Neighboring Droplets Research Articles

Hydrodynamics of Two Interacting Liquid Droplets of Aqueous Solution inside a Microchannel.

We experimentally investigated the effect of a neighboring liquid droplet on fluid convection inside a liquid droplet of aqueous solution present inside a microchannel using the microscale particle image velocimetry technique. There is no physical contact between the two droplets, and the solute concentrations of the two droplets are set at different values. Vapor concentration near the interface of the two droplets is different due to the difference in solute concentration. Water vapor evaporates from the low-concentration droplet having higher vapor pressure and condenses on the high-concentration droplet having lower vapor pressure. Evaporation and condensation induce Rayleigh convection inside the two droplets. Flow pattern shows circulating loops inside both liquid droplets. The circulations at the interacting adjacent interface of the two droplets are opposite to each other. The strength of flow inside the liquid droplets decreases with time due to decrease in the difference of solute concentration between the two droplets. The flow strength inside the two interacting droplets is also a function of separation distance between the droplets. The flow strength inside the droplets decreases with increase in separation distance.

Coupling Diffusion Welding Technique and Mesh Screen Creates Heterogeneous Metal Surface for Droplets Array

AbstractNonsilicon micromachining is a challenge, especially for large scale heterogeneous (hydrophilic/hydrophobic) surface fabrication. Here, mesh screen and diffusion welding technique are coupled to form a novel fabrication method. Mesh screen is used as mask. The diffusion welding technique sinters mesh screen on copper surface to form welding junction array. Chemical treatment of sintered package forms superhydrophobic nanostructures on surfaces except welding junction array. Separating mesh screen from copper substrate exposes hydrophilic dots array, corresponding to welding junction array. Thus, heterogeneous surface is created. By condensing wet air, main droplet and neighboring droplet occur on hydrophilic dot and superhydrophobic part, respectively. For horizontal surface, neighboring droplets initially generated on nanograsses behave consecutive events of coalescence, jump, and return, triggering contact/noncontact effect induced jumping. Thus, defect droplet area is formed and droplet size uniformity is broken. A nondimensional equation is proposed for the contact effect induced jumping analysis. Noncontact effect induced jumping is found for the first time. For vertical surface, droplet array behaves monodisperse size due to departure of merged neighboring droplets without returning, and equal opportunities of neighboring droplets captured by main droplets. The work opens a new way for large scale droplet array generation by controllable condensation.

Deposition pattern of interacting droplets

A colloid droplet having pinned contact line forms a ring pattern after drying. The ring pattern of a single droplet shows uniform deposit along the contact line. When two droplets dry adjacent to each other, the deposition pattern is influenced by the presence of the neighboring droplet. The deposition pattern at the nearest region of the two droplets shows weak deposit compared to the rest of the region. The drying pattern in two-droplets configuration is related to the convection phenomena inside the drying droplet and the influence of the neighboring droplet. We have experimentally studied the effect of a neighboring droplet on the convection pattern inside the evaporating droplet using μ-PIV technique to explain the influence of transport phenomena on the drying pattern. COMSOL simulation has also been carried out for understanding the evaporation process. The evaporation rate from the nearest region of the two droplets is lower compared to the rest of the region. Therefore, less number of particles are transported to this region due to low fluid flow causing less deposits in this region. The flow field inside a droplet in the presence of a neighboring droplet shows asymmetric convection pattern unlike single droplet where flow field is symmetric. This convection behavior correlates well with the evaporation flux distribution on the droplet surface and drying pattern.

Programmable digital droplet microfluidics using a multibarrel capillary bundle

The integration of multibarrel glass capillaries into a microfluidic co-flow configuration is demonstrated for parallel production of nanoliter scale droplets. The multibarrel system supports the generation of a continuous stream of single-phase or dual-phase droplets with digital control over droplet content, enabling the formation of dynamically-programmable and tunable emulsions. The multibarrel capillary method allows simultaneous generation of multiple droplets from parallel droplet generators, with controlled switching between selected dispersed phase or continuous phase compositions. Individual phases emerging from each capillary outlet may be selected to yield merged droplets combining continuous phase emitted from one or more neighboring droplet generators. Because the capillary emitters are configured in a two dimensional array, the multibarrel system provides greater flexibility toward producing controlled interactions between multiple solutions and phases than conventional planar microfluidic droplet generators. Using a 7-barrel capillary configuration, the ability to regulate droplet composition and particle size of individual as well as combined single and merged water-in-oil droplets is explored, together with the use of two-dimensional spatial control within the multibarrel array to generate higher order emulsions.

