Motion Of Liquid Droplets Research Articles

Additive manufacturing of heterogeneous combinatorial functional surface by plasmonic hierarchical sintering of silicon particles for active manipulation of rheological liquid motion

We present a sustainable and efficient additive manufacturing method of silicon-based heterogeneous combinatorial functional surfaces designed to actively manipulate liquid droplet motion dynamics to address advanced rheological engineering challenges and applications. This additive manufacturing enables the instantaneous formation and control of hierarchical multiscale structures with tunable wettability through instantaneous plasmonic thermophysical sintering between laser and Si particles, eliminating the need for additional masks and subsequent processing steps. Furthermore, this fabrication approach can selectively implement heterogeneous combinatorial functional surfaces in a single domain by reversibly switching extreme wettability modes (e.g., from superhydrophobic to superhydrophilic) upon laser irradiation. Continuous superhydrophilic channels in a superhydrophobic background created by selective laser re-irradiation provide sufficient local attraction to manipulate droplet motion along the channel due to van der Waals forces and Laplace pressure fields generated by the difference in wettability. Active manipulation of droplet dynamic motion, such as trajectory tracking and antigravity self-propulsion, can be realized by simply designing a laser scanning path that determines the geometry of the local channel. The manipulation platform for liquid motion dynamics can be applied to active microfluidic channels with no cavity, without the need for an external power source. This advancement has important implications for broad fluid and rheological engineering applications.

Effective Unidirectional Wetting of Liquids on Multi-Gradient, Bio-Inspired Surfaces Fabricated by 3D Printing and Surface Modification.

The movement of liquid droplets on the energy gradient surface has attracted extensive attention inspired by biological features in nature, such as the periodic spindle-shaped nodes in spider silks and conical-like barbs of cacti, and the structure-property-function relationship of multifunctional gradient surfaces. In this study, a series of specific patterns are fabricated with 3D printing technology, followed by modification via the atmospheric pressure plasma treatment and liquid phase chemical deposition, resulting in enhancing the ability of water droplets of 5 μL to travel 18.47 mm on a horizontal plane and 22.75 mm against gravity at up to a 20° tilting angle. Additionally, analysis techniques have been employed, including a contact angle analyzer, ESCA, and a laser confocal microscope to evaluate the sample performance. This work could further be applied to many applications related to microfluidic devices, drug delivery and water/fog collection.

DROPLET MOTION ON HYDROPHOBIC AND HYDROPHILIC SURFACES USING LATTICE BOLTZMANN METHOD WITH BLOOD TEST MICROPAD APPLICATION

Laboratory sciences and disease diagnostics need accelerated medical tests, ease of testing for patients, and lower diagnostic costs. In this respect, one of the most innovative methods is paper-based microfluidic analysis tools, called micro-PAD. Since the chemical-based micro-PADs need sufficient blood to show an accurate blood test, the shape of the micro-PADs, the blood quantity, and the droplet flow to reach the end of the microchannels are important. Therefore, this research showed the liquid droplet motion on a heterogeneous (hydrophobic and hydrophilic) surface using the lattice Boltzmann method, and two star-shaped micro-PADs were modeled to investigate the effect of the droplet size and the droplet deflection from the center of the micro-PAD on liquid flow in microchannels. Based on the obtained results, fluid behavior was similar in two patterns (ten-pointed pattern and four-pointed pattern). This study compared the effects of parallel, convergent, and divergent channels in a four-pointed pattern on fluid velocity. The off-center error of the droplet was considered at the initial moment. The obtained results confirmed the direct relationship between the increased deviation of the center and the lack of uniformity in the fluid penetration. The present simulation results were validated against the laboratory data.

