Passive Droplets Research Articles

Phase-separated droplets swim to their dissolution

Biological macromolecules can condense into liquid domains. In cells, these condensates form membraneless organelles that can organize chemical reactions. However, little is known about the physical consequences of chemical activity in and around condensates. Working with model bovine serum albumin (BSA) condensates, we show that droplets swim along chemical gradients. Active BSA droplets loaded with urease swim toward each other. Passive BSA droplets show diverse responses to externally applied gradients of the enzyme’s substrate and products. In all these cases, droplets swim toward solvent conditions that favor their dissolution. We call this behavior “dialytaxis”, and expect it to be generic, as conditions which favor dissolution typically reduce interfacial tension, whose gradients are well-known to drive droplet motion through the Marangoni effect. These results could potentially suggest alternative physical mechanisms for active transport in living cells, and may enable the design of fluid micro-robots.

Magnetic Nanomotors in Emulsions for Locomotion of Microdroplets.

The locomotion of droplets in emulsions is of practical significance for fields related to medicine and chemical engineering, which can be done with a magnetic field to move droplets containing magnetic materials. Here, we demonstrate a new method of droplet locomotion in the oil-in-water emulsion with the help of a nonuniform magnetic field in the case where magnetic nanoparticles (MNPs) are dispersed in the continuous phase of the emulsion. The paper analyses the motion of the droplets in a liquid film and in a capillary for various diameters of droplets, their number density, and viscosity of the continuous phase of the emulsion. It is established that the mechanism of droplet locomotion in the emulsion largely depends on the wettability of MNPs. Hydrophobic nanoparticles are adsorbed on the droplet surfaces, forming the agglomerates of MNPs with the droplets. Such agglomerates move at much higher velocities than passive droplets. Hydrophilic nanoparticles are not adsorbed at the surfaces of the droplets but form mobile magnetic clusters dispersed in the continuous phase of the emulsion. Mobile magnetic clusters set the surrounding liquid and droplets in motion. The results obtained in this paper can be used in drug delivery.

Superfast Active Droplets as Micromotors for Locomotion of Passive Droplets and Intensification of Mixing.

Micromotors are fascinating objects that are able to move autonomously and perform various complex tasks related to drug delivery, chemical processes, and environmental remediation. Among the types of micromotors, droplet-based micromotors are characterized by a wide range of functional properties related to the capability of encapsulation and deformation and the possibility of using them as microreactors. Relevant problems of micromotor utilization in the chemical processes include intensification of mixing and locomotion of passive objects. In this paper, the technique for preparation of superfast active droplets, which can be used as micromotors for effective locomotion of passive droplets in the oil-in-water emulsion, is demonstrated. The possibility of passive droplet locomotion in the emulsion is determined by a relation between the diameters of active and passive droplets. If the diameter of active droplets is larger than the diameter of passive droplets, the agglomerates form spontaneously in the emulsion and move in a straight line. In the case of the opposite relation between diameters, the agglomerates consisting of active and passive droplets rotate intensively. This makes it impossible to move the passive droplets to a given distance. Such micromotors can achieve unprecedentedly high velocities of motion and can be used to intensify mixing on the microscales.

Enzymatic control over coacervation.

The discovery of membraneless organelles (MLOs) formed by liquid-liquid phase separation raised many questions about the spatial organization of biomolecular processes in cells, but also offered a new tool to mimic cellular media. Since disordered and charged protein domains are often necessary for phase separation, coacervates can be used as models both to understand MLO regulation and to develop dynamic cellular-like compartments. A versatile way to turn passive coacervate droplets into active and dynamic compartments is by introducing enzymatic reactions that affect parameters relevant for complex coacervation, such as the charge and length of the components. However, these reactions strictly take place in a heterogeneous medium, and the complexity thereof is hardly addressed, making it difficult to achieve true control. In this chapter we help close this gap by describing two coacervate systems in which enzymatic reactions endow coacervate droplets with a dynamic character. We further highlight the technical challenges posed by the two-phase systems and strategies to overcome them.

