Investigation of droplet coalescence in nanoparticle suspensions by a microfluidic collision experiment

High efficiency of biosurfactants in stabilizing oil micro-droplets within the aging time scale of milliseconds: a microfluidic study

Rapid adsorption of surfactants onto a freshly formed interface is vital for emulsification because emulsification is a competitive process occurring between the very short time span of interface formation and surfactant mass transport. The biosurfactant surfactin has been previously reported to reach adsorption equilibrium at the hydrophobic/hydrophilic interface within hundreds of milliseconds and rapidly reduce the interfacial tension compared to chemically synthesized surfactants. According to a prior study, surfactin is expected to exhibit good performance in stabilizing micro-droplets of oil within the aging time scale of milliseconds. Herein, the stabilities of micro-droplets of n-hexadecane in the presence of a biosurfactant, surfactin (C15-SFT), and a chemically synthesized surfactant, sodium cetyl benzene sulfonate (8-SCBS), were investigated using a microfluidic method. The coalescence frequency of micro-droplets, the evolution of micro-droplet size, and the coalescence time of micro-droplets were evaluated. The results indicated that C15-SFT exhibited superiority over 8-SCBS in stabilizing the micro-droplets of n-hexadecane. Biosurfactant C15-SFT effectively reduced the fusion probability between oil droplets and elongated the coalescence time compared to 8-SCBS, and these phenomena were obvious at a shorter aging time (150 ms) and lower surfactant concentration (0.1 × critical micelle concentration). The stabilities of micro-droplets increased with aging time and the bulk concentration of surfactants. Stable micro-droplets of n-hexadecane were formed in 1 × 10-4 mol L-1 C15-SFT solution at 600 ms aging time, and the bulk concentration was 1 × 10-3 mol L-1 in the case of 8-SCBS. The micro-droplets rarely coalesced in the presence of 1 × 10-4 mol L-1 C15-SFT after 600 ms aging time, but the micro-droplets in 1 × 10-4 mol L-1 8-SCBS coalesced frequently in the midstream and downstream of the coalescence chamber, and big droplets were dominant in the emulsion. The coalescence time of micro-droplets stabilized by C15-SFT was obviously longer than that of those stabilized by 8-SCBS under the same condition, indicating that the interfacial film formed by C15-SFT has much strength to resist coalescence during collisions. This work is helpful for understanding the activity of lipopeptides in the very short early stage of the emulsification process, laying the foundation for biosurfactant research in the fields of enhanced oil recovery, bioremediation of contaminated water or soil, etc.

Contact-mediated nucleation in melt emulsions investigated by rheo-nuclear magnetic resonance

Increasing the efficiency of disperse phase crystallization is of great interest for melt emulsion production as the fraction of solidified droplets determines product quality and stability. Nucleation events must appear within every single one of the μm-sized droplets for solidification. Therefore, primary crystallization requires high subcooling and is, thus, time and energy consuming. Contact-mediated nucleation is a mechanism for intensifying the crystallization process. It is defined as the successful nucleation of a subcooled liquid droplet induced by contact with an already crystallized droplet. We investigated contact-mediated nucleation under shear flow conditions up to shear rates of 457 s^-1 for a quantitative assessment of this mechanism. Rheo-nuclear magnetic resonance was successfully used for the time-resolved determination of the solids fraction of the dispersed phase of melt emulsions upon contact-mediated nucleation events. The measurements were carried out in a dedicated Taylor-Couette cell. The efficiency of contact-mediated nucleation λsec decreased with increasing shear rate, whereas the effective second order kinetic constant kcoll,eff increased approximately linearly at small shear rates and showed a linear decrease for shear rates higher than about 200 s^-1 . These findings are in accordance with coalescence theory. Thus, the nucleation rate is optimal at specific flow conditions. There are limitations for successful inoculation at a low shear rate because of rare contact events and at a high shear rate due to too short contact time.

