Edge Of Droplet Research Articles

Dynamics of a Water Droplet over a Sessile Oil Droplet: Compound Droplets Satisfying a Neumann Condition.

We report the dynamics of compound droplets with a denser liquid (water) droplet over a less dense sessile droplet (mineral oil) that satisfies the Neumann condition. For a fixed size of an oil droplet, depending on the size of the water droplet, either it attains the axisymmetric position or tends to migrate toward the edge of the oil droplet. For a water droplet-to-oil droplet at volume ratio Vw/Vo ≥ 0.05, stable axisymmetric configuration is achieved; for Vw/Vo < 0.05, migration of water droplet is observed. The stability and migration of water droplets of size above and below critical size, respectively, are explained using the force balance at the three-phase contact line and film tension. The larger and smaller droplets that initially attain the axisymmetric position or some radial position, respectively, evaporate continuously and thus migrate toward the edge of the oil droplet. The radial location and migration of the water droplets of different initial sizes with respect to time are studied. Experiments with water droplets on a flat oil-air interface did not show migration, which signified the role of the curved oil-air interface for droplet migration. Finally, coalescence of water droplets of size above the critical size at the axisymmetric position is demonstrated. Our compound droplet studies could be beneficial for applications involving droplet transport where contamination due to direct contact and pinning of droplets on solid surfaces is of concern. Migration and coalescence of water droplets on curved oil-air interfaces could open new frontiers in chemical and biological applications including multiphase processing and biological interaction of cells and atmospheric chemistry.

Disjoining Pressure in Partial Wetting on the Nanoscale.

Partial wetting on the nanoscale may result in the formation of sessile liquid nanodroplets on flat substrates. In this case, the molecular forces generate a strong interaction between nanodroplet interfaces. This interaction is expressed in the mean-field approximation by the disjoining pressure and determines an important deviation from the spherical cap shape of the nanodroplets. This deviation is observed on the atomic force microscopy images of sessile nanodroplets of oleic acid on glass. The disjoining pressure was manipulated by hydroxylation of the glass surface. This surface modification generated a strong negative disjoining pressure due to structural forces arising from the orientation of oleic acid molecules with their polar heads toward the substrate. As a result, the shape of oleic acid nanodroplets showed large deviations from the spherical cap shape, with the liquid-vapor interface tilting angle with respect to the plane substrate having a maximum (herein considered to be the contact angle) a certain distance from the substrate, followed by its decrease to zero at the droplet edge. The integration of the augmented Young-Laplace equation, where the dependence of the negative structural disjoining pressure on the interface separation distance was assumed to be an exponential decay, yielded height profiles of droplets in good agreement with the experiment.

Some aspects to the understanding of the droplet epitaxial nano-hole formation

In this work, the self-assembled GaAs based nano-hole formation prepared by droplet epitaxial technique is discussed. Here, a qualitative explanation is given why thermal solution cannot be observed under droplet edge and can only detected under droplet middle. The thermal etching takes place far from the droplet edge because of its special quantum mechanical state, which causes melting point increase at the droplet edge. Furthermore it is explained, why thermal solution is preferred under the middle of the droplet. Here, we give a lucid interpretation for the nano-hole formation dependence on the arsenic environment. It is evidenced, that the surrounded lobe initiative originates from the remained droplet edge. We give here a qualitative description why we need somewhat ambient arsenic for the process of nano-hole formation. The material transport needs arsenic ambient and not the thermal solution.

Manipulating colloidal residue deposit from drying droplets: Air/liquid interface capture competes with coffee-ring effect

Significant efforts have been undertaken in recent years to understand, and ultimately control, the drying patterns of colloidal particle filled droplets on solid substrates. The ubiquitous coffee-ring effect has long been attributed to the drag of radial liquid flow towards the droplet edge, driven by diffusive evaporation. Here we report new results showing that there is a temporal competition between migrating colloidal particles, dispersed randomly inside a droplet, and the air/liquid interface in reaching the solid surface. Using the Lagrangian modelling approach to track the motion of colloidal particles within the evaporating droplets, we reveal a boundary line that separates colloidal particles into two groups: above it colloids are captured by the air/liquid interface prior to reaching the solid surface and vice versa. This critical line allows us to quantify the effectiveness of the two emerging routes for manipulating the coffee-ring effect, which work by controlling the interaction between colloids and either the substrate or the air-liquid interface. Our modelling results affirm recent experimental evidence that the air-liquid interface of sessile droplets plays a vital role in the formation of residual deposits. This improved fundamental understanding provides versatile strategies to control droplet deposition morphology, especially the most desired uniform deposit as well as an unusual “overhang” shape of the cross section of the ring-like deposits.

