Center Of Droplet Research Articles

Gum Arabic/Lysozyme coacervate phase similar in structure to multilamellar liposomes

It is known that the complex coacervation (CC) of biopolymers can induce conformational and structural changes of the interacting macromolecules. However, effect of such changes on the localization of biopolymers within the complex coacervate phase is unknown. In this paper confocal laser scanning, bright-field microscopy, phase analysis, dynamic light scattering, turbidimetry, circular dichroism and fluorescence measurements were used to study CC in aqueous gum arabic (GA)/lysozyme (lys)/system at various GA/lys weight ratio (q), pH 7.0 and ionic strength 0.01. CC had a binding stoichiometry of 88 lys molecules complexed per GA molecule, whereas the binding constant is 1.33⋅107 M−1. Complexation enhance with the decrease of pH from 10 to 4.6 and become weaker when the temperature increases, indicating contribution of hydrogen bonds in stabilization of complexes. We demonstrate for the first time the formation of multilamellar structure in the aqueous GA/lys coacervate phase that is similar to the structure of multilamellar liposomes. The amount of concentric vicious layers in the coacervate droplets are maximum at q values corresponding to maximum yield of complex phase.The content of GA in the layers increases with the increase of q, and in the direction from periphery to center of coacervate droplets. CC leads to a spectacular changes in the helix content of lys. We assume that hydrophilic lys molecules interacting with polydisperse GA undergo complex molecular changes leading to formation of the unique multilamellar structure.

A model for droplet heating and evaporation of water-in-oil emulsified fuel

Water-in-oil (W/O) emulsified fuel is a promising alternative fuel by inducing flash boiling of water at high temperature that can improve the atomization of fuel spray. The heating and evaporation process of emulsified fuel droplet is affected by the diffusion and coalescence of small dispersed water droplets in oil during the heating. In this study, a model is developed for the droplet heating and evaporation of W/O emulsified fuel with these key physics considered. The diffusivity of dispersed water droplets in oil is calculated using the Stokes-Einstein equation. The deactivation temperature of surfactant is selected as a criterion for the water coalescence since the water coalescence is due to the deactivation of surfactant. The process of water coalescence is simplified that the dispersed water droplets coalesce into one single water sub-droplet at the center of the oil droplet instantly as the droplet temperature reaches the deactivation temperature of surfactant. The model is validated against experimental data of single droplet under different heating temperatures, surfactant concentrations, and sizes of dispersed water droplets. Based on the proposed model, the effects of fuel properties and heating conditions on the droplet heating and evaporation are analyzed.

A novel approach for conditional measurement of droplet size distribution within droplet clusters in sprays

A novel method for conditional measurement of droplet size distribution within droplet clusters in sprays is reported in this paper. The salient feature of this approach is the application of Voronoi analysis to droplets measured by the ILIDS technique, which is capable of providing planar measurement of size of individual droplets in a spray. The principle of the present technique is based on calculation of Voronoi tessellations around the droplets identified in the ILIDS image processing. Comparison of the distribution of areas of the Voronoi cells with that from a random distribution of droplets described by a Poisson function facilitates classification of the Voronoi cells as clusters or voids. Thus, individual clusters of droplets and void areas are recognized by identifying the interconnected cells. Since the sizes of individual droplets are known, conditional measurement of droplet size distribution within clusters and voids is possible. Since droplet images are always defocused in the ILIDS technique, correct identification of droplet centers is important, which is described in the paper. It is shown that in spite of the low validation ratio in the ILIDS image processing, the results on the cluster and void characterization agree well with those by Voronoi analysis of PIV images. The novel method is demonstrated for an air-assist spray where ILIDS measurements are obtained far downstream of the injector exit. Interestingly, the characteristic droplet size within droplet clusters is found to be smaller than that in voids, indicating size-based preferential segregation of droplets due to clustering in sprays.

