Drop Dynamics Research Articles

Receding Dynamics of Jatropha Straight Vegetable Oil Drops Impacting a Heated Surface

Straight vegetable oils have a huge potential to minimize the dependence of the automobile and energy sectors on conventional fossil fuels. In the present study, the impact of a single drop of Jatropha straight vegetable oil on a stainless steel surface is investigated for various surface temperatures. The effects of the impact velocity and the surface temperature on both spreading and receding of the drops are examined. Experiments are conducted at nine surface temperatures and four impact velocities. The impact morphology reveals that the minimum surface temperature required for the initiation of the bubble nucleation is independent of the Weber number or the drops’ initial kinetic energy. The Leidenfrost point is not observed even at the highest temperature tested for the entire Weber number range. The average receding velocity is not found to depend on Weber number at a given surface temperature in the film evaporation and the film boiling regime. It increases initially with surface temperature up to 280 °C and remains constant for further higher surface temperatures. Although the receding distance increases monotonously with the surface temperature, the average receding velocity does not increase beyond 280 °C.

Automated Control Of Aspiration Of Explosion-And Fire-Hazardous Industries With Filters-Dust Collectors

The article presents a scheme for automated control of an aspiration system equipped with filters-dust collectors for production associated with the circulation of explosive dust- gas flows. As an indicator of the state of the entire aspiration, it is proposed to use the total pressure drop on the filter partition and the temperature shown by the pressure sensors. Based on the experimental study of the unrelated layer structure filter total pressure drop dynamics a mathematical description, algorithm and software for trouble-free operation of aspiration were obtained.

Influence of Microstructure Topography on the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces.

This report investigates the influence of microstructure topography on the restitution coefficient, maximum spreading diameter, and contact time of oblique drop impacts on superhydrophobic surfaces. The five surfaces tested allow for comparison of open- versus closed-cell structures, feature size and spacing, and hierarchical versus nanoscale-only surface structures. By decoupling the restitution coefficient into a normal (εn) and tangential component (εt), it is demonstrated that both εn and εt are largely independent of the microstructure topography. Instead, the restitution coefficient is governed almost exclusively by the normal Weber number. Next, a new model is presented that relates the maximum spreading diameter to an adhesion coefficient that characterizes the overall adhesive properties of the superhydrophobic microstructure during drop rebounding. Through this analysis, we discovered that surface geometries with greater microstructure roughness (i.e., overall surface area) promote a higher maximum spreading diameter than flatter geometries. Furthermore, the contact time of drop impacts on flat surfaces is positively correlated with the impact velocity due to penetration of the liquid into the porous nanostructure. However, this trend reverses for oblique impacts due to the presence of stretched rebounding behavior. Finally, substrates patterned with sparse pillar microstructures can exhibit pancake bouncing behavior, resulting in extremely low contact times. This unique bouncing mechanism also significantly influences the restitution coefficient and spreading diameter of oblique impacts.

Controlling the rebound on a solid surface by varying impact angles of ellipsoidal drops

Enhancing drop deposition on solid surfaces has received significant attention in various fields. Breaking the circular symmetry in typical impact dynamics has opportunities for altering the mass and momentum distributions significantly and improving the deposition. Here, we study the impact dynamics of ellipsoidal drops on nonwetted solid surfaces to reduce the bounce magnitude as a function of the impact angle and ellipticity. Experimental and numerical studies reveal that the ellipsoidal drop with the impact angle shows a strong reduction in the maximum bounce height, compared with the spherical drops. The oblique drop impact exhibits a remarkable feature of the off-axis aligning process caused by asymmetric retraction dynamics. Axial momentum analyses help us to interpret the underlying principle behind the peculiar retraction dynamics and establish a transition map of the rebound and deposition for varying angles and ellipticities. We believe that a breakup of the symmetry in the dynamics can provide practical implications for the control of drop deposition in diverse applications, such as spraying, coating, and cooling.

