Drop Dynamics Research Articles

Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate.

We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this scenario the triple Leidenfrost effect.

Numerical study of microfluidic emulsion dynamics under the influence of heterogeneous surface wettability

Surface wettability plays a vital role in various natural phenomena and technical applications, such as coating, printing, membrane technology, microfluidic devices, and porous media flow. We numerically investigate the dynamics and stability of liquid–liquid dispersion flowing in a microfluidic channel with heterogeneous surface wettability, using a conservative phase-field lattice Boltzmann method. First, we validate the implementation of the wetting boundary conditions. We then perform three-dimensional simulations of a single drop motion immersed in another immiscible liquid in a square microchannel with alternating surface wettability: hydrophilic, hydrophobic, and again hydrophilic sections. The viscosity of the dispersed liquid (oil) is ten times higher than that of the continuous one (water), while the two liquids have equal densities. Depending on the drop length and Capillary number, calculated based on the drop velocity and the dynamic viscosity of the continuous phase, four different flow patterns are observed when the drop passes through the hydrophobic section. The distinct flow patterns include passing without any changes in drop shape or dynamics, adhesion of the dispersed liquid to the walls, phase inversion (i.e., water becomes the dispersed phase), and drop breakage. These outcomes are in close agreement with the experimental data. A detailed explanation of the choice of the numerical parameters is presented. The critical aspect of capturing the observed drop behavior is introducing an intervening thin-film of the continuous phase of at least three diffuse interface thicknesses (between the drop and channel walls).

Dynamics of a viscous drop under an oscillatory uniaxial extensional Stokes flow

We quantify the transient deformation and breakup of a neutrally-buoyant drop with viscosity λμ∗ immersed in another Newtonian fluid with viscosity μ∗ undergoing oscillatory uniaxial extension at zero Reynolds number. The interfacial tension acting between drop phase and medium phase is γ∗. The drop is initially a sphere of radius a∗. Since the external flow oscillates harmonically with a frequency ω∗, the strength of the imposed flow is characterized by an instantaneous capillary number, Ca=Ca0cos(Det), where Ca0=μ∗ε̇a∗/γ∗ and De=ω∗μ∗a∗/γ∗ is the dimensionless frequency, or Deborah number. Here, ε̇ is the rate of extension in the imposed flow. We utilize boundary-integral computations to calculate the evolution of drop interface as a function of Ca0 and De, focusing primarily on the case where the drop and surrounding fluid have equal viscosities. The computations suggest two families of behavior for the drop deformation. First, below a critical Deborah number (which we determine to be in the interval 0.375<De<1.0), the drop breaks up in a finite time at a critical capillary number that is a function of De. At sufficiently small De the critical capillary number increases linearly with De and the breakup mode is that of “center-pinching”. On increasing De the break-up mode switches to “end-pinching”, and on further increasing De it appears that the critical capillary number diverges at a critical Deborah number between 0.375 and 1.0. This divergence signals the transition to the second family of behavior where the drop attains a long-time periodic state, or alternance, regardless of Ca0. However, at large Ca0 the drop dynamics exhibits a two time-scale behavior: the drop deforms instantaneously at a fast capillary relaxation time-scale τ∗=μ∗a∗/γ∗, whereas the maximum deformation attained during a cycle of the imposed flow grows at the slow time-scale τ∗Ca0=ε̇(μ∗a∗/γ∗)2. As such, the state of alternance is approached exceedingly slowly at large Ca0. Lastly, we perform calculations for different viscosity ratios and find the drop dynamics to be in qualitative agreement with above observations.

Modeling the dynamics of charged drop of one liquid in another under the action of an electric field

The work is devoted to the study of the features of the behavior of a group of droplets of one viscous liquid in another under the influence of various physical fields. When considering the dynamics of two drops under the action of an electric field it is assumed that a drop in the form of a sphere with radius а will be placed in an electric field with an intensity E , investigates how droplets will react to each other under the influence of an electric field. A mathematical model has been built and a computer program has been developed for the nu- merical solution of this problem. The behavior of several drops in an electric field is studied for different physical parameters of the material of the drops and the environment, as well as for different initial distribu- tions of drops and the strength of the electric field. It is shown for the first time that emulsion droplets distrib- uted in space, under the action of an electric field, begin to move and after a certain time a new stationary structure of droplets is formed. It was found that the relaxation time depends on the electric field strength, the size of the droplets and their initial distribution.