Secondary spreading of acidified aerosols on the surface of Zn

Secondary spreading of acidified sodium chloride (NaCl) aerosol droplets on the surface of zinc have been studied. The in-situ observation of the secondary spreading, corrosion morphology, corrosion products under different position and volume loss are used to understand the secondary spreading of the acidified aerosol. The results reveal that the secondary spreading effect of NaCl aerosol droplet is more pronounced compared to the acidified aerosol droplets which are mainly embodied in larger spreading region and faster spreading rate. The difference of the secondary spreading effect is associated with acidification which inhibits the increase in the pH at the periphery. Furthermore, lower corrosion rate was observed for the acidified aerosol droplets due to smaller secondary spreading area (cathodic region). The main corrosion product in the center of NaCl aerosol droplet and the aerosol droplet acidified by HCl is Simonkolleite, while for the aerosol droplet acidified by H2SO4 which shows several different corrosion morphologies in the center, the main corrosion products are Gordaite, Simonkolleite and Zn4Cl2(OH)4SO4 ∙ 5H2O. The formation of protective corrosion products Gordaite that can inhibit the anodic reaction leads to the smallest spreading area and the lowest corrosion rate for the aerosol droplet acidified by H2SO4.

Numerical study on arc-droplet coupled behavior in magnetic field controlled GMAW process

The metal transfer behavior is one of the most pivotal links of gas metal arc welding (GMAW) for improving the welding quality. Arc impacts the current path and heat input on the droplet surface, in reverse, the metal droplet detachment makes the arc to flicker. It is hard to make the metal transfer process clear with only experimental methods since the limited measuring space and high temperature in the welding region. In this work, with the help of computational fluid dynamics software ANSYS FLUENT, a magnetohydrodynamic model including argon arc plasma and steel wire and the droplet is constructed by using two independent governing equations of gas and metal phase for the mass, momentum, and energy balance. Comparing the magnetic field controlled GMAW with conventional GMAW, the temperature field, current density distribution, electric potential field, static pressure field and also metal vapor mass fraction distribution are highlighted. It is shown that the metal transfer frequency decreases and droplet diameter increases with 180 A welding current when applying an external constant axial magnetic field 0.002 T. So, the arc length decreases before the detaching moment, the current density expands in the arc region and the voltage decreases. The centrifugal movement impedes the downward flow and transfers heat from the droplet center to the outside region.

Experiments and analytical solutions of light driven flow in nanofluid droplets

Adding nanoparticles with high light response characteristics to a light-transmitting fluid medium can form a light-driven nanofluid and achieve efficient use of light energy. This paper conducts the experimental observation and theoretical analysis of the light driven nanofluid flow behavior, which is the theoretical basis for achieving the precise control of optical drive nanofluid. To realize the efficient conversion of light energy into kinetic energy, here, the motion of Fe<sub>3</sub>O<sub>4</sub> particles with a diameter of 300 nm in droplets induced by the Marangoni effect is studied under different light sources by using the particle image velocimetry (PIV). The experimental results show that when the number density of particles is higher than the critical value, the vertical vortices with symmetrical structure can be induced. At the bottom of the droplet, the particles move from the periphery to the center of droplet, and at the top of the droplet, the particles move from the center to the periphery of droplet. In addition, the frequency of light source and the number density of particles are the dominant factors in this process. Subsequently, for the light driven nanofluid experiment in this paper, the analytical solution of the flow field distribution is achieved by using the Stokes equation and the surface tension gradient boundary condition. The analytical solution of the flow field distribution obtained here is consistent with the experimental results, confirming the validity of the quantitative theory. Finally, the correlation between various driving modes, including surface tension at the top surface, surface pressure at the bottom surface or concentrated light radiation force in bulk phase, is discussed. This research provides theoretical support for the precise regulation of flow behavior and efficient conversion of light energy in the optical microfluidic system.

Multistage cooling and freezing of a saline spherical water droplet

In this paper, the thermal behaviour of a saline water droplet during flight over a marine vessel in cold weather conditions is investigated by analytical and semi-analytical techniques. To predict and analyze the droplet cooling and freezing processes, three stages are employed: a liquid cooling stage, a solidification stage, and a solid cooling stage. The theoretical model considers heat transfer via conduction inside the droplet as well as convection, evaporation (just for the liquid cooling stage), and radiation heat transfer from the droplet's surface to the ambient air. A novel semi-analytical solution technique is developed to analyze the inward moving boundary problem for the solidification stage. The results show that the liquid cooling stage is very short, and the temperature at the droplet's center remains close to the initial droplet temperature. During the solidification stage, the velocity of the inward freezing front within the droplet is approximately constant, and the temperature variations are linear when the temperature inside the droplet reaches the freezing temperature. The solid cooling stage is much longer than the other stages, and the temperature changes are non-linear. For a case study, theoretical predictions show that the average temperature of a droplet with a diameter of 1 mm at the moment of impact on the deck is approximately −1.95°C. Moreover, there is an ice shell with a thickness of 0.06 mm on the surface of the water droplet at the moment of impact.