Deformation and Approaching of Droplet Pair in a Microchannel under AC Electric Fields

Electric fields are widely used to induce the coalescence of water-in-oil emulsion droplets in droplet-based microfluidics. The dipole-dipole interaction amplifies the dielectrophorectic force experienced by two neighboring droplets to drive them toward each other. An ac electric field can act in a cumulative manner to merge the adjacent interfaces of the two droplets, even if the electric field is weak. Here we perform a combined numerical and theoretical analysis to explore the coupled flow and electric field of droplet pair in a rectangular microchannel under ac electric fields. Numerical simulations are carried out by solving of the conservation equations coupled with an improved volume-of-fluid/adaptive-mesh-refinement method and a charge-conservative leaky-dielectric model for two-phase electrohydrodynamic problems. To improve the fidelity and efficiency of the simulations, several adaptive mesh refinement criteria are used simultaneously, including gradient-based refinement along the interface, curvature-based refinement for interface with high curvature, and thickness-based refinement for thin region between confinement walls and interfaces. The evolutions of deformation parameter and droplet separation, as well as the distribution of the electric force are analyzed to reveal the responses of droplet pair to the ac field. A theoretical model with correlation parameter is established for the time needed for droplet coalescence in terms of initial droplet separation and ac field intensity. By taking account into the deformation and movement of the droplets, this model can predict more precisely the droplet manipulation for arbitrary electric field frequencies.

Backscattering measurement from a single microdroplet

Backscattering measurements for acoustically trapped lipid droplets were undertaken by employing a P[VDF-TrFE] broadband transducer of f-number = 1, with a bandwidth of 112%. The wide bandwidth allowed the transmission of the 45 MHz trapping signal and the 15 MHz sensing signal using the same transducer. Tone bursts at 45 MHz were first transmitted by the transducer to hold a single droplet at the focus (or the center of the trap) and separate it from its neighboring droplets by translating the transducer perpendicularly to the beam axis. Subsequently, 15 MHz probing pulses were sent to the trapped droplet and the backscattered RF echo signal received by the same transducer. The measured beam width at 15 MHz was measured to be 120 μ m. The integrated backscatter (IB) coefficient of an individual droplet was determined within the 6-dB bandwidth of the transmit pulse by normalizing the power spectrum of the RF signal to the reference spectrum obtained from a flat reflector. The mean IB coefficient for droplets with a 64 μ m average diameter (denoted as cluster A) was -107 dB, whereas it was -93 dB for 90-μm droplets (cluster B). The standard deviation was 0.9 dB for each cluster. The experimental values were then compared with those computed with the T-matrix method and a good agreement was found: the difference was as small as 1 dB for both clusters. These results suggest that this approach might be useful as a means for measuring ultrasonic backscattering from a single microparticle, and illustrate the potential of acoustic sensing for cell sorting.

Rapid Ejection of Giant Pluronic L121 Vesicles from Spreading Double Emulsion Droplets

We report the formation of giant Pluronic L121 vesicles with diameters of the order of 200 microm obtained by a previously unreported mechanism that occurs during the solvent evaporation method of vesicle formation. We begin with a water-oil-water double emulsion that is stabilized by dissolving the commercially available triblock copolymer Pluronic L121 in the volatile oil-phase. During the evaporation, the oil phase spreads on the surface of the continuous aqueous phase, leaving behind the aqueous inner droplet of the double emulsion droplet that eventually yields the vesicle. The spreading of the solvent mixture of the oil phase is induced either by the rapid ejection of the inner droplet out of the double emulsion droplet, or by the spreading of the oil phase of a neighboring emulsion droplet.

Nanodroplet real-time PCR system with laser assisted heating

We report the successful application of low-power (approximately 30 mW) laser radiation as an optical heating source for high-speed real-time polymerase chain reaction (PCR) amplification of DNA in nanoliter droplets dispersed in an oil phase. Light provides the heating, temperature measurement, and Taqman real-time readout in nanoliter droplets on a disposable plastic substrate. A selective heating scheme using an infrared laser appears ideal for driving PCR because it heats only the droplet, not the oil or plastic substrate, providing fast heating and completing the 40 cycles of PCR in 370 seconds. No microheaters or microfluidic circuitry were deposited on the substrate, and PCR was performed in one droplet without affecting neighboring droplets. The assay performance was quantitative and its amplification efficiency was comparable to that of a commercial instrument.

