Janus Drops Research Articles

Two-dimensional Janus drops in shear: deformation, rotation and their coupling

In this work, the dynamics of two-dimensional rotating Janus drops in shear flow is studied numerically using a ternary-fluid diffuse interface method. The rotation of Janus drops is found to be closely related to their deformation. A new deformation parameter $D$ is proposed to assess the significance of the drop deformation. According to the maximum value of $D$ ( $D_{max}$ ), the deformation of rotating Janus drops can be classified into linear deformation ( $D_{max}\le 0.2$ ) and nonlinear deformation ( $D_{max}> 0.2$ ). In particular, $D_{max}$ in the former depends linearly on the Reynolds and capillary numbers, which can be interpreted by a mass–spring model. Furthermore, the rotation period $t_R$ of a Janus drop is found to be more sensitive to the drop deformation than to the aspect ratio of the drop at equilibrium. By introducing a corrected shear rate and an aspect ratio of drop deformation, a rotation model for Janus drops is established based on Jeffery's theory for rigid particles, and it agrees well with our numerical results.

Numerical simulation of multi-functional droplet production in microfluidics

In this work, we have investigated numerically the formation of stable Janus droplets in microfluidics flow focusing device. We have examined the effect of continuous phase capillary number (Ca) as it influences the drop size and the stability of a Janus drop for a fixed dispersed phase flow rate. The effect of flow rate ratio of disperse phases has also been examined to understand the relative distribution of the material in a single Janus droplet. Numerical simulations have been performed using the commercial software Ansys Fluent 19.2 with the Volume of Fluid (VOF) method to solve discrete versions of governing equations. We have extended our method to study the formation and stability of a ternary droplet. For this, a third inlet is incorporated at the middle of the initial channel geometry and fluid combination. In the case of ternary drop, we find that different phases re-arrange themselves after drop formation before reaching the final stable configuration.

Bouncing characteristics of Janus drop impact on curved surfaces

Manipulations of “complex” drops and their interactions with “complex” surfaces have attracted much attention in recent years. Here, we hypothesize that Janus drop impact on curved superhydrophobic surfaces can be highly affected by the curvature, Weber number (We), and viscosity difference between Janus components. The main objective is to predict the splitting of the low-viscosity component from the Janus drop. The study forms regime maps for the transition of non-splitting/splitting as a function of the parameters. It is shown that the We threshold, above which splitting occurs, can be tuned by altering the surface curvature and viscosity difference between low- and high-viscosity components. Asymmetric behavior observed in the two components is elucidated by establishing the relationship of the residence time and characterizing the dynamics by means of the horizontal momenta. The underlying mechanism for bouncing on asymmetric structures can provide a meaningful guidance on the designs of liquid purification or multi-material printing.

Splitting behavior of Janus drop impact on protrusion structure

Handling tiny liquid volumes as drops is important for applications, including biochip or liquid spraying systems. Splitting of a compound drop is a challenging task in the industrial fields, but the underlying mechanism is not clearly revealed. Here, we demonstrate the dynamic characteristics of the bounce and separation of Janus drops on protrusions by using a numerical method. A regime map for the separation rate between low- and high-viscosity components is investigated for various viscosity ratios and Weber numbers, which is discussed in terms of the formation of a ring and the reduction in residence time. We predict off-centered drop impact on the protrusion structure under various offset distances and characterize the impact behavior into the ring and bifurcation bouncing. We investigate how the separation rate is affected by the protrusion's height and width. The rim dynamics occurring when a liquid film is punctured are discussed by quantifying the evolutions of the shapes and axial momenta of the drops. We expect that the splitting of the compound drop on a defect can create possibilities for the efficient control of drop manipulation and fluid purification.