Motion of self-rewetting drop on a substrate with a constant temperature gradient

Abstract

Dynamics of liquid drop on a vibrating micro-perforated plate

Abstract The present work presents the behavior of a liquid drop placed onto a surface of the perforated plate experimentally. Micro holes were created on the brass plate by a laser marking method. The actuator made of brass and piezoelectric discs driven by a sine input voltage, and it bonded rigidly to the perforated plate. This study aimed to reveal the behavior of the drop placed on the perforated plate and the effects of the initial acceleration value on the rate of change in the drop projected area onto the surface. For this purpose, a frequency-response function of the dry micro-perforated plate was measured by a laser vibrometer, and the acceleration distribution was determined on the perforated plate. Further, the drop was excited with five different initial acceleration values, and the drop projected area onto the surface was recorded by a camera during the atomization process. It was observed that droplets were atomized at the air–liquid interface on two sides of the perforated plate. The time-varying function of the drop projected area onto the surface for each initial acceleration value was presented graphically. The polynomial function was fitted to the experimental data points for each acceleration value, and thus the effects of the initial acceleration on the varying drop size were analyzed. The authors emphasized that the drop projected area onto the surface first increased and then decreased during the atomization process. The rate of change in the drop projected area onto the surface increased as the initial acceleration increased. The drop was atomized both by ejecting from the surface and sucking through the holes.

Slippage effect on interfacial destabilization driven by standing surface acoustic waves under hydrophilic conditions

The influence of the slippage phenomenon over the interfacial dynamics of a millimeter-order fluid drop exposed to surface acoustic wave atomization is numerically studied under hydrophilic conditions. Implementation of the Navier-slip model into the governing hydrodynamic equations yields an interfacial evolution equation. The solution suggests that slippage at the wall constitutes a valuable phenomenon to manipulate the parent drop geometric aspect ratio and consequently the characteristic aerosol size during the atomization process.

Effect of non-Newtonian fluid rheology on an arterial bypass graft: A numerical investigation guided by constructal design

In post-operative scenarios of arterial graft surgeries to bypass coronary artery stenosis, fluid dynamics plays a crucial role. Problems such as intimal hyperplasia have been related to fluid dynamics and wall shear stresses near the graft junction. This study focused on the question of the use of Newtonian and non-Newtonian models to represent blood in this type of problem in order to capture important flow features, as well as an analysis of the performance of geometry from the view of Constructive Theory. The objective of this study was to investigate the effects rheology on the steady-state flow and on the performance of a system consisting of an idealized version of a partially obstructed coronary artery and bypass graft. The Constructal Design Method was employed with two degrees of freedom: the ratio between bypass and artery diameters and the junction angle at the bypass inlet. The flow problem was solved numerically using the Finite Volume Method with blood modeled employing the Carreau equation for viscosity. The Computational Fluid Dynamics model associated with the Sparse Grid method generated eighteen response surfaces, each representing a severe stenosis degree of 75% for specific combinations of rheological parameters, dimensionless viscosity ratio, Carreau number and flow index at two distinct Reynolds numbers of 150 and 250. There was a considerable dependence of the pressure drop on rheological parameters. For the two Reynolds numbers studied, the Newtonian case presented the lowest value of the dimensionless pressure drop, suggesting that the choice of applying Newtonian blood may underestimate the value of pressure drop in the system by about 12.4% (Re =150) and 7.8% (Re = 250). Even so, results demonstrated that non-Newtonian rheological parameters did not influence either the shape of the response surfaces or the optimum bypass geometry, which consisted of a diameter ratio of 1 and junction angle of 30°. However, the viscosity ratio and the flow index had the greatest impact on pressure drop, recirculation zones and wall shear stress. Rheological parameters also affected the recirculation zones downstream of stenosis, where intimal hyperplasia is more prevalent. Newtonian and most non-Newtonian results had similar wall shear stresses, except for the non-Newtonian case with high viscosity ratio. In the view of Constructal Design, the geometry of best performance was independent of the rheological model. However, rheology played an important role on pressure drop and flow dynamics, allowing the prediction of recirculation zones that were not captured by a Newtonian model.

Dynamics of drop absorption by a swelling fiber.

Swelling of individual fibres exposed to a favorable solvent may affect the mechanical properties or shape of a fibrous material. We provide experimental results of the absorption dynamics of a single drop deposited on a swellable fibre and show that the total absorption time is highly dependent on the fibre geometry and drop volume. The curvature of the fibre prevents the total spreading of a drop even if the fluid is fully wetting the fibre. For drops larger than a critical volume, a local saturation of the fibre is thus reached below the drop, leading to a strong increase in the total absorption time. These observations are then rationalized with a simple pseudo-diffusive model to understand the drop absorption and fibre swelling dynamics. This minimal model provides quantitative predictions of the total absorption time.