Microfluidics as a tool to assess and induce emulsion destabilization

Microfluidic technology enables judicious control of the process parameters on a small length scale, which in turn allows speeding up the destabilization of emulsion droplets interface in microfluidic devices. In this light, microfluidic channels can be used as an efficient tool to assess emulsion stability and to observe the behavior of the droplets immediately after their formation, enabling to determine whether or not they are prone to re-coalescence. Observation of the droplets after emulsifier adsorption also allows the investigation of emulsion stability over time. Both evaluations would contribute to determine emulsion stability aiming at specific applications in food and pharmaceutical industries. Furthermore, emulsion coalescence can also be performed under extremely controlled conditions within the microfluidic devices in order to explore emulsion droplets as micro-reactors (for regulated biological and chemical assays). Such microfluidic procedures can be performed either in confined environments or under dynamic flow conditions. Under confined environments, droplets are observed in fixed positions simulating different environmental conditions. On the other hand, with the scrutiny of emulsions under dynamic flow processes, it is possible to determine the behavior of the droplets when subjected to shear forces, comparable to those experienced in conventional emulsification techniques or even in pumping operations. Given the above, this paper reviews different microfluidic techniques (such as changing channel geometry or wettability) hitherto used to destabilize emulsions, mainly focusing on the specificities of each study, whether the droplets are destabilized in confined or dynamic flow processes. Thereby, by going deeper into this review, readers will be able to identify different strategies for emulsion destabilization (in order to understand stabilizing mechanisms or even to apply these droplets as micro-reactors), as this paper shows the particularities of the most recent studies and elucidates the current state-of-the-art of this microfluidic-related application.

Emulsion characterization via microfluidic devices: A review on interfacial tension and stability to coalescence

Emulsions have gained significant importance in many industries including foods, pharmaceuticals, cosmetics, health care formulations, paints, polymer blends and oils. During emulsion generation, collisions can occur between newly-generated droplets, which may lead to coalescence between the droplets. The extent of coalescence is driven by the properties of the dispersed and continuous phases (e.g. density, viscosity, ion strength and pH), and system conditions (e.g. temperature, pressure or any external applied forces). In addition, the diffusion and adsorption behaviors of emulsifiers which govern the dynamic interfacial tension of the forming droplets, the surface potential, and the duration and frequency of the droplet collisions, contribute to the overall rate of coalescence. An understanding of these complex behaviors, particularly those of interfacial tension and droplet coalescence during emulsion generation, is critical for the design of an emulsion with desirable properties, and for the optimization of the processing conditions. However, in many cases, the time scales over which these phenomena occur are extremely short, typically a fraction of a second, which makes their accurate determination by conventional analytical methods extremely challenging. In the past few years, with advances in microfluidic technology, many attempts have demonstrated that microfluidic systems, characterized by micrometer-size channels, can be successfully employed to precisely characterize these properties of emulsions. In this review, current applications of microfluidic devices to determine the equilibrium and dynamic interfacial tension during droplet formation, and to investigate the coalescence stability of dispersed droplets applicable to the processing and storage of emulsions, are discussed.

Polymer based nanoparticles for biomedical applications by microfluidic techniques: from design to biological evaluation

The development of microfluidic technologies represents a new strategy to produce and test drug delivery systems.

Mimicking coalescence using a pressure-controlled dynamic thin film balance

The dynamics of thin films containing polymer solutions are studied with a pressure-controlled thin film balance. The setup allows the control of both the magnitude and the sign as well as the duration of the pressure drop across the film. The process of coalescence can be thus studied by mimicking the evolution of pressure during the approach and separation of two bubbles. The drainage dynamics, shape evolution and stability of the films were found to depend non-trivially on the magnitude and the duration of the applied pressure. Film dynamics during the application of the negative pressure step are controlled by an interplay between capillarity and hydrodynamics. A negative hydrodynamic pressure gradient promoted the thickening of the film, while the time-dependent deformation of the Plateau border surrounding it caused its local thinning. Distinct regimes in film break-up were thus observed depending on which of these two effects prevailed. Our study provides new insight into the behaviour of films during bubble separation, allows the determination of the optimum conditions for the occurrence of coalescence, and facilitates the improvement of population balance models.

Insights into the Microscale Coalescence Behavior of Surfactant-Stabilized Droplets Using a Microfluidic Hydrodynamic Trap

Coalescence of micrometer-scale droplets is impacted by several parameters, including droplet size, viscosities of the two phases, droplet velocity, angle of approach, as well as interfacial tension and surfactant coverage. The thinning dynamics of films between coalescing droplets can be particularly complex in the presence of surfactants, due to the generation of Marangoni stresses and reduced film mobility. Here, a microfluidic hydrodynamic "Stokes" trap is used to gently steer and trap surfactant-laden micrometer-sized droplets at the center of a cross-slot. Water droplets are formed upstream of the cross-slot using a microfluidic T-junction, in heavy and light mineral oils and stabilized using SPAN 80, an oil-soluble surfactant. Incoming droplets are made to coalesce with the trapped droplet, yielding measurements of the film drainage time. Film drainage times are measured as a function of continuous phase viscosity, incoming droplet speed, trapped droplet size, and surfactant concentrations above and below the critical micelle concentration (CMC). As expected, systems with higher surfactant concentrations and slower incoming droplet speed exhibit longer film drainage times. At low surfactant concentrations, the drainage time is longer for the more viscous heavy mineral oil in the continuous phase, whereas at high surfactant concentrations, the dependence on continuous phase viscosity vanishes. Perhaps more surprisingly, larger droplets and high confinement also result in longer film drainage times, potentially due to deformation of the droplet interfaces. The results are used here to determine critical conditions for coalescence, including both an upper and a lower critical capillary number. Moreover, it is shown that induced surfactant concentration gradient effects enable coalescence events after the droplets had originally flocculated, at surfactant concentrations above the CMC. The microfluidic hydrodynamic trap provides new insights into the role of surfactants in film drainage and opens avenues for controlled coalescence studies at micrometer length scales and millisecond time scales.