Dynamic characteristics of ellipsoidal Janus drop impact on a solid surface

Impinging Janus drops can be stably produced by adding a high-viscosity drop to a low-viscosity drop. Here, we investigate the dynamic features of bouncing Janus drops on a solid substrate for an exploration of the effects of the viscosity ratio, initial drop shape, and impact velocity on altering the hydrodynamics. Numerical results show that the low-viscosity component evolves into liquid alignment along the principal direction with the help of a preferential flow, thereby resulting in the partial detachment from the mother Janus drops. We establish a regime map of the separation ratio of the drop and discuss how the parameters affect the asymmetry in the bounce and separation behavior. The low-viscosity components can be more likely to be detached from Janus drops as the viscosity ratio, drop's ellipticity, and/or impact velocity increase. This phenomenon is explained by the residence time and breakup of symmetry in the horizontal momentum between the low- and high-viscosity components. The peculiar dynamic characteristics of the Janus drop can provide potential for various applications, such as liquid purification and separation.

Characterizing the Bounce and Separation Dynamics of Janus Drop on Macrotextured Surface.

Janus drops are thermodynamically stable when a high-viscosity fluid is imposed on a low-viscosity fluid. To understand physical mechanisms in Janus drop impact on macrotextured surfaces, several challenges in finding parameters or strategies still remain. Here, this study investigates the asymmetric bounce and separation of impinging Janus drops on non-wettable surfaces decorated with a macroridge to explore the effect of the drop size, viscosity ratio, and ridge size on the dynamics. Through numerical simulations, we determine the threshold Weber number, above which separation occurs, by varying drop diameters and viscosity ratios of the Janus drops. We investigate the initial bouncing directions of separated drops as a function of the impact velocity and viscosity ratio. We also predict how the separation efficiency is affected by the ridge’s height and width. The asymmetric impact dynamics of Janus drops on macrotextured surfaces can provide new strategies to control drop bouncing in applications, such as liquid separation and purification.

The role of viscosity ratio in Janus drop impact on macro-ridge structure

An interaction of liquid and solid surfaces upon impact has made great progress in understanding the principle behind impinging compound drops, such as single-interface Janus and core–shell configurations, for controlling drop mobility on the surfaces. Despite advancement of recent technologies, fundamentals of how viscosity ratios of Janus drops affect post-impact dynamics on anisotropic surfaces are still unknown. Here, we numerically investigate the asymmetric impact dynamics of Janus drops on a non-wettable ridged surface to demonstrate the feasibility of the separation of the low-viscosity part from the high-viscosity part by reducing the residence time. The separation is investigated for various viscosity ratios, Weber numbers (We), and initial angle, which are discussed in terms of the temporal evolution of the mass and momentum distributions. A regime map for the separation reveals that the low-viscosity parts are more likely to be separated from high-viscosity parts as the viscosity ratio increases. The phenomenon can be related to a retraction time, which is explained by a hydrodynamic model for the low-viscosity part. This study suggests that We thresholds for the separation can be significantly reduced with the help of center-assisted retraction along the ridge. The asymmetric bouncing of Janus drops on a ridged surface can open up possibilities for the efficient control of liquid separation.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Focal conic flowers, dislocation rings, and undulation textures in smectic liquid crystal Janus droplets.

Liquid crystalline phases of matter often exhibit visually stunning patterns or textures. Mostly, these liquid crystal (LC) configurations are uniquely determined by bulk LC elasticity, surface anchoring conditions, and confinement geometry. Here, we experimentally explore defect textures of the smectic LC phase in unique confining geometries with variable curvature. We show that a complex range of director configurations can arise from a single system, depending on sample processing procedures. Specifically, we report on LC textures in Janus drops comprised of silicone oil and 8CB in its smectic-A LC phase. The Janus droplets were made in aqueous suspension using solvent-induced phase separation. After drop creation, smectic layers form in the LC compartment, but their self-assembly is frustrated by the need to accommodate both the bowl-shaped cavity geometry and homeotropic (perpendicular) anchoring conditions at boundaries. A variety of stable and metastable smectic textures arise, including focal conic domains, dislocation rings, and undulations. We experimentally characterize their stabilities and follow their spatiotemporal evolution. Overall, a range of fabrication kinetics produce very different intermediate and final states. The observations elucidate assembly mechanisms and suggest new routes for fabrication of complex soft material structures in Janus drops and other confinement geometries.