Actuating droplets with electrowetting: Force and dynamics

AbstractElectrowetting on dielectric (EWOD) allows rapid movement of liquid droplets on a smooth surface, with applications ranging from lab‐on‐chip devices to micro‐actuators. The in‐plane force on a droplet is a key indicator of EWOD performance. This force has been extensively modeled but few direct experimental measurements are reported. We study the EWOD force on a droplet using two setups that allow, for the first time, the simultaneous measurement of force and contact angle, while imaging the droplet shape at 6000 frames/s. For several liquids and surfaces, we observe that the force saturates at a voltage of approximately 150 V. Application of voltages of up 2 kV, that is, 10 times higher than is typical, does not significantly increase forces beyond the saturation point. However, we observe that the transient dynamics, localized at the front contact line, do not show saturation with voltage. At the higher voltages, the initial front contact line speed continues to increase, the front contact angle temporarily becomes near zero, creating a thin liquid film, and capillary waves form at the liquid–air interface. When the localized EWOD forces at the contact line exceed the capillary forces, projectile droplets form. Increasing surface tension allows for higher droplet forces, which we demonstrate with mercury.

Triboelectric Nanogenerator Array as a Probe for In Situ Dynamic Mapping of Interface Charge Transfer at a Liquid-Solid Contacting.

Contact between water droplets with hydrophobic surfaces is a common phenomenon at functional interfaces, and it has been extensively studied. However, quantifying the charge transfer between the liquid-solid interfacial contacting, especially for the charge density distribution throughout the movement of liquid droplet on a dielectric surface, remains to be investigated. Here, we developed a pixeled droplet triboelectric nanogenerator (pixeled droplet-TENG) array with high-density electrode array as a probe for measuring the charge transfer at a liquid-solid interface when a water drop moves on the hydrophobic surface. To intuitively observe the charge transfer between the liquid-solid interface, we "imaged" the transferred charges along movement trajectory of a water droplet as it slides along a tilted solid surface at a spatial resolution of 0.4 mm and time sensitivity of 0.02 s. Our study shows that the transferred charges are not uniformly distributed along the path, which is possibly due to the two-step model of electron transfer and ion adsorbed on the solid surface, and thus the formation of an electric double layer will inevitably shield the net surface on the solid surface. Our study presents a probe technology with potential applications in surface chemistry, physics, material science, and cell biology.

Electron Tractor Beam: Deterministic Manipulation of Liquid Droplets on Solid Surfaces

AbstractThe motion of liquid droplets is studied across many physics subfields and manipulation of nanoscale droplets on demand holds great promise in, e.g., nanotechnology. In this study, AuGe droplets on a germanium substrate are manipulated by an electron beam in a scanning electron microscope. The electron beam exposure creates a local temperature gradient in a substrate and induces a directional thermomigration of droplets nearby. To quantitatively analyse this phenomenon and to reveal the mechanism behind it, experimental observations under different conditions (beam current, sample temperature, scanning strategy) and simulations are combined. The obtained insights suggest that the droplet motion is limited by the dissolution of the substrate below the droplet. In addition, it is shown that the electron tractor beam is not only able to control the droplet motion but can also be used to split the droplets, thus opening new possibilities for droplet manipulation inside an SEM.

Modeling the dynamics of partially wetting droplets on fibers

We study the motion of liquid droplets moving along partially wettable fibers. Employing lattice Boltzmann simulations, we observe three dynamic regimes: compact droplet, droplet breakup, and droplet oscillation. The transitions between these regimes depend not only on the droplet Bond number but also on the fiber curvature. In particular, droplet breakup is promoted with increasing fiber curvature. Furthermore, we can identify different scaling laws for the droplet velocity when the droplet size is large or small compared to the fiber radius.

Highly efficient liquid droplet manipulation via human-motion-induced direct charge injection

The current electrowetting mechanisms show a low efficiency, although manipulating liquid droplets is essential to biological and chemical fields. Herein, we propose a highly efficient droplet manipulating method using direct charge injection (DCI) via human motion induced triboelectricity. A triboelectric nanogenerator (TENG) is used to provide both the charges and strong electric fields to drive the movement of liquid droplets. Using this method, the charge quantity and average velocity of the droplet (10 μL) reach 0.25 nC and 255 mm s−1, respectively, over 6 times higher than those of traditional methods (0.03 nC and 43.2 mm s−1). Alternative charge injection was demonstrated to enable both reciprocating and jumping motions of the droplet. Finally, we also successfully devised and demonstrated a platform with versatile system-level functions including droplets transportation, positioning, merging, and cleaning. This work advances the current droplet manipulation field via introducing a compelling approach using human-motion-induced direct charge injection.