How do groups of red imported fire ants (Hymenoptera: Formicidae) feed on a droplet of sugar water?

Many previous studies have focused on the foraging behaviors and strategies of the red imported fire ants, Solenopsis invicta Buren on solid food or granular bait; little attention has been paid to how liquid sugar is fed upon. In the present study, behavioral responses of S. invicta to 25% sucrose water droplets were observed. Five foraging patterns were identified in S. invicta colonies under laboratory conditions: (i) no feeding, no sucrose water feeding was observed; (ii) surround feeding, ants surrounded and fed along the edge of the sucrose droplet; (iii) stacked feeding, ants stacked and fed along the edge of the sucrose droplet; (iv) droplet-break feeding, ants broke the liquid droplet and sucked sucrose water that spread on surface of the substance or soil particles previously transported by ants; and (v) cover feeding, whole surface of the sucrose droplet was covered by layers of feeding ants. This is the first time cover feeding in S. invicta has been reported, which obviously requires more ants compared to the other patterns. In addition, individual ants were tracked in videos under laboratory conditions, and behavioral repertoires that led to stacking, covering and droplet-breaking were identified and described. The field investigation showed that surround feeding was most frequently performed by S. invicta foragers; however, cover feeding was not observed under field conditions during this study. Both laboratory and field studies showed colony-level variations in sugar-water feeding.

The Image Processing of Droplet for Evaporation Experiment in SJ-10

We have completed an experiment for droplet evaporation processing using Young-Laplace fitting, exponent fitting, polynomial fitting and ellipse fitting, which could be used for multiple shapes of droplets. The droplet evaporation experiment test was an important science experiment in SJ-10. In order to get the change process of the physical parameter, such as the touching edges and the droplet evaporation rate, we had gained the contour edge image of the droplet and used mathematic method to do the fitting analysis. The accuracy of the physical parameter was depended on the accuracy of the mathematic fitting. Using the original Young-Laplace fitting method could not process all the images of evaporation and liquid interface from the space experiment facility of SJ-10, especially the smaller droplet images. We could get more accurate contour fitting and result using the new method described in this article. This article proposes a complete solution, including edge detecting and contour fitting. In edge detecting, Canny detector was applied to extract droplet edge. In contour fitting, Young-Laplace fitting, exponent fitting, polynomial fitting and ellipse fitting are designed to fit the contour of droplets, which make the solution apply to all of droplets in SJ-10.

Discrete Element Model for Suppression of Coffee-Ring Effect

When a sessile droplet evaporates, coffee-ring effect drives the suspended particulate matters to the droplet edge, eventually forming a ring-shaped deposition. Because it causes a non-uniform distribution of solid contents, which is undesired in many applications, attempts have been made to eliminate the coffee-ring effect. Recent reports indicated that the coffee-ring effect can be suppressed by a mixture of spherical and non-spherical particles with enhanced particle-particle interaction at air-water interface. However, a model to comprehend the inter-particulate activities has been lacking. Here, we report a discrete element model (particle system) to investigate the phenomenon. The modeled dynamics included particle traveling following the capillary flow with Brownian motion, and its resultant 3D hexagonal close packing of particles along the contact line. For particles being adsorbed by air-water interface, we modeled cluster growth, cluster deformation, and cluster combination. We found that the suppression of coffee-ring effect does not require a circulatory flow driven by an inward Marangoni flow at air-water interface. Instead, the number of new cluster formation, which can be enhanced by increasing the ratio of non-spherical particles and the overall number of microspheres, is more dominant in the suppression process. Together, this model provides a useful platform elucidating insights for suppressing coffee-ring effect for practical applications in the future.