Stability of Acidic Egg White Protein Emulsions Containing Xanthan Gum

The influence of xanthan gum concentration on the physicochemical stability of model oil-in-water emulsions prepared with egg white protein at pH 3.8 and containing 150 mM NaCl was investigated by following droplet aggregate formation, rheological changes, and serum separation with storage time. Egg white emulsions were more strongly flocculated and exhibited higher stability against creaming than those of yolk, irrespective of the presence or absence of xanthan. Depletion effects, originating from the presence in the continuous phase of the emulsions of nonadsorbing xanthan molecules, intensified droplet-droplet flocculation effects and resulted in large droplet flocs. At relatively low xanthan contents, the emulsions exhibited higher stability against creaming compared to the respective control emulsions probably due to the formation of a continuous droplet aggregate network structure. At higher xanthan contents, less extensive droplet interactions, due to slowly evolving microstructure of phase-separated xanthan-rich and xanthan-depleted regions, resulted in emulsions exhibiting increased stability against creaming. The role of interactions between protein molecules adsorbed on neighboring droplets in these changes and their effect on emulsion aging are discussed.

Statistico-probabilistic approach to taking account of the vapor depletion in the kinetics of homogeneous nucleation: A free-molecular regime of droplet growth

We propose a statistico-probabilistic approach to investigate the process of homogeneous formation of droplets in a vapor phase in the presence of an already formed and growing droplet under free-molecular regime of droplet growth after the instantaneous creation of initial vapor supersaturation. We find the probability density for the formation of a new, nearest (neighbor) droplet in the vicinity of an initially formed droplet. The mean distance between two neighboring droplets is also determined, as well as the average time lag for the formation of the nearest (neighbor) droplet; the latter quantity serves as an estimate for the duration of the nucleation stage. An estimate for the average number of droplets forming in unit volume by the end of the nucleation stage is also given. Our results are compared with the predictions of classical nucleation theory which assumes the density uniformity of a metastable phase. Where the proposed approach is applicable, there is observed qualitative agreement between the results. The underlying cause of this agreement is analyzed and the limits of applicability of the uniformity approximation are clarified.

Flame propagation of n-decane spray in microgravity

Flame propagation experiments on n-decane spray were performed in microgravity to investigate theflame propagation mechanism of a spray for less volatile fuels. After the spray was dispersed into an acrylic propagation tube with an inner diameter of 62 mm and a length of 535 mm, about 5 s were required for a quiescent spray to form under conditions of microgravity. The spray was ignited by an electrically heated nichrome wire just after the Sauter mean diameter (SMD) and the concentration of spray were measured by a laser droplet analyzer. The variation of flame propagation speed with the SMD was determined. Results showed that the change of the flame propagation speed with the SMD had a maximum for a constant equivalence ratio. This can be reasonably explained by analogy with the flame spread from a droplet to a neighboring droplet in a droplet array, that is, the flame spread of a droplet array and the flame propagation speed of spray reach a maximum when the spacing between a droplet and a neighboring droplet is approximately equal to the flame radius of the droplet. Therefore, all experimental data could be arranged on one curve when the lateral axis was taken as the spacing between droplets divided by the flame diameter. Even when methane of 1 vol %, which is outside the flammability limit, was added to spray with a constant overall equivalence ratio, the flame propagation speeds fell on the integrated curve.

MATHEMATICAL MODELING OF DROPLET HEATING, VAPORIZATION, AND IGNITION INCLUDING DETAILED CHEMISTRY

A two-dimensional computer code for the simulation of heating, vaporization, ignition, and subsequent combustion of cold droplets injected in a hot uniform gas flow is developed. The numerical simulation of the processes is performed in an axisymmetric configuration for spherical droplets with boundary fitted grid point systems. Detailed models for the relevant processes are employed; in particular, detailed chemical reaction systems are used. Both methanol droplets in hot air as well as liquid oxygen droplets in gaseous hydrogen are studied. The first chemical system is relevant in diesel engine combustion, and typical conditions at 30 bar are investigated. The liquid oxygen/hydrogen system is studied at 10 bar, and the inlet liquid oxygen (LOX) droplet temperature is cryogenic (85 K)—this condition is found in liquid rocket propulsion. The computer code accounts for physical properties in this temperature range through addition of a database for LOX/hydrogen to the commonly used NASA polynomials in the temperature range between 300 and 5000 K. For the LOX droplets in gaseous hydrogen, two neighboring droplets are also considered, and the influence of droplet spacing on ignition is investigated.