Drop dynamics and size distribution in a dense spray produced by a twin-fluid atomizer

Drop dynamics and size distribution in a dense spray produced by a twin-fluid atomizer

Influence of lubricant addition on dynamics of a single drop impacting onto a very hot substrate

Influence of lubricant addition on dynamics of a single drop impacting onto a very hot substrate

Newtonian and Non-Newtonian Effects on the Collision Dynamics of a Liquid Drop with a Static Drop Located on Smooth Solid Surface

Newtonian and Non-Newtonian Effects on the Collision Dynamics of a Liquid Drop with a Static Drop Located on Smooth Solid Surface

Drop Bouncing Dynamics on Ultrathin Films

For drops to contact various surfaces, the removal of the interstitial fluid is the prerequisite to contact. While the conventional understanding is for drops to irreversibly spread on a film made of the same substance, we describe the dynamics of drops initiating contact yet carrying enough momentum to completely lift off of the substrate which we label as contact bouncing. We report new experimental results of the dynamics between drops impacting thin films described by the ratio of the liquid film hL to the drop with diameter D0 for the range of 0.004 < hL/D0 < 0.08. Using high-speed interferometry, we visualize the interfacial gas layer spatiotemporal signatures across the various film thicknesses and Weber numbers. We find that while increasing the deformability of the thin films enhances the gas entrainment phenomenon at early times, it also increases the rate of the gas purging rate, increasing the chance of contact just prior to the gas film retraction and drop lift off sequence. Drops which contact the liquid film during the retraction stage are able to bounce with <5% volume loss.

Dynamics of Drops as Confined Systems Containing Nanoparticles-Reformation of Nanoparticles and Evaporation of Water

Dynamics of Drops as Confined Systems Containing Nanoparticles-Reformation of Nanoparticles and Evaporation of Water

Numerical investigation of respiratory drops dynamics released during vocalization.

Release of drops from a human body has been the focus of many recent investigations because of the current COVID-19 pandemic. Indirect virus transmission from asymptomatic individuals has been proved to be one of the major infectious routes and difficult to quantify, detect, and mitigate. We show in this work a detailed and novel numerical investigation of drops released during vocalization from a thermal manikin using a large eddy simulation coupled with Lagrangian tracking of drops. The vocalization experiment was modeled using existing data from the literature for modeling exhaled airflow, emission rate, and size distribution. Particular focus was on the definition of the boundary conditions for the exhalation process. Turbulence was compared with experimental data for the near mouth region for 75 exhalation breathing cycles and showed the sensitivity of different modeling assumptions at the mouth inlet. The results provide insights of special interest for understanding drop dynamics in speech-like exhalation modes, modeling the mouth inlet boundary conditions, and providing data for verifying other more simplified models.

Ellipsoidal drop impact on a single-ridge superhydrophobic surface

A water drop impacting on a superhydrophobic surface exhibits a complete bounce after a certain residence time bounded by a theoretical time scale. Recent studies have demonstrated that the residence time is modified below the time scale through an asymmetric bouncing on a ridge surface. However, previous studies still assumed initially spherical shapes of drops, focusing on a high impact velocity for a desirable residence time. Herein, we propose asymmetric bouncing dynamics of ellipsoidal drops on a single-ridge superhydrophobic surface to create synergistic cooperation between the initial drop shapes and the ridge structures. Numerical simulations demonstrate the feasibility of reducing the residence time by approximately 55% using a volume-of-fluid method, compared with a conventional drop impact on flat surfaces. We examine how the drop's ellipticity and impact velocity affect the spreading, retraction, and splitting and interpret the underlying mechanism behind the residence time reduction by quantifying the evolutions of the shape and axial momenta of the drops. We also predict the bouncing directions of the drops and the effect of the off-centred distances and ridge sizes in the height and width on the residence time. These findings are expected to illuminate the fundamentals of asymmetric bouncing on a ridge and provide beneficial guidance to strategies such as self-cleaning and anti-icing.