Prompting Splash Impact on Superamphiphobic Surfaces by Imposing a Viscous Part.

It is widely acknowledged that splash impact can be suppressed by increasing the viscosity of the impinging drop. In this work, however, by imposing a highly viscous drop to a low‐viscosity drop, it is demonstrated that the splash of the low‐viscosity part of this Janus drop on superamphiphobic surfaces can be significantly promoted. The underlying mechanism is that the viscous stress exerted by the low‐viscosity component drives the viscous component moving in the opposite direction, enhancing the spreading of the low‐viscosity side and thereby its rim instability. The threshold velocity, above which splashing occurs, can be tuned by varying the viscosity ratio of the Janus drop. Moreover, the impact of the Janus drop can be employed to verify the mechanism of splash.

Microfluidic Production of Multiple Emulsions

Microfluidic devices are promising tools for the production of monodispersed tuneable complex emulsions. This review highlights the advantages of microfluidics for the fabrication of emulsions and presents an overview of the microfluidic emulsification methods including two-step and single-step methods for the fabrication of high-order multiple emulsions (double, triple, quadruple and quintuple) and emulsions with multiple and/or multi-distinct inner cores. The microfluidic methods for the formation of multiple emulsion drops with ultra-thin middle phase, multi-compartment jets, and Janus and ternary drops composed of two or three distinct surface regions are also presented. Different configurations of microfluidic drop makers are covered, such as co-flow, T-junctions and flow focusing (both planar and three-dimensional (3D)). Furthermore, surface modifications of microfluidic channels and different modes of droplet generation are summarized. Non-confined microfluidic geometries used for buoyancy-driven drop generation and membrane integrated microfluidics are also discussed. The review includes parallelization and drop splitting strategies for scaling up microfluidic emulsification. The productivity of a single drop maker is typically <1 mL/h; thus, more than 1000 drop makers are needed to achieve commercially relevant droplet throughputs of >1 L/h, which requires combining drop makers into two-dimensional (2D) and 3D assemblies fed from a single set of inlet ports through a network of distribution and collection channels.

Coalescence of Janus droplets prepared by one-step vibrational mixing

Janus emulsions are prepared by one-step vibrational mixing of two immiscible oils, silicone oil, SO, with density of 0.964 g/cm3 and sunflower oil saturated with dimethyl phthalate, VO (DMP), with density of 0.991 g/cm3, and an aqueous solution of sodium dodecyl sulfate as continuous phase. The destabilization of Janus emulsion is followed by the change of layer dimension as well as the microscope image, illustrating the two main aspects of creaming and coalescence. The fundamental impasse of the final separation of the Janus drop oils against unfavorable free energy change is resolved by a numerical analysis on a model emulsion with realistic interfacial tension values to show a thermodynamically favored path to the final destabilization process.

Interfacial Tension; a Stabilizing Factor for Janus Emulsions of Silicone Bixa Orellana Oils

AbstractThe destabilization process was investigated for a Janus emulsion of silicone and Bixa Orellana oils stabilized by polyoxyethylene sorbitan monooleate (Tw 80) and carboxymethyl cellulose. The emulsion stabilized with Tw 80 showed significant and fast creaming, a process that was prevented by the addition of the polymer. During the extensive coalescence of the emulsions stabilized by Tw 80, the Janus topology was retained for months of storage until, finally, separation of the oils occurred. This result strongly indicates an unexpected stabilizing action of the i nterfacial free energy. This conclusion was supported by a calculation for a realistic model system of the interfacial energy difference between two cases of coalescence. In the first case, the two coalescing Janus drops united into a larger Janus drop, while in the second case two drops formed, each with only one oil. The first case gave a spontaneous reaction (reduced interfacial energy), while the second one meant an increase of energy, i.e. it cannot happen without adding energy. The authors are aware that this stabilization is a new phenomenon in emulsion science with potential ramifications in future emulsion technology. However, it is essential to realize that the stabilization is of temporary occurrence in the destabilization process, and the free energy to give a final emulsion state with separated oils is overwhelmingly dominant. In short, Janus emulsions will, in the end, separate into layers of the liquids, like all emulsions.