Numerical estimation of droplet motion on linear wettability gradient surface in microgravity environment

Self-propelled motion of liquid droplets on wettability gradient surface (WGS) is an emerging field due to its various applications. Simulations are performed to investigate droplet motion on WGS under gravity and microgravity conditions. Herein, transient, laminar, 3-dimensional Continuity and Navier-Stokes equation are solved for the computational domain consisting of liquid water droplet on WGS in presence of air, applying no-slip, constant contact angle boundary conditions at surface using ANSYS FLUENT® for the computational domain. Simulation results are validated with the available experimental and simulation results and are found to be in good agreement. Further, a parametric study is performed to exemplify the effect of gravity, initial contact angle, drop volume and surface gradient on droplet velocity, spreading, position and surface shear stress. Present study demonstrates, shape evolution of droplet moving on WGS is attributed to caterpillar alike inching motion in both gravity and microgravity conditions. Droplet is almost stagnant on surface with initial superhydrophobic angle (θhigh > 150°) with low gradient (Δθe = 0.2° and 1°). However, despite of higher spreading droplet accelerates on hydrophobic surfaces (θhigh = 100°) with higher gradient (Δθe = 10°). The effect of gravity is negligible for small droplets. However, substantially affects large droplet due to increased value of Bond number. Present results are helpful for designing and optimizing wettability graded surfaces which endures droplet splitting and coalescence in various microgravity engineering systems. In addition, the WGS are also advantageous for heat transfer augmentation.

Lateral drift of liquid droplets sliding down substrates with nonuniform dissipation

Understanding and controlling the motion of liquid droplets is important both from a fundamental point of view and in many applications. In this work, we theoretically analyze the dynamics of a liquid droplet sliding down a ramp under the action of gravity. We show that when the dissipation (which we account for by means of an effective contact line viscosity) has a gradient in the direction orthogonal to gravity, the droplet, while sliding down, drifts laterally in the direction opposite to the dissipation gradient. We validate our analytical results, obtained in the limit of low Bond numbers, by a numerical solution of our model.

Driving Droplets on Liquid Repellent Surfaces via Light‐Driven Marangoni Propulsion

AbstractMarangoni flow, a surface shear flow, is a promising option for propulsion when controlling liquid droplet motion on solid substrates. However, the applicability of Marangoni flow induced by heating the substrate for droplet manipulation is limited by low precision of motion control and a narrow range of substrate types. Herein, a novel noncontact light‐driven droplet manipulation method by using photothermally active droplets is introduced. Marangoni flow is induced by local photothermal heating via near‐infrared (NIR) irradiation resulting in an internal flow that drives the droplets. The photothermally active droplets slide away from the NIR light and the direction of motion can be precisely controlled by changing the irradiation position remotely. In addition, it is demonstrated that the addition of a miscible liquid to the droplets can reverse the direction of motion. Moreover, the authors show that spherical droplets on conventional liquid repellent surfaces move through a rolling mechanism instead of sliding. It is believed that this droplet manipulation method can provide a general way of droplet transportation on solid surfaces.

