Droplet Motion Research Articles

A programmable magnetic digital microfluidic platform integrated with electrochemical detection system

Digital microfluidic (DMF) technology is widely used in bioanalysis and chemical reactions due to its accuracy and flexibility in manipulating droplets. However, most DMF systems usually rely on complex electrode fabrication and high driving voltages. Sensor integration in DMF systems is also quite rare. In this study, a programmable magnetic digital microfluidic (PMDMF) platform integrated with electrochemical detection system was proposed. It enables non-contact, flexible droplet manipulation without complex processes and high voltages, meeting the requirements of automated electrochemical detection. The platform includes a magnetic control system, a microfluidic chip, and an electrochemical detection system. The magnetic control system consists of a microcoil array circuit board, a N52 permanent magnet, and an Arduino control module. N52 magnets generate localized magnetic fields to drive droplet movement, while the Arduino module enables programmable control for precise manipulation. The maximum average velocity of the droplet is about 3.9 cm/s. The microfluidic chip was fabricated using 3D printing and the superhydrophobic surface of chip was fabricated by spray coating. The electrochemical detection system consists of the MoS2@CeO2/PVA working electrode, Ag/AgCl reference electrode, and carbon counter electrode. To evaluate the practical value of the integrated platform, glucose in sweat was automatically and accurately detected. The proposed platform has a wide linear detection range (0.01–0.25 mM), a lower LOD (6.5 μM), a superior sensitivity (7833.54 μA·mM−1·cm−2), and excellent recovery rate (88.1–113.5%). It has an extensive potential for future application in the fields of medical diagnostics and point-of-care testing.

Anisotropic Wetting Behavior on Gradient Surface Structures Fabricated by Direct Laser Interference Patterning on Stainless Steel

AbstractIn this work, anisotropic line‐like textures with gradually increasing spatial periods from 2.0 to 4.8 µm are fabricated on stainless steel by two‐beam Direct Laser Interference Patterning using a ps‐laser source. Three manufacturing strategies are developed to fabricate these so‐called gradient structures achieving different surface roughness Rz and overall texture uniformity. The influence of unidirectional texture gradients on anisotropic wetting properties is studied by depositing 1 µL water droplets. The results show that the optimum‐designed gradient patterns induce in all cases the spontaneous motion of water droplets in the direction of increasing periodicity, or equivalently, increasing hydrophilicity. It is also observed a direct correlation between the gradient ratio and the traveling distance, which reaches a maximum of 0.99 mm for a period gradient of 2.24 µm mm−1. Furthermore, multidirectional gradient textures are produced by combining gradient areas with opposing gradient orientations. The anisotropic wetting characteristics on such complex textures are measured, showing a strong local dependency of the water contact angle and droplet displacement. The findings from this work provide new design rules for customizing gradient surfaces able to realize predictable, position‐dependent wetting properties and directional movement of microliter water droplets.

Nonmonotonic Motion of Sliding Droplets on Strained Soft Solids

Soft materials are ubiquitous in technological applications that require deformability, for instance, in flexible, water-repellent coatings. However, the wetting properties of prestrained soft materials are only beginning to be explored. Here we study the sliding dynamics of droplets on prestrained soft silicone gels, both in tension and in compression. Intriguingly, in compression we find a nonmonotonic strain dependence of the sliding speed: mild compressions decelerate the droplets, but stronger compressions lead again to faster droplet motion. Upon further compression, creases nucleate under the droplets until, finally, the entire surface undergoes the creasing instability, causing a “run-and-stop” motion. We quantitatively elucidate the speed modification for moderate prestrains by incremental viscoelasticity, while the acceleration for larger prestrains turns out to be linked to the solid pressure, presumably through a lubrication effect of expelled oligomers. Published by the American Physical Society 2025

Comment on “Brownian motion of droplets induced by thermal noise”

Comment on “Brownian motion of droplets induced by thermal noise”