Dynamics and rheology of Janus drops in a steady shear flow

The behavior and rheology of a dispersion of Janus drops (or Janus emulsion) under a steady shear flow are explored in the infinite dilution limit. To achieve analytical progress, the Janus drops are assumed to consist of a pair of fluids bounded to hemispherical domains of equal radii. At ‘freely’ suspended conditions the Janus drops undergo periodic orbits in a shear flow that are intermediate to that of a solid sphere and a disk that depend on the viscosities of the internal fluids. Non-Newtonian behavior is found for this system on account of the anisotropic hydrodynamics of the Janus drops. The viscosity of the Janus emulsion that corresponds to the minimum energy of dissipation is analogous to that derived by Taylor (1932) for a dispersion of simple drops. It is also found that an external force can induce the Janus drops to adopt a preferential orientation in a shear flow. Interestingly, a neutrally buoyant Janus drop with a displaced center of gravity can migrate lateral to the undisturbed shear flow; it is inferred that this phenomenon can lead to spatial-dependent rheology in pressure-driven flows.

An unexpected stabilization factor during destabilization of a Janus emulsion

An aqueous Janus emulsion of a vegetable/silicone oil combination, stabilized by Tween 80, showed an unexpected stability of the Janus topology per se during several weeks of creaming. This result indicated thermodynamics as a stabilizing factor and the free interfacial energy was calculated for a model Janus drop. The results illustrated the expected fact that the coalescence of two Janus drops to form a larger drop of the same configuration is thermodynamically favored. Conversely, the process, forming two single oil drops actually would necessitate an increase of the free energy. The free energy difference between the two cases was several orders of magnitude greater than that for kT, illustrating the importance of thermodynamics for the process.

On the anisotropic response of a Janus drop in a shearing viscous fluid

The force and couple that result from the shearing motion of a viscous, unbounded fluid on a Janus drop are the subjects of this investigation. A pair of immiscible, viscous fluids comprise the Janus drop and render it with a ‘perfect’ shape: spherical with a flat, internal interface, in which each constituent fluid is bounded by a hemispherical domain of equal radius. The effect of the arrangement of the internal interface (drop orientation) relative to the unidirectional shear flow is explored within the Stokes regime. Projection of the external flow into a reference frame centred on the drop simplifies the analysis to three cases: (i) a shear flow with a velocity gradient parallel to the internal interface, (ii) a hyperbolic flow, and (iii) two shear flows with a velocity gradient normal to the internal interface. Depending on the viscosity of the internal fluids, the Janus drop behaves as a simple fluid drop or as a solid body with broken fore and aft symmetry. The resultant couple arises from both the straining and swirling motions of the external flow in analogy with bodies of revolution. Owing to the anisotropic resistance of the Janus drop, it is inferred that the drop can migrate lateral to the streamlines of the undisturbed shear flow. The grand resistance matrix and Bretherton constant are reported for a Janus drop with similar internal viscosities.