Decreasing the Weight-Size Parameters of Gas-Steam Turbine Plant by Increasing the Efficiency of Thermodynamic Processes in the Condenser

The aim of this work is decreasing the weight-size parameters of the contact gas-steam turbine plant and contact condenser elements by increasing the efficiency of thermal-gas dynamic processes of condensation through rational irrigation of countercurrent gas-steam flow. To achieve the goal the total efficiency of water-return drops ranging from 0.1 to 1 mm at different initial velocities from 5 to 35 m/s emitted by the multi-nozzle sprinkler was determined by mathematical modeling of the liquid droplet movement processes, heat and mass transfer between the liquid droplet and gas-vapor mixture, and gas-vapor mixture pressure loss. The effect of increasing the gas-steam mixture velocity from 3.3 to 6 m/s on the overall efficiency of water return was determined. The novelty of the obtained results was defined by an increase in the water return into cycle from 12 to 13% with a droplet diameter of 0.3 -- 0.4 mm and the initial velocity from the sprinkler of 5--10 m/s. The velocity of the mixture was to 6 m/s at rational correlations of the initial velocity of the droplets’ escape, which increased the total amount of heat withdrawn to 11%. The positive effect conditions of irrigation processes on thermogasdynamic and weight-size parameters of the condenser elements for the contact gas and steam turbine plant at full pressure recovery coefficients of over 0.967 were substantiated. The most significant result was the reduction of the weight-size parameters of the marine infrastructure object power plant from 8 to 19%.

Movement of liquid droplets in a shear flow

The problem of the motion of a particle around a particle in a shear flow for the conditions of weightlessness and normal earth gravity is considered. The cases where the particle density is greater or less than the density of the main buffer moving fluid (oil particles in water and water particles in oil) are presented. On the basis of numerical modeling, the nature of the influences of gravitational, viscous, lifting and frontal forces on the trajectory of fluid particles in a shear fluid flow are shown. of diameter particles. For the considered liquid particles and stratification of the shear velocity field It is established that the minimum distance of approach of liquid droplets is equal approximately 0.07 of diameter particles.

Droplet motion and oscillation on contrasting micro-striated surfaces

Spontaneous motion of liquid droplets can occur on hydrophobic, micro-structured, solid surfaces comprising a structural gradient. In this study, we examine such motion experimentally and explain our observations by invoking variable droplet–surface interactions (both actuation and resistance forces) arising from the structural gradient. The oscillatory motion of the droplet constitutes an integral aspect of the behaviour and this is incorporated into the overall modelling. The theoretical model features a truncated spheroid for the drop shape (flattened in the region of solid contact) coupled with the oscillatory and alternate leading and trailing motion of the contact line. Results from the model and experiments provide good qualitative and quantitative agreement. The component of the vertical oscillation is found to help overcome wetting hysteresis and actuate the motion, this being a key element for the completeness of the model.

Effect of Pore Shape and Spacing on Water Droplet Dynamics in Flow Channels of Proton Exchange Membrane Fuel Cells

Effective water management increases the performance of proton exchange membrane fuel cells (PEMFCs). The liquid droplet movement mechanism in the cathode channel, the gas-liquid two-phase flow pattern, and the resulting pressure drop are important to water management in PEMFCs. This work employed computational fluid dynamics (CFD) with a volume of fluid (VOF) to simulate the effects of two operating parameters on the liquid water flow in the cathode flow channel: Gas diffusion layer (GDL) pore shape for water emergence, and distance between GDL pores. From seven pore shapes considered in this work, the longer the windward side of the micropore is, the larger the droplet can grow, and the duration of droplet growth movement will be longer. In the cases of two micropores for water introduction, a critical pore distance is noted for whether two droplets coalesce. When the micropore distance was shorter than this critical value, different droplets coalesce after the droplets grew to a certain extent. These results indicate that the pore shape and the distance between pores should be accounted for in future simulations of PEMFC droplet dynamics and that these parameters need to be optimized when designing novel GDL structures.