Measurement of droplet sizes in bubbly oil-in-water flows using a fluid-sampling device

Abstract One important characteristic of bubbly oil-in-water two-phase flows is the droplet size distribution, which has a significant effect on the heat/mass transfer between phases and the turbulence characteristics. In this study, a sampling device of dynamic fluid is designed to trap the morphological structures of the bubbly oil-in-water flows with low flow rate and high water cut. In the experiment, oil droplets in vertical upward pipe are forced into a sampling slot which is specifically designed to ensure that the passing oil droplets are not overlapped. The images of the oil droplets are recorded using a high speed camera when the mixture flows through the sampling slot. The methods of multi-scale edge detection and watershed segmentation are employed to extract the edges of the solitary and adjoining oil droplets in the flow images. Based on the detected oil droplet edges the equivalent oil droplet diameters are derived and compared with a model of maximum droplet diameter. Additionally, time-frequency entropy and the total energy of measured conductance signals are extracted using Adaptive Optimal Kernel Time-Frequency Representation (AOK TFR) to characterize the droplet size distribution of bubbly oil-water two-phase flows.

Organization of SiO2 and TiO2 Nanoparticles into Fractal Patterns on Glass Surface for the Generation of Superhydrophilicity

The superhydrophilic surfaces have many important practical applications such as antifogging, antifouling, self-cleaning, etc. The present study demonstrates a simple and facile template-assisted dip-coating approach for the organization of silica (SiO2) and titania (TiO2) nanoparticles (NPs) into fractal patterns on the glass surface. The pure NPs suspension showed a “coffee ring effect” and did not form any organized pattern on the glass surface after drying. In this reported method, NPs were organized into fractal patterns using a template consisting of sodium carboxymethyl cellulose (CMCNa) and oxalic acid mixture in the presence of a cationic surfactant (CTAB). The presence of surfactant plays a major role to alter the coffee ring effect because of a Marangoni flow in the direction of droplet edge to its center induced by the surface tension gradient and surface potential of the particles, which eventually helps to get a uniform fractal pattern. Finally, the fractal patterns of only SiO2 and TiO2 NPs...

Effects of interfacial heat transfer, surface tension and contact angle on the formation of plasma-sprayed droplets through simulation study

Abstract A comprehensive numerical study of the effects of interfacial heat transfer, surface tension (1.8 N/m, 1.647 N/m, 1.35 N/m and temperature-dependent values) and contact angles (60°, 90° and 120°) on the droplet spreading behaviour in the formation of the plasma-sprayed splats was conducted. The evolution of splat morphologies with time was accurately captured using a volume of fluid (VOF) model in a 2-D computational domain. The results show that bubbles form at the time of impact and decrease the heat transfer efficiency at the contact points. During spreading, solidification occurs at the droplet edge before maximum spreading. The rapid growth of the underlying solidified layer induces fluid instabilities for the upper liquid layer, which triggers material jetting. The interfacial heat transfer is the key parameter influencing the droplet cooling process and the final splat morphology. Increasing the substrate preheating temperatures (from 27 °C to 300 °C) delays solidification but increases the potential of substrate melting. A low surface tension (1.35 N/m) readily promotes liquid projections, while the contact angle is of less importance in changing the surface morphologies.

Observations of internal flow inside an evaporating nanofluid sessile droplet in the presence of an entrapped air bubble.

Using a unique, near-field microscopy technique, fringe patterns and nanoparticle motions are visualized immediately following a nanofluid droplet deposition on a glass substrate in which an air bubble is entrapped. The nanofluid consists of DI-water, 0.10% Aluminum Oxide nanoparticles with an average diameter of 50 nm, and 0.0005% yellow-green polystyrene fluorescent particles of 1 μm diameter. High-speed, fluorescent-mode confocal imaging enables investigation of depth-wise sectioned particle movements in the nanofluid droplet inside which a bubble is entrapped. The static contact angle is increased when a bubble is applied. In the presence of the bubble in the droplet, the observed flow toward the center of the droplet is opposite to the flow observed in a droplet without the bubble. When the bubble is present, the evaporation process is retarded. Also, random motion is observed in the contact line region instead of the typical evaporation-driven flow toward the droplet edge. Once the bubble bursts, however, the total evaporation time decreases due to the change in the contact line characteristics. Moreover, the area of fringe patterns beneath the bubble increases with time. Discussed herein is a unique internal flow that has not been observed in nanofluid droplet evaporation.

Sub-Micron Patterning of Polymer Brushes by Controllable Deposition of Polyelectrolyte Monolayers

The controlled deposition of polyelectrolyte macroinitiators for polymer brush growth is investigated. High resolution patterns are created for periodic array gratings towards responsive polymer brush sensors. Further, controlling the segregation of the macroinitiator to the droplet edge by careful manipulation of the constituent solutes leads to sub-micron patterning of polymer brushes.