Concentration Distribution of Photosensitive Liquid in a Droplet Under Ultraviolet Light

Abstract A semi-analytical solution for the concentration of photosensitive suspension is developed in a hemispherical droplet illuminated with ultraviolet (UV) laser. A biharmonic equation in stream function is analytically solved using toroidal coordinates, which is used to solve the transport equation for concentration. Flow pattern and photosensitive material concentration are affected by the peak location of the UV light intensity, which corresponds to the surface tension profile. When the laser beam is moved from the droplet center to its edge, a rotationally symmetric flow pattern changes from a single counter clockwise circulation to a circulation pair and finally to a single clockwise circulation. This modulation in the orientation of circulation modifies the concentration distribution of the photosensitive material. The distribution depends on both diffusion from the droplet surface and the Marangoni convection. The region beneath the droplet surface away from the UV light intensity peak has low concentration, while the region near the downward dividing streamline has the highest concentration. When the UV light peak reaches the droplet edge, the concentration is high everywhere in the droplet.

The Features of Ring-Shaped Deposit Formation upon Evaporation of Magnetic Colloid Droplets in a Magnetic Field

The structure of deposits resulting from the evaporation of magnetic colloid droplets has been experimentally studied. It has been shown that the deposit has the largest thickness in the center and at the boundary of a droplet; i.e., a ring-shaped deposit is formed. The width of the ring-shaped (boundary) region of the deposit has been measured and shown to depend on colloid concentration and the action of a nonuniform external magnetic field. In particular, it has been found that, under the action of a magnetic field having a gradient directed to the droplet center, the thickness and width of the ring-shaped region of the deposit decrease.

Numerical investigation of a burning fuel droplet pair with different spacings and sizes

The combustion of double fuel droplets with different initial centre spacings and different droplet radii in convective environment are studied numerically by using the VOF (volume of fluid) model. The influences of these two factors on the droplet burning rates in the process of spray combustion are studied. The combustion processes of the two droplets with different initial centre spacings (shown as d2-t curves) and with different droplet radii (shown as (d/d0)2-t curves) are constructed respectively. Then functional analysis that reflects the global characteristic of combustion curves is proposed to process the simulation results. By analysing the functional values of combustion curves of the double droplets with different droplet centre spacings and radii, some results are obtained as follows: (1) The burning rates of the front droplets are always greater than those of the back droplets in convective environment, regardless the different droplet spacings or droplet radii. (2) The functional values of the combustion curves of the front droplets decrease nonlinearly as the initial centre spacing increases with a rapid decline first and a slow decline later, while the functional values of the combustion curves of the back droplets decrease linearly as the initial centre spacing increases. (3) The functional values of droplets combustion curves increase linearly as the droplet radius increases. In this case, the different initial centre spacings do not obviously affect the trends of the droplet burning rates versus the droplet radius.

Tunable magnetism of a hexagonal Anderson droplet on the triangular lattice

Motivated by recent progress on quantum engineered Kondo lattices, we numerically investigated the local magnetic properties of a hexagonal Anderson droplet consisting of multiple rings of magnetic atoms periodically arrayed on a triangular lattice. We demonstrated the tunability of the magnetic properties via their evolution with the droplet geometry for two types of systems with distinct local orbital occupancy profile. We found that the local susceptibility of the droplet center of some types of droplets can be remarkably enhanced in contrast to the conventionally rapid decrease due to spin correlations of surrounding droplet rings. The tunability of the magnetic properties is attributed to the charge redistribution with varying the droplet geometry enforced by the confined lattice with open boundary. Our simulations complements the exploration of the novel artificial tunability of engineered lattice systems.

FluoroCellTrack: An algorithm for automated analysis of high-throughput droplet microfluidic data.