Study on Mold Flux Entrapment by Numerical Simulation, Water Modeling, and High‐Temperature Quantitative Velocity Measurement

A mathematical model for tracking the mold flux entrapment (MFE), and mold flux droplet (MFD) movement and capture is established to investigate the influences of the casting speed (CS) and submerged entry nozzle side hole inclination angle (SEN‐SHIA) on MFE. The numerical simulation results of the surface velocity present excellent alignment with the high‐temperature measurement and water model experiment results, showing that the surface velocity increases as CS increases and decreases as SEN‐SHIA increases. The numerical simulation reveals three MFE mechanisms of vortex, shear force, and bubble, presenting excellent alignment with the observed results in the water model. When SEN‐SHIA is 15°, with increasing CS from 1.4 to 1.6 and 1.8 m min−1, the mass of MFE within 40 s increases from 12.30 to 45.65 and 90.50 g. Among them, the mass of MFDs trapped by walls increases from 0.50 to 2.39 and 4.86 g. At CS of 1.8 m min−1, with increasing SEN‐SHIA from 15° to 30° and 45°, the mass of MFE within 40 s decreases from 90.50 to 38.90 and 5.30 g. Among them, the mass of MFDs trapped by walls decreases from 4.86 to 1.94 and 0.05 g.

The diffuse solid method for wetting and multiphase fluid simulations in complex geometries

We develop a diffuse solid method that is versatile and accurate for modeling wetting and multiphase flows in highly complex geometries. In this scheme, we harness N+1-component phase field models to investigate interface shapes and flow dynamics of N fluid components, and we optimize how to constrain the evolution of the component employed as the solid phase to conform to any pre-defined geometry. Implementations for phase field energy minimization and lattice Boltzmann method are presented. Our approach does not need special treatment for the fluid–solid wetting boundary condition, which makes it simple to implement. To demonstrate its broad applicability, we employ the diffuse solid method to explore wide-ranging examples, including droplet contact angle on a flat surface, particle adsorption on a fluid–fluid interface, critical pressure on micropillars and on Salvinia leaf structures, capillary rise against gravity, Lucas-Washburn's law for capillary filling, and droplet motion on a sinusoidally undulated surface. Our proposed approach can be beneficial to computationally study multiphase fluid interactions with textured solid surfaces that are ubiquitous in nature and engineering applications.

Theoretical model for droplet self-motion in hydrophilic and hydrophobic microchannels with wettability gradient surfaces

Spontaneous liquid transport in microchannels driven by wettability gradient surfaces has received considerable interest for microfluidic applications. In this paper, a theoretical model was developed to investigate the self-motion behaviors of droplet in square microchannels featured by wettability gradient surfaces using 3D (three dimensional) morphological construction and virtual work principle. The effects of wettability gradient and contact angle range on droplet movement were compared and evaluated. A larger wettability gradient contributes to a higher droplet velocity, and the droplet velocity could reach several centimeters per second. In a wettability-gradient microchannel, the droplet velocity increased and decreased along the channel length in the hydrophobic and hydrophilic channels, respectively. For a droplet moving in a microchannel with its surface wettability continuously changing from hydrophobic to hydrophilic zones, the maximum droplet velocity appears at the contact angle of 90°, and it could be about 2.7 cm/s in the 100 μm width channel under a wettability gradient of 4°/mm. At a same wettability gradient, a lower initial contact angle is associated with higher droplet velocity for hydrophobic channels, while it is opposite in hydrophilic channels. Dimensionless correlations were developed to predict the self-motion velocities in hydrophobic and hydrophilic microchannels, and droplet self-motion characteristics at different wettability gradient conditions were elucidated. The droplet velocities obtained from the theoretical model are in good agreement with that from a 3D numerical simulation, but featured by tens of times smaller in time consumption of computation, showing the reliability and high computation efficiency of the theoretical model.