Release of Oxygen from a Nano-sized Water Droplet Observed using Molecular Dynamics

Dissolved oxygen is necessary for many biological processes as well as many industrial practices. Dissolved oxygen released from water in dissolved air flotation (DAF) systems can be have many different applications. However, DAF systems are very costly to operate. To develop more efficient DAF systems, a deeper understanding of the process of oxygen being released from water is required. In this study, molecular dynamics (MD) simulations were used to simulate 100 oxygen molecules surrounded by 31002 water molecules at temperatures ranging from 0℃ to 100℃. Simulations were carried out for 10 ns, during which, in most cases, all the oxygen molecules were released from the water droplet. With MD simulations, visualization of the molecules escaping the water droplet was possible, which aided the understanding of the interactions between molecules at the nano-scale. The results showed that as the oxygen molecules moved near the edge of the water droplet that the oxygen molecules hesitated before escaping the water droplet or returned to the interior of the water droplet. This was because of the attractive forces between the water and oxygen molecules. Moreover, after most of the oxygen molecules were released from the droplet, some were found to return to the droplet's edge or even the interior of the droplet. It was also confirmed that oxygen molecules were released at a faster rate at higher temperatures.

Controlling coffee ring structure on hydrophobic polymer surface by manipulating wettability with O2 plasma

A simple and novel method is firstly reported for controlling coffee ring structure on polystyrene (PS) film surface by O2 plasma. O2 plasma treatment leads to the wettability change of PS surface from hydrophobic to hydrophilic. For hydrophilic PS surface the coffee ring structure is avoided relying on the motion of contact line (CL) while SiO2 microspheres are left. The motion of the CL is produced based on the viscosity and Marangoni effect with the addition of polymer additives. For hydrophobic PS surface coffee ring structure still persists even with polymer additives because SiO2 microspheres transfer with the motion of the CL at the beginning of droplet evaporation and accumulate at the droplet edge at late stage with the pinning of the CL. As a result, uniform and macroscale SiO2 microspheres deposition without coffee ring structure and SiO2 microspheres deposition with coffee ring structure are controlled by O2 plasma. This method provides a new way to tune coffee ring structure with smart surface and may be potentially useful for a range of application at material deposition and diagnosing diseases.

Droplets boiling crisis of ethanol water solution on duralumin substrate with SiO2 nanoparticles coating

Boiling crisis and evaporation of droplets of ethanol water solution on a horizontal ceramic heating surface were studied experimentally. The experiment was performed on duralumin substrate with SiO2. The coating was obtained by nanoparticles plasma spraying. Measurements of the contact angle of the aqueous solution onto the ceramic surface were performed. Most evaporation relates to an increase in the area of the wetting droplet surface and only 10–20% of evaporation relates to the effect of diffusion and change in the thermal–physical coefficients. Maximal instability of the bubble microlayer for alcohol solution with C0=20–40mass% is observed. The critical heat flux behaves non-monotonously at a change in alcohol concentration in solution, and there are two extremes. The maximal value of sustainability coefficient at evaporation of droplets of ethanol solution corresponds to alcohol concentration of 20–30mass%. Concentration gradient leads to instability of a triple contact line at the bubble bottom and a decrease in the dry spot area; this promotes higher critical heat flux levels (CHF). The behavior of a binary mixture depends not only on the Marangoni effect at the droplet edges, as thought previously, but also on the unsustainable behavior of the bubbles contact line. The heat transfer coefficient of ethanol solution in the spheroidal state decreases with a rise of wall overheating and spheroid diameter. Experimental dependence of the vapor layer height on wall overheating at boiling crisis was derived. The height of this layer (60–80μm) at Leidenfrost temperature manifold exceeds the surface microroughness value (1–1.5μm).

Drop impact on natural porous stones

Drop impact and spreading on three natural porous stones are experimentally determined using high-speed imaging and compared with spreading over an impermeable steel surface. The dynamic non-wetting behavior during spreading and the hydrophobic contact angle >90° is attributed to the presence of an air layer between the droplet and the porous substrate. As the contact line pins at maximum spreading on the porous stone, the maximum spreading determines the liquid contact area on such substrate. The droplet gets pinned when the air layer is broken at the contact line and capillary forces develop in fines pores at the droplet edge, pinning the droplet. Maximum spreading on porous stones increases with impact velocity but does not scale with Weber number at low impact velocity. It is demonstrated that dynamic wetting plays an important role in the spreading at low velocity and that the dynamic wetting as characterized by the dynamic contact angle θD has to be taken into account for predicting the maximum spreading. Correcting the maximum spreading ratio with the dynamic wetting behavior, all data for porous stones and non-porous substrate collapse onto a single curve.