High-throughput droplet microfluidic devices with fluorescence detection systems provide several advantages over conventional end-point cytometric techniques due to their ability to isolate single cells and investigate complex intracellular dynamics. While there have been significant advances in the field of experimental droplet microfluidics, the development of complementary software tools has lagged. Existing quantification tools have limitations including interdependent hardware platforms or challenges analyzing a wide range of high-throughput droplet microfluidic data using a single algorithm. To address these issues, an all-in-one Python algorithm called FluoroCellTrack was developed and its wide-range utility was tested on three different applications including quantification of cellular response to drugs, droplet tracking, and intracellular fluorescence. The algorithm imports all images collected using bright field and fluorescence microscopy and analyzes them to extract useful information. Two parallel steps are performed where droplets are detected using a mathematical Circular Hough Transform (CHT) while single cells (or other contours) are detected by a series of steps defining respective color boundaries involving edge detection, dilation, and erosion. These feature detection steps are strengthened by segmentation and radius/area thresholding for precise detection and removal of false positives. Individually detected droplet and contour center maps are overlaid to obtain encapsulation information for further analyses. FluoroCellTrack demonstrates an average of a ~92–99% similarity with manual analysis and exhibits a significant reduction in analysis time of 30 min to analyze an entire cohort compared to 20 h required for manual quantification.

Inverse cascade of the vortical structures near the contact line of evaporating sessile droplets

Microscopic imaging as well as the particle image velocimetry (PIV) are carried out to evaluate the concentration, velocity and vorticity fields near the contact line of the nano-particles-laden evaporating sessile droplets. After the onset of the linear thermocapillary instabilities due to the Marangoni perturbations, the non-linear state sets in and the micro-scale jet-like vortex structures are ejected from the contact line towards the center of the droplet. Afterwards, the jet-like vortical structures expand in the spanwise directions and start to interact with the neighbouring structures. Two types of the inverse cascade mechanisms are found to occur. In the first kind, the vortices of the similar length scale merge and continuously produce larger vortices and corresponding wavelength growth. The second inverse cascade mechanism takes place due to the entrainment of the smaller vortices into the larger structures. Both inverse cascade processes are identified as the continuous feeding of the kinetic energy from the smaller scales to the larger scales. For individual micro-jets the velocity field characterizes the jet-like vortex structures ejected from the contact line towards the droplet center opposing the bulk flow from the center towards the contact line. In addition, the vorticity field overlaid by the velocity streamlines identify the sense of rotation of the low pressure zones on either side of the micro-jet as well as the high pressure stagnation point at the tip.

Aqueous Corrosion of Deformed Steel Under Simulated Diluted Bitumen

The effect of predeformation on corrosion of ASTM A106B pipeline steel exposed to 1.7 mM sodium chloride droplets covered by simulated diluted bitumen was evaluated. The microstructures of ASTM A106B pipeline steel with and without predeformation were examined by electron backscatter diffraction and optical microscope. Corrosion of pipeline steel under the chloride droplet covered by simulated diluted bitumen for 5 min was studied with and without predeformation using scanning electron microscopy. Corrosion was initiated at the surface of ASTM A106B pipeline steel after 5 min of exposure. The predeformation increased the number of pits initiated at the steel surface and the number of partially dissolved inclusions. Scanning electron microscopy, profilometry, and x-ray photoelectron spectroscopy measurements were used to characterize the corrosion of the specimens with and without prior deformation after 24 h of exposure to an oil-covered droplet. The corrosion products coalesced and formed a small circular ring which deviated from the geometric center of the droplet. The diameters of the circular rings for the unbent and pre-bent specimens were 2.371±0.125 mm and 2.465±0.046 mm, respectively; the distances between the circular ring centers and droplet centers were 0.599±0.124 mm and 0.620±0.190 mm, respectively. The average corrosion penetration of the predeformed specimen was 1.18±0.09 times higher than that of the specimen without predeformation.

In vivo superresolution photoacoustic computed tomography by localization of single dyed droplets

Photoacoustic (PA) computed tomography (PACT) is a noninvasive hybrid imaging technique that combines optical excitation and acoustic detection to realize high contrast, high resolution, and deep penetration in biological tissues. However, the spatial resolution of PACT is limited by acoustic diffraction. Here, we report in vivo superresolution PACT, which breaks the acoustic diffraction limit by localizing the centers of single dyed droplets that are flowing in blood vessels. The droplets were prepared by dissolving hydrophobic absorbing dye in oil, followed by mixing with water. The dyed droplets generate much higher-amplitude PA signals than blood and can flow smoothly in vessels; thus, they are excellent tracers for localization-based superresolution imaging. The in vivo resolution enhancement was demonstrated by continuously imaging the cortical layer of a mouse brain during droplet injection. The droplets that were flowing in the vessels were localized, and their center positions were used to construct a superresolution image that exhibits sharper features and more finely resolved vascular details. An improvement in spatial resolution by a factor of 6 has been realized in vivo by the droplet localization technique.