The motion and mass growth of droplets with phase transitions in a homogeneous medium

The motion and mass growth of droplets with phase transitions in a homogeneous medium

Molecular insights into the motion of oil droplets in aqueous solutions of ester- and amide-containing cationic surfactants

Molecular insights into the motion of oil droplets in aqueous solutions of ester- and amide-containing cationic surfactants

Propelled motion of droplets on a free liquid surface

Abstract This manuscript presents a novel phenomenon pertaining to the propelled motion of droplets on a free liquid surface. Following the testing of a series of systems and a comparison of their physical properties, it is proposed that such propelled motion requires the liquids to exhibit low interfacial tensions, high evaporation rates, and to be miscible under disturbance. The presence of low interfacial tension ensures the formation of a film of the pool on the surface of the droplet. The rapid evaporation of the droplet and the film results in disruptions that may cause the liquid film and the droplet to merge and dissolve into one another, ultimately forming an inner phase droplet. The ejection of the inner phase droplet generates a substantial reaction force, propelling the droplet in the process. Fluorescence experiments indicate that the primary component of the inner phase of the droplet is the liquid present in the liquid pool.

Cytoskeleton regulates lipid droplet fusion and lipid storage by controlling lipid droplet movement.

Cytoskeleton regulates lipid droplet fusion and lipid storage by controlling lipid droplet movement.

Laser-Induced Photothermal Pulling of Dyed Droplets on a Superhydrophobic Surface.

Understanding the interaction between laser beams and liquid droplets has significant implications for applications in microfluidics and optical manipulation. Laser beams have previously been reported to act as tractor beams to pull microscopic particles toward the light source or serve as a heating source when directed from above to induce lateral droplet motion via photothermal effects. However, it remains unknown whether a laser beam can move a droplet toward its source. In this study, we show that a laser beam can pull a dyed droplet on a superhydrophobic surface toward the light source through a sequence of photothermal effects. By directing a green laser beam near the bottom front of a dyed droplet, we observe that the droplet moves toward the light source in two distinct stages. Initially, the dyed droplet advances due to contact angle hysteresis and coalescence with condensation satellite droplets. Subsequently, the droplet motion is stimulated by iterative bubble bursting, coalescence, and relaxation, a combination of effects not reported earlier. We experimentally investigate this motion phenomenon and analyze the influence of laser power and focal point position on droplet motion, offering new insights into laser-induced droplet manipulation.

Motion of a deformable droplet in a rectangular, straight channel

The motion and deformation of a neutrally buoyant drop in a rectangular channel experiencing a pressure-driven flow at a low Reynolds number has been investigated both experimentally and numerically. A moving-frame boundary-integral algorithm was used to simulate the drop dynamics, with a focus on steady-state drop velocity and deformation. Results are presented for drops of varying undeformed diameters relative to channel height ( $D/H$ ), drop-to-bulk viscosity ratio ( $\lambda$ ), capillary number ( $Ca$ , ratio of deforming viscous forces to shape-preserving interfacial tension) and initial position in the channel in a parameter space larger than considered previously. The general trend shows that the drop steady-state velocity decreases with increasing drop diameter and viscosity ratio but increases with increasing $Ca$ . An opposite trend is seen for drops with small viscosity ratio, however, where the steady-state velocity increases with increasing $D/H$ and can exceed the maximum background flow velocity. Experimental results verify theoretical predictions. A deformable drop with a size comparable to the channel height when placed off centre migrates towards the centreline and attains a steady state there. In general, a drop with a low viscosity ratio and high capillary number experiences faster cross-stream migration. With increasing aspect ratio, there is a competition between the effect of reduced wall interactions and lower maximum channel centreline velocity at fixed average velocity, with the former helping drops attain higher steady-state velocities at low aspect ratios, but the latter takes over at aspect ratios above approximately 1.5.