Control of Particle Adsorption for Stability of Pickering Emulsions in Microfluidics

Studying the stability of Pickering emulsion is of great interest for applications including catalysis, oil recovery, and cosmetics. Conventional methods emphasize the overall behavior of bulk emulsions and neglect the influence of particle adsorbing dynamics, leading to discrepancies in predicting the shelf-life of Pickering emulsion-based products. By employing a microfluidic method, the particle adsorption is controlled and the stability of the Pickering emulsions is consequently examined. This approach enables us to elucidate the relationship between the particle adsorption dynamics and the stability of Pickering emulsions on droplet-level quantitatively. Using oil/water emulsions stabilized by polystyrene nanoparticles as an example, the diffusion-limited particle adsorption is demonstrated and investigated the stability criteria with respect to particle size, particle concentration, surface chemistry, and ionic strength. This approach offers important insights for application involving Pickering emulsions and provides guidelines to formulate and quantify the Pickering emulsion-based products.

Interactions between micro-scale oil droplets in aqueous surfactant solution determined using optical tweezers

HYPOTHESIS

The stability of the emulsions is crucial, which relies on a well-developed understanding of dynamic interaction forces between single dispersed droplets. In the previous studies, many interests focus on the oil droplets of size range of 20-200 µm. However, emulsion droplets with diameter below 10 µm are rarely mentioned, which is the size scale of real emulsion droplets in various applications, such as toners, spacers for liquid crystal displays, and materials in biomedical and biochemical analysis. The micro-scale droplets have many differences on the deformation, internal pressure and hydrodynamic effects. It is necessary to understand the interaction mechanisms between two real size scales of oil droplets for guiding practical production and application.

EXPERIMENTS

In this work, tetradecane was chosen as the model oil phase in all experiments. The interaction forces of two tetradecane droplets with the diameter of 5.0 µm in water in the presence of surfactant and salt solution were directly measured using optical tweezers. The force-distance curves were established, and the zeta potential of tetradecane droplets was studied using Zetasizer Nano ZSP.

FINDINGS

The absolute value of zeta potential of tetradecane droplets was found to decrease with the increase of salt concentration and increase with the increase of surfactant concentration. The repulsive force between two tetradecane droplets was found to decrease with the increase of salt concentration because the electrostatic double-layer force was suppressed gradually with the increase of salt concentration. The "hydrodynamic suction" effect during the process of retraction becomes more pronounced due to the corresponding increase in the hydrodynamic force with the increase of the approaching velocity between the tetradecane droplets. Furthermore, we found the existing model for the measurement of large droplets by atomic force microscope (AFM) is invalid for the measurement of micro-scale droplets by optical tweezers. The deformation of colliding micro-scale droplets can be safely ignored, which is quite different from the large droplets. Our results provide a useful method to study the interaction forces between micro-scale emulsion droplets with pN force resolution, and gives a deep insight of the stabilization mechanism of real size scale of O/W emulsions. These findings have significant implications on the stability of emulsions in many food, cosmetics, medicine, and advanced materials.

Light and Magnetic Dual-Responsive Pickering Emulsion Micro-Reactors

Emulsion droplets can serve as ideal compartments for reactions. In fact, in many cases, the chemical reactions are supposed to be triggered at a desired position and time without change of the system environment. Here, we present a type of light and magnetic dual-responsive Pickering emulsion microreactor by coadsorption of light-sensitive titania (TiO₂) and super paramagnetic iron oxide (Fe₃O₄) nanoparticles at the oil-water interface of emulsion droplets. The droplets encapsulating different reactants in advance can be driven close to each other by an external magnetic field, and then the chemical reaction is triggered by UV illumination due to the contact of the isolated reactants as a result of droplet coalescence. An insight into the incorporation of hydrophobic TiO₂ and hydrophilic Fe₃O₄ nanoparticles simultaneously at the emulsion interface is achieved. On the basis of that, an account is given of the coalescence mechanism of the Pickering emulsion microreactors. Our work not only provides a novel Pickering emulsion microreactor platform for triggering chemical reactions in a nonintrusive and well-controlled way but also opens a promising avenue to construct multifunctional Pickering emulsions by assembly of versatile building block nanoparticles at the interface of emulsion droplets.

