The Evolution of Active Droplets in Chemorobotic Platforms

There is great interest in oil-in-water droplets as simple systems that display astonishingly complex behaviours. Recently, we reported a chemorobotic platform capable of autonomously exploring and evolving the behaviours these droplets can exhibit. The platform enabled us to undertake a large number of reproducible experiments, allowing us to probe the non-linear relationship between droplet composition and behaviour. Herein we introduce this work, and also report on the recent developments we have made to this system. These include new platforms to simultaneously evolve the droplets’ physical and chemical environments and the inclusion of selfreplicating molecules in the droplets

Self-organised droplet flow patterns in microchannels

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.In this work, we have investigated the generation and behaviour of self-organised droplet flow patterns in microchannels. The water droplets, which are generated at a T-junction where the carrier is oil, move into an expanded channel and are self reorganised into various flow patterns: single-profile, double-helix-profile, triple-helix-profile, and more. We find that increasing water/oil flow rate ratio and Capillary number lead to more densely packed droplet flow patterns. The channel geometry also plays an essential role where the 300-μm-deep expansion channel can form multiple layers of droplets while only single layer of droplets can be observed in the 200-μm-deep expansion channel

Self-propulsion of pure water droplets by spontaneous Marangoni stress driven motion

We report spontaneous motion in a fully bio-compatible system consisting of pure water droplets in an oil-surfactant medium of squalane and monoolein. Water from the droplet is solubilized by the reverse micellar solution, creating a concentration gradient of swollen reverse micelles around each droplet. The strong advection and weak diffusion conditions allow for the first experimental realization of spontaneous motion in a system of isotropic particles at sufficiently large P\'eclet number according to a straightforward generalization of a recently proposed mechanism. Experiments with a highly concentrated solution of salt instead of water, and tetradecane instead of squalane, confirm the above mechanism. The present swimming droplets are able to carry external bodies such as large colloids, salt crystals, and even cells.Comment: 5 pages, 5 figure

Spontaneous Interfacial Fragmentation of Inkjet Printed Oil Droplets and Their electrical characterization

This work presents the fabrication of femtoliter-scale oil droplets by inkjet printing based on a novel mechanism for the spontaneous fragmentation at the interface with an immiscible water phase and the electrical characterization of the resulting immersed “daughter” droplets. [1] In particular, picoliter-scale fluorinated oil droplets impact on surfactant laden water phase at moderately high Weber number (101), and are subjected to spreading and capillary instabilities at the water/air interface which ultimately lead to rupture in smaller sized droplets, according to reported models for macroscale droplets systems - [2] the emerging fragmentation results in “daughter” droplets having volumes of about 10-30 % with respect to the initial droplet volume. Remarkably, the picoliter scale downscaling leads to a novel surfactant-driven fragmentation due to the low Bond number (around 10-4-10-5), meaning that droplet immersion is dependent on surface tension forces and not on gravitational forces. In fact, the non-ionic Polyoxyethylene (20) sorbitan monolaurate was observed to permit the droplet immersion in the water phase only if spiked in the water phase at concentrations equal or higher than its critical micellar concentration (i.e. around 0.003% v/v). The resulting oil “daughter” droplets are characterized by a chip with integrated microelectrodes, permitting to extract number, velocities and diameter distribution (peaked at about 3 m) employing electrical impedance measurements. In accordance with reported models, the electrical characterizations show that the droplets have volumes in the femtoliter scale and are subjected to inertial focusing. [3] This work can be considered an important advancement for understanding the effects of downscaling on fragmentation phenomena at immiscible interfaces, leading to a knowledge platform for a tailored oil droplets fabrication applicable for drug encapsulation, pharmaceutic preparations, and thin-film wrapping around droplets. [4] Bibliography 1. D. Spencer, F. Caselli, P. Bisegna and H. Morgan., Lab Chip, 2016, 16, 2467. 2. H. Lhuissier, C. Sun, A. Prosperetti, and D. Lohse, Phys. Rev. Lett., 2013, 110, 3. G. Arrabito, V. Errico, A. De Ninno, F. Cavaleri, V. Ferrara, B. Pignataro, and F.Caselli, Langmuir, 2019, 35, 4936. 4. D. Kumar, J. D. Paulsen, T. P. Russell, N. Menon, Science, 2018, 359, 775