THE EFFECT OF SUBSTRATE SURFACE ON THE DYNAMICS OF SESSILE DROP UNDER AXISYMMETRIC VIBRATIONS

THE EFFECT OF SUBSTRATE SURFACE ON THE DYNAMICS OF SESSILE DROP UNDER AXISYMMETRIC VIBRATIONS

Innovations in Non-Linear Oscillations of a Pendent Drop From a Capillary Tip During Formation and Detachment - An LBM Simulation

Individual drops are suitable tools to study the liquid-fluid interfacial properties. In this work, forcedisplacement equation and non-linear oscillations of a pendent drop are numerically investigated. The presented novel force-displacement function allows following the dynamics of a pendent drop and realizing its elastic behavior. The growth and detachment of drop, which is pending due to gravity from a capillary tip, is considered (assuming high density and high viscosity ratios and immiscible two-phase flows). Twodimensional multi-relaxation time lattice Boltzmann method (MRT-LBM) was used to simulate growth, detachment, and oscillations of the drop using a conservative model for high-density ratio. The forcedisplacement function of a pendent drop (FDFPD), which is non-linear, was introduced. Using FDFPD, the non-linear elastic specifications of the pendent drop were determined. It was realized that the drop shows three different elastic behaviors simultaneously (hardening, linear, and softening). The drop superharmonic and subharmonic frequencies were calculated, using the natural frequency of the linear portion of FDFPD. Besides, the drop would grow as long as its displacement is between the extrema of FDFPD. In addition, a dynamic criterion for the onset of detachment was established. Also, increasing the Bond number from 0.11 to 1.96, while keeping Reynolds number equal to 0.023, accelerates the drop detachment and increases the linear portion of FDFPD. It was shown that increasing Capillary number from 1.8E-5 to 7.3E-4, while keeping Reynolds number equal to 0.023, accelerates the drop detachment and increases the non-linear portions of FDFPD.

Investigations into the Complete Spreading Dynamics of a Viscoelastic Drop on a Spherical Substrate.

We study the spreading dynamics of a sphere-shaped elastic non-Newtonian liquid drop on a spherical substrate in the capillary-driven regime. We use the simplified Phan-Thien-Tanner model to represent the rheology of the elastic non-Newtonian drop. We consider the drop to be a crater on a flat substrate to calculate the viscous dissipation near the contact line. Following the approach compatible with the capillary-viscous force balance, we establish the evolution equation for describing the temporal evolution of the contact line during spreading. We show that the contact line velocity obtained from the theoretical calculation matches well with our experimental observations. Also, as confirmed by the present experimental observations, our analysis deems efficient to capture the phenomenon during the late stage of spreading for which the effect of line tension becomes dominant. An increment in the viscoelastic parameter of the fluid increases the viscous dissipation effect at the contact line. It is seen that the higher dissipation effect leads to an enhancement in the wetting time of the drop on the spherical substrate. Also, we have shown that the elastic nature of the fluid leads to an increment in the dynamic contact angle at any temporal instant as compared to its Newtonian counterpart. Finally, we unveil that the phenomenon of the increasing contact angle results in the time required for the complete wetting of drop, which becomes higher with increasing viscoelasticity of the fluid. This article will fill a gap still affecting the existing literature because of the unavailability of experimental investigations of the spreading of the elastic non-Newtonian drop on a spherical substrate.