Dynamically asymmetric and bicontinuous morphologies in active emulsions

The morphology of a mixture made of a polar active gel immersed in an isotropic passive fluid is studied numerically. Lattice Boltzmann method is adopted to solve the Navier–Stokes equation and coupled to a finite-difference scheme used to integrate the dynamic equations of the concentration and of the polarization of the active component. By varying the relative amounts of the mixture phases, different structures can be observed. In the contractile case, at moderate values of activity, elongated structures are formed when the active component is less abundant, while a dynamic emulsion of passive droplets in an active matrix is obtained for symmetric composition. When the active component is extensile, aster-like rotating droplets and a phase-separated pattern appear for asymmetric and symmetric mixtures, respectively. The relevance of space dimensions in the overall morphology is shown by studying the system in three dimensions in the case of extensile asymmetric mixtures where interconnected tube-like structures span the whole system.

Lamellar ordering, droplet formation and phase inversion in exotic active emulsions

We study numerically the behaviour of a two-dimensional mixture of a passive isotropic fluid and an active polar gel, in the presence of a surfactant favouring emulsification. Focussing on parameters for which the underlying free energy favours the lamellar phase in the passive limit, we show that the interplay between nonequilibrium and thermodynamic forces creates a range of multifarious exotic emulsions. When the active component is contractile (e.g., an actomyosin solution), moderate activity enhances the efficiency of lamellar ordering, whereas strong activity favours the creation of passive droplets within an active matrix. For extensile activity (occurring, e.g., in microtubule-motor suspensions), instead, we observe an emulsion of spontaneously rotating droplets of different size. By tuning the overall composition, we can create high internal phase emulsions, which undergo sudden phase inversion when activity is switched off. Therefore, we find that activity provides a single control parameter to design composite materials with a strikingly rich range of morphologies.

Surface Fluctuations and Coalescence of Nucleolar Droplets in the Human Cell Nucleus

The nucleolus is a membraneless organelle embedded in chromatin solution inside the cell nucleus. It is a site of ribosomal biogenesis, plays a key role in cell cycle progression and stress response, and acts as an indicator in many human diseases and aging. However, the physical nature of the human nucleolus remains an open question. Elucidating nucleolar dynamics, such as their shape fluctuations and fusion, could not only help answer this question, but also provide insight into the material properties of the surrounding nucleoplasm. In this work, we analyze the surface dynamics and fusion kinetics of nucleoli in live human cells[1]. Nucleolar surface fluctuations inform on the nucleolus-nucleoplasm interface, while nucleolar fusion might resolve whether nucleoli aggregate like solid particles or coalesce like liquid droplets. We find that the surface exhibits subtle, but measurable, fluctuations and that the neck connecting two fusing nucleoli grows as r(t)∼t1/2. This is consistent with liquid droplets with low surface tension ∼10−6 Nm−1 coalescing in an outer fluid of higher viscosity ∼103 Pa s, i.e. chromatin solution. We hypothesize that the high viscosity of chromatin solution slows down the nucleolar coalescence kinetics to facilitate transcription inside nucleoli. Interestingly, we find the neck velocity, dr/dt, to be comparable to the velocity of chromatin solution[2]. Surprisingly, nucleolar coalescence occurs in an active fluid, yet can be described by coalescence theory for passive liquid droplets, suggesting that the measured viscosity and surface tension might be effective quantities. Our study presents a noninvasive approach of using natural probes, such as nucleoli, and their dynamics to investigate material properties of the cell and its constituents[1]. 1. Caragine CM et al., Phys. Rev. Lett., In Press, 2018. 2. Zidovska A et al., Proc. Natl. Acad. Sci., 110(39), 15555-60, 2013.

Spontaneous motility of passive emulsion droplets in polar active gels.