Expansion and Fragmentation of a Liquid-Metal Droplet by a Short Laser Pulse

We report an experimental and numerical investigation of the fragmentation mechanisms of micrometer-sized metal droplet irradiated by ultrashort laser pulses. The results of the experiment show that the fast one-side heating of such a droplet may lead to either symmetric or asymmetric expansion followed by different fragmentation scenarios. To unveil the underlying processes leading to fragmentation we perform simulation of liquid-tin droplet expansion produced by the initial conditions similar to those in experiment using the smoothed particle hydrodynamics (SPH) method. Simulation demonstrates that a thin heated surface layer generates a ultrashort shock wave propagating from the frontal side to rear side of the droplet. Convergence of such shock wave followed by a rarefaction tale to the droplet center results in the cavitation of material inside the central region by the strong tensile stress. Reflection of the shock wave from the rear side of droplet produces another region of highly stretched material where the spallation may occur producing a thin spallation layer moving with a velocity higher than expansion of the central shell after cavitation. It is shown both experimentally and numerically that the threshold laser intensity necessary for the spallation is higher than the threshold required to induce cavitation in the central region of droplet. Thus, the regime of asymmetrical expansion is realized if the laser intensity exceeds the spallation threshold. The transverse and longitudinal expansion velocities obtained in SPH simulations of different regimes of expansion are agreed well with our experimental data.

Reversible Solubilization of Pyrene by a Gas Switchable Surfactant Investigated by Molecular Dynamics Simulation.

The reversible solubilization behavior of pyrene by a CO2/N2 switchable surfactant (named N'-dodecyl- N, N-dimethylacetamidinium bicarbonate (DDAB)) was investigated with molecular dynamics (MD) simulations. We first individually simulated the aggregation of the inactive surfactant N'-dodecyl- N, N-dimethylacetamidines (DDA) and effective surfactant DDAB in water. Detailed structural properties analysis showed that DDAB molecules aggregated into a micelle, while the aggregation of DDA molecules was considered to be an oil droplet that was separated from the water phase. MD simulations revealed that pyrene molecule was solubilized in the interior hydrophobic region of the micelle as expected. Pyrene was adsorbed on the surface of the oil droplet which is due to the dense packing of DDA molecules inside the oil droplet. The simulated release process showed that the solubilized pyrene in the interior was squeezed out when the micelle was changed to an oil droplet. Reduced density gradient (RDG) function was used to study the weak interactions and explore the molecular driving force behind the reversible solubilization. The results demonstrated that repulsion effects of water molecules on the DDA headgroups play an important role on the pyrene release. Because of the persistent molecular motion of DDA molecules into the droplet center, pyrene was finally repelled out of the oil droplet. Our study provided a molecular mechanism into the reversible solubilization of a gas-controlled switchable surfactant. This is expected to be useful for surfactant-enhanced remediation (SER) experiments.

A phenomenological approach to the deposition pattern of evaporating droplets with contact line pinning

When an evaporating droplet of colloidal suspension dries on a solid surface with the contact line pinned, the solute particles are driven by the solvent flow toward the edge and form a ring-like deposition pattern. In this work, we take into account the contact angle hysteresis and incorporate it into the effective model of Man and Doi (2016 Phys. Rev. Lett. 116 066101) which is based on Onsager’s variational principle. We show that single-ring pattern is formed when the contact line pinning and/or friction are sufficiently strong. We demonstrate that there exists an appropriate range for contact line pinning and friction in which two rings can be formed in the deposition pattern, one at the initially pinned contact line and the other a bit closer to the center of droplet.