Hydro-Vortex Classification of Composite Microparticles

The use of nanomaterials for the implementation of a new set of functional properties has no alternative when creating refractory composite materials. Fine-dispersed Al2O3 is a necessary component of special high-quality cements, heat-resistant inert refractories and abrasive materials. Equations are obtained for the average values of the Euler and Reynolds criteria, the relaxation time of liquid droplets with integrated nanoparticles depending on their median size in the process of hydro-vortex classification. The tests carried out confirmed that under the conditions of self-similar hydro-vortex coagulation in the process of hydro-vortex classification, the inertial forces acting on the unsteady hydro-vortex motion of the dispersed system a drop of liquid — a microparticle of material, significantly affect the trajectory of its motion in comparison with the forces acting in the direction of motion of the fluidized bed of dispersed material. In this case, the control action provides a constant relaxation time, which significantly increases the efficiency of the influence of the angular velocity of droplet rotation on the classification process. Thus, inertial hydro-vortex classification in the self-similarity mode makes it possible to control the trajectories of motion of microparticles. At the same time, the energy of the translational motion of liquid droplets, determined by the characteristics of the hydro-vortex nozzles of the aerator, ensures the constancy of the time of the inlet manifold of the receiving hopper for relaxation, allowing the median diameter range of 0.5 × 10–6 – 5 × 10–6 m to be provided with a dispersion of no more than 20 % on their median diameter with a larger width of the inlet header of the receiving hopper.

Bio-inspired semi-infused adaptive surface with reconfigurable topography for on-demand droplet manipulation

The topography of magnetoresponsive semi-infused adaptive surface is reversibly switched between semi-infused and oil-accumulated states for stimuli-free pinning of droplets in a tilted state and on-demand motion of liquid droplets.

Fabrication of patterned solid surfaces with highly controllable wettability.

Precisely controlling the wettability of a solid surface is vital for a wide range of applications such as control of liquid droplet motion, water collection and the directional transport of fluids. However, fabricating a large-area solid surface with highly controllable wettability in a low-cost way is still challenging. Here we present a cost-effective method to fabricate patterned solid surfaces with highly controllable wettability by combining chemical etching technique, chemical vapor deposition technique and laser direct writing technique. We experimentally demonstrated that the contact angle of water droplets on the patterned surfaces of a porous nanofilm fabricated using the presented fabrication method can be adjusted from 94.4° to 168.2° by changing the duty ratio of the periodic pattern on the patterned surfaces. Furthermore, we experimentally demonstrated that the contact angle of water droplets on the patterned surfaces is almost independent of the shape of the unit cell of the patterns. In addition, we propose an effective surface model to accurately calculate the contact angle of water droplets on patterned solid surfaces. Using the effective surface model, the wettability of a patterned solid surface can be precisely controlled by designing the duty ratio of its periodic patterns.

High‐Performance Tubular Electricity Generators Operated by Magnetically Driving Movement of Droplet

AbstractElectricity generators by driving movement of liquid droplet or flow on the surface of nanocarbon materials represent an emerging means to harvest energy from nature and environment, which have widely potential applications in the fields of sensors, biomedicine, energy conversion, and so on. Herein, a tubular electricity generator with magnetic droplet encapsulated in a continuous graphene tube is developed. The droplet in tubular electricity generators can be easily driven by magnetic force, during which a potential difference ranging from 196 to 419 mV can be realized by tuning the salt species in the droplet. The value of the generated potential difference is among the highest range, compared with other electricity generators reported previously. In addition, in‐series connecting effect could be achieved with multi‐droplet encapsulated in one single electricity generator, resulting in multifold output voltage of device with one droplet. The work provides a powerful strategy to build electricity generator with high output voltage and easy manipulation, and the tubular devices are promising to be used for biomedical and/or wearable electronics.

Bidirectional motion of droplets on gradient liquid infused surfaces

The current paradigm of self-propelled motion of liquid droplets on surfaces with chemical or topographical wetting gradients is always mono-directional. In contrast, here, we demonstrate bidirectional droplet motion, which we realize using liquid infused surfaces with topographical gradients. The deposited droplet can move either toward the denser or the sparser solid fraction area. We rigorously validate the bidirectional phenomenon using various combinations of droplets and lubricants, and different forms of structural/topographical gradients, by employing both lattice Boltzmann simulations and experiments. We also present a simple and physically intuitive analytical theory that explains the origin of the bidirectional motion. The key factor determining the direction of motion is the wettability difference of the droplet on the solid surface and on the lubricant film.