We study by computer simulations the dynamics of a droplet of passive, isotropic fluid, embedded in a polar active gel. The latter represents a fluid of active force dipoles, which exert either contractile or extensile stresses on their surroundings, modelling for instance a suspension of cytoskeletal filaments and molecular motors. When the polarisation of the active gel is anchored normal to the droplet at its surface, the nematic elasticity of the active gel drives the formation of a hedgehog defect; this defect then drives an active flow which propels the droplet forward. In an extensile gel, motility can occur even with tangential anchoring, which is compatible with a defect-free polarisation pattern. In this case, upon increasing activity the droplet first rotates uniformly, and then undergoes a discontinuous nonequilibrium transition into a translationally motile state, powered by bending deformations in the surrounding active medium.

Generation and manipulation of “smart” droplets

We report the generation and manipulation of electrorheological (ER) droplets that exhibit the giant ER effect. The experiments were carried out on specially designed microfluidic chips, in which the ER droplets were generated by using the microfluidic flow-focusing approach. Both the size and formation rate of these droplets can be controlled through digitally applied electrical signals. The principle of droplet manipulation is based on the electrical responsiveness of ER droplets and hence the denotation of “smart” when the electrical signals can be triggered by sensing/control devices. Due to the unique characteristics of the GER effect, the smart droplets can deform and even stop the microfluidic channel flow under an applied electric field. The pressure difference induced by the smart droplets inside the micro-channel is controllable by varying the field strengths, droplet sizes and particle concentrations in the GER suspension. By trapping and timed release of smart droplets in different micro-branch channels, we demonstrate that the smart droplets generated upstream cannot only be stored or displayed in the desired downstream channel(s) and thereby offer the potential of micro-droplet display, but also be useful in counting, flow directing and sorting the desired number of passive droplets sandwiched between two smart droplets. Such capabilities of smart droplets will enable the programmable control of discrete processes in bio-analysis, chemical reactions, digital microfluidics, and digital droplet display.

Transport of Droplets Expelled by Coughing in Ventilated Rooms

This study aims to understand the transport and dispersal of droplets produced by coughing in a ventilated room. Experiments were conducted to measure the initial velocity and the duration of a coughing burst. Computational fluid dynamics with a Eulerian—Lagrangian model was used to investigate the transport characteristics of evaporating droplets expelled into the ventilated room. The simulation results indicate that if expelled horizontally the droplets finally settle if their initial size is >100 μm, while if expelled upwards the droplets only settle if their initial size is ≥300 μm. Different ventilation set-ups vary in their ability to remove droplets produced by coughing. The mixing ventilation system has an almost equal efficiency for removing small passive droplets and the nuclei of droplets whose original size was 100 and 80 μm, respectively. For displacement ventilation, although it has high efficiency in removing small passive droplets, it has difficulty in removing the nuclei of large droplets. In the displacement system, small droplets show a two-zonal distribution in the room, but the nuclei of large droplets are subjected to gravitational force and tend to settle in the lower part of the room.

Stretch and Shape Distributions of Droplets with Interfacial Tension in Chaotic Mixing

Abstract A numerical simulation is developed to study the time-dependent shapes of droplets in chaotic mixing, as a function of interfacial tension and droplet-to-matrix viscosity ratio. The two-dimensional, time-periodic Newtonian flow between eccentric cylinders is used as a prototype mixing flow. The microstructure is modeled as three-dimensional ellipsoidal droplets, ignoring breakup and coalescence. A Lagrangian particle method is used to follow the microstructure. When interfacial tension is small (global capillary number is large), the major axes of the droplets exhibit the same stretching statistics as passive fluid elements and droplets with zero interfacial tension in chaotic flows: the geometric average of the stretch ratio grows exponentially with time, at a rate equal to the Lyapunov exponent of the flow, while the log of the major-axis stretch of the droplets, when scaled by its instantaneous mean and standard deviation, has a time-invariant, Gaussian global distribution and a non-uniform, fractal, and time-invariant spatial distribution. In this regime the stretch of the longest droplet axis is insensitive to interfacial tension, but the shape of the cross section is very sensitive: initially spherical droplets deform first into ribbons or sheets, but eventually transform into axisymmetric threads. The larger the global capillary number, the longer the sheet morphology persists, but sheet-like structures are always transient, and the sheets relax to threads if mixing goes on too long.