Equilibrium Janus Drop Topology. 2. A Single Drop in a Spherical Continuous Phase

The equilibrium topology is analyzed of a single Janus drops of oils O1 and O2 in a spherical continuous aqueous phase, W, co-centered with O1. The radius of the O1 oil cap is equal to unity, while that of the oil O2 is varied and the cap centered individually. When the space of O2 exceeds the limit of W, the excess volume is discarded and the border surface of W is considered without interfacial energy. Earlier findings, when increasing the O2 on O1 coverage on a single Janus drop in an infinite aqueous phase, brought to light a selective O2 inversion from an emulsion (O1 + O2)/W to (O1 + W)/O2 at a well-defined surface coverage of O1 by O2. This specific inversion did not take place with the Janus drop in an aqueous phase of limited dimensions. Instead the inversion was perceived to take place, when the volume fraction of O2, VO2 = (VO1 + Vw + VO2) exceeded 0.5. The information from the earlier publication was used to calculate the variations of volumes VO2 and VW with the radius of the W sphere. The results partially contradicted earlier results.

Silicone/vegetable oil Janus emulsion: Topological stability versus interfacial tensions and relative oil volumes

Several aspects were studied of the formation and destabilization in bulk of silicone/vegetable oil, SO/VO, Janus emulsions, stabilized by Tween 80. In the formation of the emulsions, it was unexpectedly found that the dispersions tended to contain both single and flocculated drops irrespective of the emulsification intensity. Microscopy of the emulsions with no cover glass revealed flocculated drops of a large (200–500μm) central SO drop with many small VO drops attached. Applying a cover glass did not significantly change the drop size; instead two-oil Janus drops of well-defined contact angle were found. The emulsions showed rapid creaming irrespective of the preparation method, but a few days storage did not significantly change the drop size in the creamed layer, nor was separation of the oils detected. The total interfacial free energy of the Janus drops at equilibrium was compared to the two relevant alternatives; engulfed and separate drops. The Janus drop free energies were found less for all volume ratios of the oils, when the surfactant concentrations in the aqueous phase was sufficient to prevent spreading of VO on SO. Changing the surfactant concentration to bring the interfacial tensions closer to the critical value for spreading gave declining interfacial free energy difference to that of engulfed drops.

Destabilization of a dual emulsion to form a Janus emulsion

A vegetable oil (VO) was added to an emulsion of silicone oil in water (SO/W) with mixing limited to once turning the test tube upside down. Initially, the VO was dispersed into virtually centimeter-sized drops and the emulsion contained effectively no Janus drops, while after 1 h of agitation at a low level to prevent creaming, drops of 50–100-μm size of the two oils were observed: in addition to an insignificant number of Janus drops. The topology of the latter showed them to emanate from flocculated individual drops of the two oils, but with no discernible effect by the interfacial tension equilibrium on the drop topology. Continued gentle mixing gave increasing fraction of Janus drops of increased size with a topology gradually approaching the one expected from the interfacial equilibrium at the contact line. The spontaneous formation of Janus drops indicated a reduction of the interfacial free energy in the process and the interfacial energy difference between separate and Janus drops was calculated for an appropriate range of interfacial tensions and for all oil fractions. The calculations enabled a distinction of the decrease due to interfacial area changes from the reduction of interfacial tensions per se, with the latter only a minor fraction.

Janus Emulsions from a One-Step Process; Optical Microscopy Images

The optical microscopy images of an emulsion are commonly distorted when viewed between a cover glass and a planar microscopy slide. An alternative method is to place the sample on a slide with a cavity, which in turn suffers from incomplete information for high internal phase ratio (HIPR) emulsions, due to the inevitable crowding of the drops. This problem is particularly acute for more complex emulsions, such as those with Janus drops, for which a detailed image of the drop is essential. A number of publications have recently described Janus emulsions prepared by a one-step high energy emulsification process with microscopy images obtained by the sample between a planar slide and a cover glass. The correlation to the morphology of emulsions in bulk of these images is critical, but, so far, a potential equivalence has not been established. Since the images are central in order to understand why Janus emulsions should form under such conditions, the need to ascertain any such association is urgent. With this contribution, we compare images from different microscopy methods to those of gently diluted HIPR emulsions. The results reveal that the images of the emulsion samples between a cover glass and a planar microscope slide actually present a realistic representation of the drop topology in bulk emulsions.