Amphiphilic nanoparticles suppress droplet break-up in a concentrated emulsion flowing through a narrow constriction

This paper describes the break-up behavior of a concentrated emulsion comprising drops stabilized by amphiphilic silica nanoparticles flowing in a tapered microchannel. Such geometry is often used in serial droplet interrogation and sorting processes in droplet microfluidics applications. When exposed to high viscous stresses, drops can undergo break-up and compromise their physical integrity. As these drops are used as micro-reactors, such compromise leads to a loss in the accuracy of droplet-based assays. Here, we show droplet break-up is suppressed by replacing the fluoro-surfactant similar to the one commonly used in current droplet microfluidics applications with amphiphilic nanoparticles as droplet stabilizer. We identify parameters that influence the break-up of these drops and demonstrate that break-up probability increases with increasing capillary number and confinement, decreasing nanoparticle size, and is insensitive to viscosity ratio within the range tested. Practically, our results reveal two key advantages of nanoparticles with direct applications to droplet microfluidics. First, replacing surfactants with nanoparticles suppresses break-up and increases the throughput of the serial interrogation process to 3 times higher than that in surfactant system under similar flow conditions. Second, the insensitivity of break-up to droplet viscosity makes it possible to process samples having different composition and viscosities without having to change the channel and droplet geometry in order to maintain the same degree of break-up and corresponding assay accuracy.

Liquid–liquid microflow reaction engineering

Engineering characteristics of liquid–liquid microflow and its advantages in chemical reactions.

Light-Triggered Release from Pickering Emulsions Stabilized by TiO2 Nanoparticles with Tailored Wettability

In this work, a new strategy for developing light-triggered Pickering emulsions as smart soft vehicles for on-demand release is proposed. Initially, UV-induced tailored wettability allows anchoring of TiO2 nanoparticles at the interface to prepare stable water in oil emulsions. Such emulsions show the efficacy of microencapsulation and controlled release by demulsification due to the hydrophilic conversion of the TiO2 nanoparticles using a noninvasive light irradiation trigger. A molecule of interest is selected as a model cargo to quantitatively evaluate the as-prepared Pickering emulsions for their encapsulation and release behaviors. Moreover, light-responsive emulsion destabilization mechanism is studied as a function of particle concentration, light wavelength, and light intensity, respectively, determined by drop diameter evolution and droplet coalescence kinetics plots. For consideration of application in life sciences, Pickering emulsions sensitive to visible light are also established based on nitrogen doping of TiO2 nanoparticle emulsifiers.

Numerical analysis of Pickering emulsion stability: insights from ABMD simulations

The issue of the stability of Pickering emulsions is tackled at a mesoscopic level using dissipative particle dynamics simulations within the Adiabatic Biased Molecular Dynamics framework. We consider the early stage of the coalescence process between two spherical water droplets in a decane solvent. The droplets are stabilized by Janus nanoparticles of different shapes (spherical and ellipsoidal) with different three-phase contact angles. Given a sufficiently dense layer of particles on the droplets, we show that the stabilization mechanism strongly depends on the collision speed. This is consistent with a coalescence mechanism governed by the rheology of the interfacial region. When the system is forced to coalesce sufficiently slowly, we investigate at a mesoscopic level how the ability of the nanoparticles to stabilize Pickering emulsions is discriminated by nanoparticle mobility and the associated caging effect. These properties are both related to the interparticle interaction and the hydrodynamic resistance in the liquid film between the approaching interfaces.

In situ dispersion of non-aqueous Fe3O4 nanocolloids by microdroplet coalescence and their use in the preparation of magnetic composite particles

Monodispersity and size uniformity are critical issues for nanoparticles, especially for the inorganic particles dispersed in organic carriers serving as the precursor of composites. Herein, for the first time, we have developed a method based on flow-induced droplet coalescence for in situ dispersion of surface-modified Fe3O4 nanoparticles to prepare Fe3O4/polystyrene (Fe3O4/PS) composite particles. A plate-type microchannel was constructed to initiate droplet coalescence for reducing the water-oil interfacial area and for dispersing Fe3O4 nanoparticles into the precursor suspensions. Under optimized conditions, the precursor suspensions could be composed of monodispersed Fe3O4 nanoparticles with an average size of approximately 12 nm. In this case, the saturation magnetization of the prepared superparamagnetic composites was as high as 4.012 emu g(-1) at a magnetite content of 5 wt%. The method is simple and has great potential to be tailored for the preparation of non-aqueous suspensions with uniform and monodispersed nanoparticles.