Disk to dual ring deposition transformation in evaporating nanofluid droplets from substrate cooling to heating.

Substrate temperature plays an important role in deposited morphologies formed after the evaporation of nanofluid droplets. The deposited patterns are shown to vary from a uniform disk-like profile to a dual ring from cooling to heating of the substrate. The droplet on the substrate with a relatively low temperature reveals three primary stages. Stage I features an outward transport of nanoparticles along the liquid-air interface near the droplet edge. Meanwhile some nanoparticles form sediment on the solid surface with a certain distance to the contact line. In the central region nanoparticles are dominated by Brownian motion so they fluctuate around their positions. Stage II is characterized by an increasing outward transport of nanoparticles in the bulk so the coffee ring is gradually enhanced. Most particles in Stages I and II are central-concentrated, leaving an annular gap sparsely covered adjacent to the outer ring. In Stage III, the pattern is homogenized by filling the gap with the arrival of the interior nanoparticles. Upon increasing the substrate temperature, the accompanied flow pattern exhibits a transition when the substrate still remains cooler than the atmosphere. It is resulted from the evaporative cooling at the droplet apex counteractive to the applied temperature gradient by substrate cooling. Above the transition temperature, the induced inward Marangoni flow takes place earlier at a higher substrate temperature, and in conjunction with the outward radial flow a dual ring pattern is formed.

Bubble boiling in droplets of water and lithium bromide water solution

The dynamics of growth and interaction of vapor bubbles in droplets of pure water and LiBr water solution on a horizontal wall were investigated in a wide superheating range. The growth rates of bubbles were determined both in a distillate droplet and in a salt solution droplet. The bubble growth rate in a pure water droplet at the final stage is somewhat lower than in pool boiling. The bubble growth rate in a salt solution is substantially lower than for pure water. Due to the bubble density maldistribution, the vapor flow density is appreciably higher at the droplet edges than on the droplet axis. Collective behavior of the bubbles possesses both stochastic character and elements of self-organization. The thermal measurements were carried out by means of high-speed video and blowup thermal imager.

Median ellipse parameterization for robust measurement of fuel droplet size

The combustion properties of blended fuel combinations can be characterized by performing single droplet fuel combustion experiments. These combustion experiments are visualized using high speed image acquisition. Once the high speed images are obtained, the burn rate and other characteristics of combustion, such as the occurrence of microexplosions, can be characterized. Currently these quantities are either measured manually or are measured using automated software. However, the current software packages used for this task are limited in that they can only measure droplets that are elliptical in shape and manual corrections often have to be made to avoid significant errors in the measurement. An automated droplet tracking algorithm is presented that can automatically track droplet size without manual intervention due to its robustness to the presence of missing or extra edges in the images. In addition, the proposed method can track shapes more general than ellipses, which is required in order to track the droplet during microexplosions. The proposed algorithm starts by fitting ellipses to numerous five point subsets from the droplet edge data. The closed contour is parameterized by determining the median perimeter of the set of ellipses. The resulting curve is not an ellipse, allowing arbitrary closed contours to be parameterized. The performance of the proposed algorithm and the performance of existing algorithms are compared to a ground truth segmentation of the fuel droplet images. This comparison demonstrates that the median ellipse parameterization algorithm has improved performance for both area quantification and edge deviation.

Motion and deformation of a droplet in a microfluidic cross-junction

In this paper we investigate computationally the transient deformation of a droplet flowing along the centerline of a microfluidic cross-junction device. We consider naturally buoyant droplets with size smaller than the cross-section of the square channels comprising the cross-junction, and investigate low-to-strong flow rates and a wide range of fluids viscosity ratio. Our investigation shows that the intersecting flows at the cross-junction act like a constriction, and thus the droplet shows a rich deformation behavior as it passes through the micro-junction. Our work highlights the three-dimensional effects of the asymmetric microfluidic geometry on the droplet deformation, and the different effects of the viscosity ratio on the droplet’s overall length scales and the local length scales at the droplet edges. The large edge curvatures and thus the small local length scales developed transiently, especially at the tail of low-viscosity droplets, reveal that the current investigation is a multi-length interfacial dynamics problem.