Modeling and experimental analysis of thermocapillary effect on laser grooved surfaces at high temperature

Spreading behaviors of oil on laser grooved stainless steel 316L surfaces were studied theoretically and experimentally at high temperature with consideration of thermocapillary effect. A mathematic model of droplet spreading on grooved surfaces was developed taking into account of thermocapillary, groove capillary, thermal viscous effect and surface tension. Then, numerical analysis and experiments were carried out on laser grooved surfaces with different temperature gradient and change of droplet center temperature from 24 to 200 °C accordingly. It was shown that the present model agreed well with the experiment in predicting oil droplet thermocapillary spreading. The initial acceleration phase of oil spreading was proved more and more significant with temperature gradient increasing, producing maximum velocity form 5.8 to 15.93 mm/s with the increasing of temperature gradient from 0 to 3.72 °C/mm. Besides, the oil spreading in the opposite direction of temperature gradient was faster than that in the direction of temperature gradient, and the distance difference increased form 0.06 mm to 1.16 mm with the increasing of temperature gradient from 0.45 to 2.12 °C/mm after 1 s spreading, which illustrated that thermocapillary effect was gradually weakened with the corresponding temperature rising. Further, oil thermal viscous effect was proved as the main reason for this weakening. Groove capillary should be the active force for the directional spreading and even directing oil to high temperature regions, especially under high temperature gradient conditions.

Corrosion Driven Drying Phenomena Occurring during the Atmospheric Corrosion of Magnesium Containing Galvanized Steel

The corrosion behaviour of various Zn-Mg-Al hot-dip galvanised coatings on steel substrates were studied using a combination of time-lapse photography (TLP), the scanning Kelvin probe (SKP) and non-contact height profiling. In a standard experiment 1uL electrolytic droplets comprising varying concentrations of aqueous NaCl were deposited onto the surface of the Zn-Mg-Al alloy coating held in a humid air atmosphere maintained in isopiestic equilibrium (equilibrium with respect to the vapour pressure of water) with a large remote reservoir of aqueous electrolyte which had the same composition as the original droplet. The progress of atmospheric corrosion was then followed using TLP with photographs taken at 2-minute intervals for a period of up to 60 hours. When required, in-situ SKP was used to produce area maps of electrochemical potential if in the vicinity of the electrolytic droplets over a similar period of time. The SKP probe could also be used as a non-contact height profilometer which allowed the topography of the electrolytic droplet and the surrounding Zn-Mg-Al alloy surface to be mapped. This made it possible to study time-dependent changes in droplet volume and shape as atmospheric corrosion proceeds. In most cases the phenomenon known as cathodic secondary spreading was observed at the droplet periphery, where a thin film (microns) of electrolyte develops and advances away from the droplet as time proceeds. The presence of this thin electrolyte film changes the reflectivity of the Zn-Mg-Al alloy surface with respect to visible light and also produces a marked reduction (drop) in local electrochemical potential. The kinetics of secondary spreading could therefore be followed using a combination of TLP and in-situ SKP. Results are presented which show the dependence of spreading rate on: time, original NaCl concentration and Zn-Mg-Al alloy composition. The role of aluminium and magnesium in modifying (reducing) rates of secondary spreading is discussed in relation to their relative electrocatalytic activities for cathodic oxygen reduction and their distribution within the microstructure of the various alloy coatings. It is shown, using in-situ profilometry, that at lower initial concentrations of NaCl (and consequently higher relative humidity under isopiestic conditions) the original electrolyte droplets dry out within approximately 2-3 hours of being deposited on the alloy surface. The mechanism of this drying phenomenon is discussed in relation to the relevant mechanisms of atmospheric corrosion. It is proposed that as corrosion proceeds all the Na+ cations originally present in the droplet migrate out into the region of secondary cathodic spreading. Conversely, Cl- anions migrating towards the anodic sites at the droplet centre become sequestered as insoluble (or poorly soluble) zinc corrosion products such as Simonkolleite, Zn(OH)Cl. (The presence of Simonkollite is confirmed using grazing angle Z-ray diffraction.) As these corrosion products precipitate the activity of water in the droplet (and its associated vapour pressure) increase and water evaporates from the droplet as it tries to maintain isopiestic equilibrium with the experimental atmosphere. Corrosion-driven drying was also seen for higher initial concentrations of NaCl but at considerably slower rates. The drying effect, caused by the secondary spreading phenomena, may be an important self-limiting mechanism of atmospheric corrosion affecting Zn-Mg-Al alloy coatings.