Coupling between wetting dynamics, Marangoni vortices, and localized hot cells in drops of volatile binary solutions

HypothesisA sessile drop comprising a mixture of volatile solvents supports spatial variations in interfacial energy, which gives rise to solutal Marangoni flow, alongside evaporative loss of drop mass. Both the Marangoni flow and evaporation bring about a dance of concurrent and inter-connected phenomena: internal Marangoni vortices, localized hot cells, and complex wetting dynamics. ExperimentWe employ Particle Image Velocimetry and Infra-Red Microscopy to visualize Marangoni vortices, temperature variations, and the wetting dynamics of drops of toluene and ethanol mixtures. FindingsThe intensity of the measured phenomena vary concurrently in time and in like manner according with the initial composition of drops. In particular, we observe maximum intensity levels when the initial toluene proportion in the drops is 60%, and none of these phenomena in the case of pure toluene. Moreover, the drops initially expand on the solid in response to Marangoni flow, then contract due to evaporation; between these dynamic wetting regimes, we further observe a regime of one or periodic wetting/de-wetting cycles at low toluene concentrations. Our findings indicate that both the solutal Marangoni flow and evaporation drive the different phenomena we observe and confirm the connection between Marangoni vortices and the formation of localized hot cells.

Electric field modulated deformation dynamics of a compound drop in the presence of confined shear flow

Dynamics of compound drops is central in several emerging applications including emulsion-fueled direct injection, targeted drug delivery, and the development of mechano-sensitive artificial cells. These applications are commonly hallmarked by high shear rates in confined fluidic environments. In the present work, we depict the role of the transverse electric field in controlling the resulting morpho-dynamics, including non-monotonic topological evolution and pinch-off phenomenon. In a tightly confined domain, we further show that the critical value of the electric field for triggering the pinch-off phenomenon may be greatly reduced as compared to larger-sized conduits. Finally, we portray a non-trivial variation of the drop pinch-off time with the degree of domain confinement, as attributed to the complex electro-hydrodynamic coupling over small scales. These results may turn out to be critical in manipulating the shape evolution of bio-mimetic soft matter in physiologically relevant fluidic pathways and on-chip applications.

Enhancing the Asymmetry of Bouncing Ellipsoidal Drops on Curved Surfaces.

Reducing the residence time of drops on solids has been attracting much attention in a wide range of industrial methods, such as self-cleaning and anti-icing. Classical drop dynamics is generally confined to circular symmetry and a theoretical limit of the bouncing time. In this study, we investigate the bouncing dynamics of ellipsoidal drops on cylindrical surfaces. Experimental and numerical results show that, compared with spherical ones, ellipsoidal shapes create the synergy effect of a preferential flow along the curved side, thereby leading to a significant reduction in the residence time when the drop's major axis coincides with the cylinder's axial direction. The effects of the drop ellipticity and surface curvature on the bouncing dynamics are investigated for several Weber numbers and discussed through momentum analyses. The proposed concave/convex decorated models demonstrate the feasibility of the further reduced residence time by enhancing the asymmetry in the mass and momentum distributions. This study can provide a new perspective to shape-dependent impact dynamics by emphasizing the importance of the geometric configuration between ellipsoidal drops and anisotropic surfaces in determining the extent to which the dynamics are asymmetric.

Dynamics of an elastoviscoplastic droplet in a Newtonian medium under shear flow

The dynamics of a single elastoviscoplastic (EVP) drop immersed in plane shear flow of a Newtonian fluid is studied by 3D direct numerical simulations using a finite-difference and level-set method combined with the Saramito model for the EVP fluid. This model gives rise to a yield stress behavior, where the unyielded state of the material is described as a Kelvin-Voigt viscoelastic solid and the yielded state as a viscoelastic Oldroyd-B fluid. Yielding of an initially solid drop of Carbopol is simulated under successively increasing shear rates. We proceed to examine the roles of nondimensional parameters on the yielding process; in particular, the Bingham number ($Bi$), capillary number ($Ca$), Weissenberg number ($Wi$), and ratio of solvent and total drop viscosity. We find that all of these parameters, and not only $Bi$, have a significant influence on the drop dynamics. Numerical simulations predict that the volume of the unyielded region inside the droplet increases with $Bi$ and $Wi$, while it decreases with $Ca$ at low $Wi$ and $Bi$. A new regime map is obtained for the prediction of the yielded, unyielded, and partly yielded modes as a function of $Bi$ and $Wi$. The drop deformation is studied and explained by examining the stresses in the vicinity of the drop interface. The deformation has a complex dependence on $Bi$ and $Wi$. At low $Bi$, the droplet deformation shows a nonmonotonic behavior with an increasing $Wi$. In contrast, at moderate and high $Bi$, droplet deformation always increases with $Wi$. Moreover, it is found that the deformation increases with $Ca$ and with the solvent to total drop viscosity ratio. A simple ordinary differential equation model is developed to explain the various behaviours observed numerically. The presented results are in contrast with the heuristic idea that viscoelasticity in the dispersed phase always inhibits deformation.