Analysis of COVID-19 Aerosol Dispersion in a Car Cabin Due to Driver's Cough

The current coronavirus outbreak, attributable to SARS-CoV-2, has triggered widespread interest in the risk of infection associated with confined spaces. This study focuses on establishing a model for simulating the spread of COVID-19 within a car cabin due to a driver’s cough using computational fluid dynamics (CFD) simulation in the ANSYS Fluent software. Previous studies have mainly focused on the transmission of the virus over large areas; however, attention has shifted to confined areas, including cars. This study fills this gap by modelling the flow regions, velocity, and pressure fields of virus-containing aerosols. This includes constructing a three-dimensional model of a car cabin and visualising cough droplet movement. While tracking the dispersal of viral particles, the discrete phase model was applied with the stipulation of boundary conditions to mimic real-life coughing. The verification used in the study was a grid independence test, after which the simulations examined the particle residence time, streamline, and velocity field in the car. Studies have shown that viral particles can persist in the air for quite a while and circulate to various parts of the car cabin, thus putting the occupants at a higher risk of infection. The streamline analysis revealed possible propagation routes and recirculation zones that may amplify the transmission from the driver to the rest of the car occupants. The velocity distribution emphasises the potential exposure risk. The findings of this study corroborate previous research on the increased exposure of COVID-19car interiors, with particular emphasis on the transmission from the driver to the other occupants of the car. The outcomes of the research have drawn conclusions about the effectiveness of ventilation as well as other preventive measures to decrease the rate of airborne transmission in vehicles, thus responding to the objectives of the study.

Triboelectric charge-separable probes for potential single-droplet biochemical sensing.

Biochemical sensors have found widespread applications in the fields of health and environment. As the number of biochemical sensors continues to increase, their energy supply has emerged as a challenge. Self-powered biochemical sensors based on triboelectric generators (TENGs) offer a promising solution to this challenge. However, current self-powered sensors for in situ detection of liquid samples either suffer from low output signals, resulting in insufficient sensitivity, or require relatively large sample volumes. To address these challenges, this work introduces a TENG with charge separation capability by incorporating electrodes that can directly contact the solution at both ends of a fluidic channel. Unlike traditional devices, this device utilizes the reciprocating motion of a single droplet within the device to achieve charge accumulation in both the electrodes and the liquid sample. By utilizing the characteristic that biochemical substances contained within the droplet affect its charge storage capacity, the concentration of these biochemical substances in the droplet is reflected by the value of the output voltage when it reaches a stable state. The device functions as a triboelectric charge-separable probe and demonstrates responsiveness to solution pH, salt concentration, and the concentration of nanoparticles or Escherichia coli, showcasing its potential as a biochemical sensor.

Predator-Prey Behavior of Droplets Propelling Through Self-Generated Channels in Crystalline Surfactant Layers.

Motile droplets provide an attractive platform for liquid matter-based applications and protocell analogues displaying life-like features. The functionality of collectively operating droplets increases by the advance of well-designed (physico)chemical systems directing droplet-droplet interactions. Here, we report a strategy based on crystalline surfactant layers at air/water interfaces, which sustain the propulsion of floating droplets and at the same time shape the paths for other droplets attracted by them. First, we show how decylamine forms a closed, crystalline layer that remains at the air/water interface. Second, we demonstrate how aldehyde-based oil droplets react to decylamine in the crystalline layer by forming an imine, causing the droplets to move through the layer while leaving behind an open channel (comparable to "Pac-Man"). Third, we introduce tri(ethylene glycol) monododecylether (C12E3) droplets in the crystalline layer. The crystalline layer suppresses the motion of the C12E3 droplets, however, the aldehyde droplets create surface tension gradients upon depletion of surfactants from the air/water interface, thereby driving Marangoni flows that attract the C12E3 droplets as well as the myelin filaments they grow: Causing the C12E3 droplets to chase, and ultimately catch, the aldehyde droplets along the channels they have created, featuring a predator-prey analogy established at an air/water interface.

