic behavior driven by soft particle interactions. Published by the American Physical Society 2024

The present study investigated experimentally the effect of electric field on droplet formation process in a co-flow microchannel. A silicone oil (viscosity of 4.6 mPa · s) and an aqueous solution (mixture of 52% w/w water–glycerol) were used as the continuous and dispersed phases, respectively, while the applied voltage varied from 0 kV to 10 kV. It was found that both electric field and flow rates of the two phases affected the droplet formation process. When no electric field was utilized, the droplet size decreased with increasing continuous phase flow rate while the formation time decreased with increasing dispersed phase flow rate. When an electric field was applied, both the droplet size and formation time decreased with increasing voltage. Three droplet formation patterns, namely squeezing, dripping, and jetting, were identified. Under the effect of electric field, the range of flow rates where the squeezing pattern appeared reduced. The droplet size was related to the electric Bond number and the flow rate ratio of the continuous phase to the total flow rate with exponents that depended on whether the electric field affected the droplet formation process. A flow pattern map with phase flow rate ratio and electric Bond number as coordinates was also built.

The displacement flow of an organic Newtonian fluid by a pure viscoelastic aqueous solution is experimentally investigated inside a circular microchannel of 200 μm. Displacement is commonly encountered in many industrial applications, from cleaning and decontamination to enhanced oil recovery. In this study, a pure viscoelastic fluid with no shear-thinning properties (known as Boger fluid) made up of polyethylene oxide, polyethylene glycol, and zinc chloride is used to displace an immiscible organic liquid (silicone oil). The results were compared against those from displacement with a Newtonian fluid of similar density and viscosity as the viscoelastic one. High-speed imaging is used to track both the residual film thickness of the organic phase and the interface deformations during displacement. It is found that the Boger fluid displacing phase produces a thinner displaced phase film compared to the Newtonian fluid, particularly at high capillary numbers. A correlation is proposed for the film thickness, which includes the Weissenberg number for the viscoelastic case. After the displacement front, the interface becomes unstable with two modes of instability identified. In the case of the Boger fluid, the two modes of instability are core shifting, which is also present in the Newtonian case, and a periodic instability from the elastic stresses during displacement. Additionally, the shape of the interfacial instabilities switches freely from asymmetric to axisymmetric ones throughout the flow. The frequency of the periodic instabilities increases with the displacing phase flow rate. It was also found that microchannel bends downstream of the observation point affect the shape and frequency of the instabilities.