Benchmark solutions for flows with rheologically complex interfaces

Abstract Complex fluid–fluid interfaces determine for a large part the macroscopic material properties of foams and emulsions that appear in applications such as food, materials processing and consumer care products. As a step towards predicting these properties, a 2D axisymmetric and a 3D finite element model have been developed for simulating the dynamics of a single Newtonian drop in a Newtonian matrix fluid with a rheologically complex sharp interface in between. Interfaces with constant interfacial tension and with viscous, elastic and viscoelastic extra interfacial stresses are considered. The model has been validated by means of the method of manufactured solutions and by comparison with results from other studies using different discretisation methods. Higher-order convergence in space and time is obtained, demonstrating correct implementation of our numerical methods. Benchmark solutions for a drop with a Kelvin–Voigt interface under simple shear flow have been provided. Compared to a viscous interface, the drop deformation becomes smaller and the drop becomes less oriented in the direction of flow if interfacial elasticity is added.

Statics and dynamics of drops spreading on a liquid-liquid interface

Lattice Boltzmann simulations along with a diffuse interface model are used to simulate the capillary driven, symmetric spreading of a fluid drop on another fluid-fluid interface. While the equilibrium shapes of the drops with negative spreading coefficient are comprehensively represented on a phase diagram, the dynamics leading to equilibrium shapes are shown to be universal with power-law behaviors exhibited only in the limit of the small spreading coefficient.

Deposits from evaporating emulsion drops

The processes in which droplets evaporate from solid surfaces, leaving behind distinct deposition patterns, have been studied extensively for variety of solutions. In this work, by combining different microscopy techniques (confocal fluorescence, video and Raman) we investigate pattern formation and evaporation-induced phase change in drying oil-in-water emulsion drops. This combination of techniques allows us to perform drop shape analysis while visualizing the internal emulsion structure simultaneously. We observe that drying of the continuous water phase of emulsion drops on hydrophilic surfaces favors the formation of ring-like zones depleted of oil droplets at the contact line, which originate from geometrical confinement of oil droplets by the meniscus. From such a depletion zone, a “coffee ring” composed of surfactant molecules forms as the water evaporates. On all surfaces drying induces emulsion destabilization by coalescence of oil droplets, commencing at the drop periphery. For hydrophobic surfaces, the coalescence of the oil droplets leads to a uniform oil film spreading out from the initial contact line. The evaporation dynamics of these composite drops indicate that the water in the continuous phase of the emulsion drops evaporates predominantly by diffusion through the vapor, showing no large differences to the evaporation of simple water drops.

Analysis of the drop dynamics of control rod assembly in a LBE-cooled research reactor

The lead-based fast reactor is very attractive as one of the Generation-IV nuclear energy systems due to high-power density, heavy liquid metal coolants with a high boiling point as well as harmless interaction with air or water. Nuclear heat is generated from very high neutron flux that is regulated and controlled by the drop of the Control Rods (CRs) for safe operation. The resisting effect of the Lead Bismuth Eutectic (LBE) due to high density on the driving force of the CR Assembly (CRA) during the fall is crucial, which some delays may lead to unavoidable accidents. The mathematical approach has been applied to investigate the CRA dynamics of lead-based research reactor. The non-linear differential equations governing the motion of the CRA as well as the LBE flow have been ascertained to capture the pertinent parameters. Using the Fourth-Order Runge-Kutta iteration method, a computer program was developed with MATLAB and it can be used to calculate the drop time of the CRA and other important parameters described and presented in the study. Compared with the results of the experiment in the available literature, the program developed was proved to be applicable and reliable. The results showed that among the LBE flow characteristics, the density is the most influential parameter on the drop time and tends to have a positive relationship with the drop time. The total CRA mass contributes to the driving force of the CRA to reduce drop time when increased. Finally, the result of the theoretical model will serve as a basis to analyse the auxiliary mechanism and thus, help make informed choice on LBE flow parameters during modification to improve safety.