Coarsening mechanisms of droplet spinodal decomposition in binary fluid mixtures

We demonstrate here the experimental results indicating the existence of interdroplet interactions between droplets even in the late stage of droplet spinodal decomposition. On the basis of these results, we discuss a few possible coarsening mechanisms of droplet spinodal decomposition. A droplet of finite curvature has long-range concentration fields around it to satisfy the boundary condition given by the Gibbs–Thomson relation. For a pair of neighboring droplets, thus, the closed isoconcentration lines including the two droplets are generally formed in the matrix phase in the late stage of spinodal decomposition. Thus, the droplet can feel its neighboring droplet via the concentration gradient produced by the latter. The excess bulk and gradient energy associated with the diffuse concentration changes around droplets may cause a long-range attractive interaction between the droplets, which induces the interdroplet collisions. We call this mechanism ‘‘gradient-induced-coupling mechanism.’’ The gradient force associated with an interface is likely responsible for the motion of droplets under the incompressibility condition. In fluid mixtures, this coupling of concentration fields can further be enhanced by the violation of the local equilibrium due to the quick hydrodynamic coalescence of colliding droplets. In such a case, there is a possibility that the concentration diffusion cannot follow this quick geometrical coarsening. This interface quench effect probably leads to the phenomenon of ‘‘collision-induced collision via diffusion’’: A droplet experiencing collision, which accompanies strong diffusion fields, has a higher probability of the subsequent collisions. The hydrodynamic flow induced by droplet coalescence also leads to the subsequent collisions, which we call ‘‘collision-induced collision via flow.’’ Further, the geometrical configuration of droplets also leads to ‘‘geometrical collision-induced collision.’’ All the above mechanisms likely make the coarsening of droplet spinodal decomposition faster than the prediction of Brownian-coagulation mechanism.

Lipidic mitochondrial inclusions in experimentally-induced in vitro myopathy of mouse diphragm

1. Treatment of murine skeletal muscle with the Ca2(+)-channel agonist, Bay K 8644, induces myopathic changes including configurational changes of the mitochondria. 2. During the progression of the myopathy the mitochondria apparently engulf neighboring lipid droplets. 3. Mitochondrial changes are most marked when damage to the muscle is most severe, are prevented under conditions that should reduce Bay K 8644-induced Ca2(+)-influx, and are not modified by various maneuvers designed to prevent the generation of free radical species.

Electric coalescence of two neighboring water droplets supported in oil phase under a microscope.

油中水滴型乳化液の電気解乳化機構の基礎研究として顕微鏡下の油相電場中に対向して保持された2滴の微小水滴の合一挙動について調べた. 実験は1) 2つの帯電水滴の合一, 2) 平等電場中の2つの非帯電水滴の合一について行った,観察した2滴の合一挙動は前報の顕微鏡下の電場中における油中水滴の観察の結果と一致するものであった. 合一機構を1) の場合はTaylorのモデルにより, 2) の場合はLatham and Roxburghによるモデルにより検討した.

Explosive vaporization of a large transparent droplet irradiated by a high intensity laser

Shadowgraph studies of the explosive vaporization of a transparent water droplet after irradiation by a high intensity beam show that dielectric breakdown occurs within the droplet shadow face and generates a dense plasma, which absorbs the laser pulse. The convective forces expel the vapor from the droplet shadow face. We have deduced (1) the vapor propagation velocities, (2) the recoil velocity of the remaining droplet, and (3) the deformation rate of the illuminated face. Droplets are noted to eject fingerlike material from the surface facing the single laser-vaporized droplet when the asymmetrical vapor intercepts the neighboring droplets.

Highly concentrated emulsions. II. Real systems. The effect of film thickness and contact angle on the volume fraction in creamed emulsions

Several aspects of a previously developed two-dimensional theory are extended to real, though monodisperse, emulsions. Specifically, the volume fraction φ 0 of the dispersed phase in close-packed, uncompressed emulsions is predicted as a function of the thickness of the film of continuous phase separating neighboring droplets, and of the contact angle between the film and its adjacent Plateau border. When the film thickness and contact angle are both zero, φ 0 equals 0.7405. A finite film thickness tends to reduce this value, while a finite contact angle tends to raise it as a result of spontaneous deformation of the drops. In principle, φ approaches unity as the contact angle increases to 35.26°. Experiments with cream layers formed at the top of initially more dilute oil-in-water emulsions, stabilized with anionic surfactants such as sodium dodecyl sulfate, confirm these theoretical predictions, at least semiquantitatively. The effects of the type of anionic emulsifier, counterion, and oil phase were studied. In some systems, contact angles as high as 65° were observed. Whenever the contact angle is large, it is difficult to realize the high volume fractions predicted theoretically, because of the formation of a rather rigid floc network, trapping pockets of continuous phase that are not readily expelled, even by centrifugation. For compressed emulsions, the relationship between the volume fraction φ (> φ 0) on the one hand and the drop shape and the disjoining pressure in the films on the other (which could be developed rigorously for cylindrical systems) could be extended to real systems only for the limiting case of extremely high volume fractions, where the continuous phase is present in the form of straight Plateau borders bounded by cylindrical surfaces.