The role of oscillation in ellipsoidal drop impact on a solid surface

Ellipsoidal shapes of drops can significantly modify the impact dynamics and suppress the rebound by inducing symmetry breaking in the mass and momentum distributions compared to the axisymmetric dynamics of typical drops. However, the previous works have assumed that the drop oscillation at the moment of impact only slightly affects the post-dynamics although the oscillation must be involved in the spreading. Here, we study the impact dynamics of the oscillating ellipsoidal drops on non-wetting surfaces as a function of the ellipticity, oscillation phase, and Weber number (We) experimentally and numerically. The spreading dynamics show notable hysteretic features in the maximal spreading diameters at the four regions of the oscillation phase. The hysteresis appears more prominently in prolate drops than in oblate drops and becomes remarkably suppressed at the four phases as We increases. Momentum analysis shows that the phases for shaping the drops spherically can drive higher asymmetry in the horizontal momenta than the other phases for shaping the drops ellipsoidally. The momentum asymmetry in the horizontal axes indicates that the oscillation phase as well as the ellipticity can play an important role in altering the hydrodynamics and reducing the bounce magnitude.

Drops on the Underside of a Slightly Inclined Wet Substrate Move Too Fast to Grow

Pendant drops suspended on the underside of a wet substrate are known to accumulate fluid from the surrounding thin liquid film, a process that often results in dripping. The growth of such drops is hastened by their ability to translate over an otherwise uniform horizontal film. Here we show that this scenario is surprisingly reversed when the substrate is slightly tilted (≈2°); drops become too fast to grow and shrink over the course of their motion. Combining experiments and numerical simulations, we rationalize the transition between the conventional growth regime and the previously unknown decay regime we report. Using an analytical treatment of the Landau-Levich meniscus that connects the drop to the film, we quantitatively predict the drop dynamics in the two flow regimes and the value of the critical inclination angle where the transition between them occurs.

Thermocapillary effects on eccentric compound drops in Poiseuille flows

We analytically study the motion and deformation of an eccentric compound drop subject to an externally imposed temperature gradient and suspended in a Poiseuille flow, using a bispherical coordinate system. The temperature gradient alters surface tensions at the drop interfaces and triggers strong Marangoni flows, altering the drop dynamics. While a positive temperature gradient speeds up both drops, a negative gradient can potentially lead to a stable equilibrium where both drops move with the same velocity. The Marangoni effect also enhances deformation, particularly in the inner drop, which may take either a prolate or an oblate shape.

Bouncing dynamics of spheroidal drops on macro-ridge structure

Bouncing drops on solid surfaces have gained increasing attention for various practical applications, such as self-cleaning and anti-icing strategies. Breaking the circular symmetry in bouncing dynamics on a ridge enables drop dynamics to be modified significantly and the residence time of drops on surfaces to be reduced. Here, we numerically investigate the asymmetric bouncing dynamics of oblate and prolate spheroidal drops on a superhydrophobic surface decorated with a rectangular ridge to demonstrate the feasibility of further reducing the residence time by shaping raindrop-like drops. The residence time is investigated for various aspect ratios and Weber numbers, which are discussed based on impact stages of spreading, splitting, and retraction. The underlying principle behind the residence time reduction is analyzed by quantifying the temporal variations in the widths and the axial momenta of the drops. The bouncing directions of the spheroidal drops are closely related to the momentum distributions during the retraction. We investigate how to change the residence time for ridges of different heights and widths. The symmetry-breaking bouncing of the spheroidal drops on ridge surfaces will provide fundamental and practical inspiration for the efficient control of drop mobility.