Experimental and numerical study of the water-in-oil emulsions in porous media

In different industries and environments, water-in-oil emulsions are complicated mixtures of multiple phases. They are useful for enhanced oil recovery, petroleum refining, and oil spill remediation. The behavior and properties of water-in-oil emulsions through porous media depend on several factors, such as interfacial tension, contact angle, and oil viscosity. In this work, modeling of water-in-oil emulsions was performed using COMSOL Multiphysics and validated using experimental data. The utilized experimental method included a T-junction microfluidic device to visualize and measure how the water droplets in water-in-oil emulsions differ in size, shape, and displacement. A sensitivity analysis was conducted to evaluate the impacts of interfacial tension, contact angle, oil and water channel size, and oil viscosity on the water and oil droplet sizes, distribution, and mobility in porous media. The results show the effects of water salinity, flow rates, and asphaltenes on the interfacial tension and water droplet size in water-in-oil emulsions using a T-junction microfluidic device. The size of droplets water-in-oil emulsions is influenced by the water’s salinity, the interfacial tension between water and oil, and the flow rate within each phase. The optimal water droplets were obtained by the seawater diluted two times (SW#2), and the droplet shape and breakup were influenced by the shear rate, reynolds number and weber number. The rates of flow affect the shaping and division of droplets, while the suggested modeling approach can precisely depict the behavior and structure of water-in-oil emulsions within porous media. The findings of this research provide valuable insights for optimizing the performance and efficiency of water-in-oil emulsion processes.Document Type: Original articleCited as: Zarin, T., Aghajanzadeh, M., Riazi, M., Ghaedi, M., Motealeh, M. Experimental and numerical study of the water-in-oil emulsions in porous media. Capillarity, 2024, 13(1): 10-23. https://doi.org/10.46690/capi.2024.10.0

Oil Droplet Coalescence in W/O/W Double Emulsions Examined in Models from Micrometer-to Millimeter-Sized Droplets

Water-in-oil-in-water (W$_{1}$/O/W$_{2}$) double emulsions must resist W$_{1}$–W$_{1}$, O–O and W$_{1}$–W$_{2}$ coalescence to be suitable for applications. This work isolates the stability of the oil droplets in a double emulsion, focusing on the impact of the concentration of the hydrophilic surfactant. The stability against coalescence was measured on droplets ranging in size from millimeters to micrometers, evaluating three different measurement methods. The time between the contact and coalescence of millimeter-sized droplets at a planar interface was compared to the number of coalescence events in a microfluidic emulsion and to the change in the droplet size distributions of micrometer-sized single and double emulsions. For the examined formulations, the same stability trends were found in all three droplet sizes. When the concentration of the hydrophilic surfactant is reduced drastically, lipophilic surfactants can help to increase the oil droplets’ stability against coalescence. This article also provides recommendations as to which purpose each of the model experiments is suited and discusses advantages and limitations compared to previous research carried out directly on double emulsions

Marangoni-Induced Reversal of Meniscus-Climbing Microdroplets

Small water droplets or particles located at an oil meniscus typically climb the meniscus due to unbalanced capillary forces. Here, we introduce a size-dependent reversal of this meniscus-climbing behavior, where upon cooling of the underlying substrate, droplets of different sizes concurrently ascend and descend the meniscus. We show that microscopic Marangoni convection cells within the oil meniscus are responsible for this phenomenon. While dynamics of relatively larger water microdroplets are still dominated by unbalanced capillary forces and hence ascend the meniscus, smaller droplets are carried by the surface flow and consequently descend the meniscus. We further demonstrate that the magnitude and direction of the convection cells depend on the meniscus geometry and the substrate temperature and introduce a modified Marangoni number that well predicts their strength. Our findings provide a new approach to manipulating droplets on a liquid meniscus that could have applications in material self-assembly, biological sensitive sensing and testing, or phase change heat transfer.Comment: submitted to Soft Matte

Computer simulation of turbulent electrocoalescence

Offshore wells produce some water, and the ratio of water increases during the lifetime of a well, in particular when water is injected to increase the extraction rate. Hence, oil companies demand techniques that enhance the separation of oil and water. A speed-up of the separation process is achieved by applying electric fields to turbulent-flow water-in-oil emulsions. The electric field gives rise to attractive forces between close droplets and increases the probability of coalescence at contact, while the turbulence enhances the frequency of droplet collisions. To improve the understanding of the mutual interaction between the turbulence and the electric field, this thesis presents a framework for computer simulation of turbulent electrocoalescence. The framework is based on the Eulerian-Lagrangian approach where each droplet is tracked and the electric and the hydrodynamic interactions between the droplets are handled. The forces working between two droplets in stagnant oil are modelled and compared with experimental data. It was found that the electric dipole-dipole forces and the filmthinning forces dominate at small droplet spacings. The turbulence felt by the droplets is modelled by a stochastic differential-equation model. A new model is proposed to correlate the fluid velocities seen by close droplets, and this is important for the prediction of the collision velocity, the collision frequency, and the clustering of droplets. Two algorithmic improvements are made: An adaptive cell structure and the cluster integration method. The proposed adaptive cell structure adapts to the number density of droplets and ensures an efficient computation without any input from the user regarding the cell structure. The cluster integration method assembles clusters of droplets that interact and integrates each cluster separately using a variable step-size Runge-Kutta method. A significant speed-up compared to traditional approaches is reported. Finally, the results obtained by computer simulations of turbulent electrocoalescence agree qualitatively with experimental observations in the literature.PhD i energi- og prosessteknikkPhD in Energy and Process Engineerin