Effects of surfactant solubility on the hydrodynamics of a viscous drop in a dc electric field

The physico-chemistry of surfactants (amphiphilic surface active agents) is often used to control the dynamics of viscous drops and bubbles. Surfactant sorption kinetics has been shown to play a critical role in the deformation of drops in extensional and shear flows, yet to the best of our knowledge these kinetics effects on a viscous drop in an electric fieldhave not been accounted for. In this paper we numerically investigate the effects of sorption kinetics on a surfactant-covered viscous drop in an electric field. Over a range of electric conductivity and permittivity ratios between the interior and exterior fluids, we focus on the dependence of deformation and flow on the transfer parameter $J$, and Biot number $\text{Bi}$ that characterize the extent of surfactant exchange between the drop surface and the bulk. Our findings suggest solubility affects the electrohydrodynamics of a viscous drop in distinct ways as we identify parameter regions where (1) surfactant solubility may alter both the drop deformation and circulation of fluid around a drop, and (2) surfactant solubility affects mainly the flow and not the deformation.

Reagent-free hemoglobin estimation on a spinning disc

Quantitative estimation of hemoglobin concentration is a commonly required clinical test to assess various manifestations of pathological conditions in blood. While several invasive and non-invasive approaches have been introduced for rapid low-cost determination of hemoglobin levels in patients, these methods have been reported to suffer from various constraints, limiting their applicability for precise quantitative analysis. Methods reporting high levels of quantitative accuracy, on the other hand, are severely challenged for their deployment in resource-limited settings, as these necessitate specialized reagents that demand exclusive storage and transportation facilities as well as trained human resources for deployment in the field. Circumventing these constraints, here we demonstrate a low cost (<$0.01 each test) reagent-free method for hemoglobin estimation on a simple portable unit that can be implemented in under-served locations without sacrificing the fundamental principle of direct evaluation of hemoglobin extracted from the human blood sample. Exploring the fundamental principle of osmotic hemolysis for extraction of hemoglobin from red blood cells, the method harnesses the dynamics of a blood drop on a rotating platform and simple imaging to come up with results in a turn-around time of about 13 min. The efficacy of the device has been justified by validating with established pathological gold standards. These results are likely to pave the pathway of establishing the paradigm of a first-principle based reagent-free evaluation of blood pathology in health and disease.

Pairwise interactions of surfactant-covered drops in a uniform electric field

The effect of surfactant on the pairwise interactions of drops in an applied uniform DC electric field is studied using a combination of numerical simulations and an analytical theory assuming small drop deformations. We show that the surfactant weakens the electrohydrodynamic flow and thus dielectrophoretic interactions play a more prominent role in the dynamics of surfactant-covered drops compared to clean drops. If drop conductivity is the same as the suspending fluid, a nondiffusing surfactant can arrest the drops' relative motion, thereby effectively preventing coalescence.

The Forced Oscillations of an Oblate Drop Sandwiched Between Different Inhomogeneous Surfaces under AC Vibrational Force

The dynamics of an incompressible fluid drop under the action of non-uniform electric field are considered. The drop is bounded axially by two parallel solid planes, which in the examined case are considered to be heterogeneous. The external electric field acts as an external force which causes motion of the contact line. In equilibrium, the drop has the form of a circular cylinder. The equilibrium contact angle is $$0.5\pi$$ . In order to describe the motion of the contact line the modified Hocking boundary condition is applied: the velocity of the contact line is proportional to the deviation of the contact angle and the rate of fast relaxation process, the frequency of which is proportional to twice the frequency of the electric field. The Hocking parameter depends on the polar angle $$\alpha$$ , i.e. the coefficient of the interaction between the plate and the fluid (the contact line) is a function of the plane coordinates. The focus of this study is on a special case, in which this function is proportional to $$|\cos (\alpha )|$$ .

Hydrodynamics of detachment, free falling and impact of drops

The results of optical measurements of drop dynamics are presented. Oscillations and waves on its surface were traced. The pattern of the secondary droplets falls onto an immersed drop and the discrete distribution of the substance of a uniformly colored drop in the targeted liquid was visualized. The important role of energy transport processes, both fast – local atomic-molecular – and slow that are translational and dissipative, in the formation of flow patterns is highlighted. A system of fundamental equations of fluid mechanics with equations of state for the Gibbs potential and density is applied. Ligaments – thin trickles with scales from atomic-molecular and macroscopic sizes – are investigated. Their images in the family of intrinsic solutions of the fundamental system and drop impact phenomena are shown.