Self-Swimming Droplets Driven by the Synergistic Effects of the Fast Interfacial Polymerization and Hydrolysis Reactions.

Self-swimming droplets, as an intriguing phenomenon in the microscopic realm, have traditionally been achieved by harnessing the interfacial properties of small-molecule emulsifiers. Herein, we successfully prepared self-swimming droplets by employing polymers. Leveraging the synergistic effects of fast interfacial polymerization and hydrolysis reactions, a polymer layer is rapidly formed at the oil-water interface, followed by the generation of weakly amphiphilic substances that establish an interfacial tension gradient. Thus, strong Marangoni circulation flows were induced to propel the droplets to move spontaneously. The interfacial behavior of the self-swimming droplets was investigated, revealing regular oscillations and the spontaneous formation of microdroplets on the larger droplet surface. Comparative experiments confirmed the synergistic effects of the two types of reactions in forming self-swimming droplets, as the absence of either reaction hindered autonomous movement. The movement time reaches 758 s, and the maximum speed of droplet movement reaches 5.08 mm/s. This work not only enriches the variety of self-swimming droplets but also provides theoretical guidance for designing and preparing innovative self-swimming systems.

Predator‐Prey Behavior of Droplets Propelling through Self‐Generated Channels in Crystalline Surfactant Layers.

Motile droplets provide an attractive platform for liquid matter‐based applications and protocell analogues displaying life‐like features. The functionality of collectively operating droplets increases by the advance of well‐designed (physico)chemical systems directing droplet‐droplet interactions. Here, we report a strategy based on crystalline surfactant layers at air/water interfaces, which sustain the propulsion of floating droplets and at the same time shape the paths for other droplets attracted by them. First, we show how decylamine forms a closed, crystalline layer that remains at the air/water interface. Second, we demonstrate how aldehyde‐based oil droplets react to decylamine in the crystalline layer by forming an imine, causing the droplets to move through the layer while leaving behind an open channel (comparable to “Pacman”). Third, we introduce tri(ethylene glycol) monododecylether (C12E3) droplets in the crystalline layer. Whereas the crystalline layer suppresses the motion of the C12E3 droplets, the aldehyde droplets create surface tension gradients upon depletion of surfactants from the air/water interface, thereby driving Marangoni flows that attract the C12E3 droplets as well as the myelin filaments they grow: Causing the C12E3 droplets to chase, and ultimately catch, the aldehyde droplets along the channels they have created, featuring a predator‐prey analogy established at an air/water interface.

Spontaneous motion of liquid droplets on soft gel surfaces with non-uniform cross-linking densities

We report an experimental investigation of the spontaneous motion of liquid droplets on soft gels with a cross-linking gradient. By systematically adjusting the spatial difference in cross-linking density, we observed that millimeter-sized liquid droplets moved along the elastic modulus gradient and even climbed inclined slopes against gravity. Unlike the wetting dynamics of micro-droplets, which are governed by elastocapillary effects, we demonstrated that the observed spontaneous movements of millimeter-sized droplets were driven by the surface energy difference resulting from the variations in cross-linking density. Using in situ confocal microscopy imaging, we analyzed the viscoelastic dissipation induced by the moving wetting ridges near dynamic contact lines. Our findings provide a novel strategy for controlling droplet dynamics on soft and dissipative interfaces, based on the relationship between cross-linking density and surface energy of soft gels.

Motion and separation characteristics of oil droplets in high-pressure oil-water separation processes

A small-scale separator was used for controlled experiments to further analyze oil droplet motion and separation characteristics under high pressure. The separation process was monitored at varying pressures, focusing on stratification interface dynamics. The oil droplet size distribution in the water phase was measured realistically, and its pressure-dependent variation was predicted. Numerical simulations incorporating the predicted droplet size distribution were performed and validated against experimental data. The results confirmed that increasing pressure reduced oil droplet size and concentrated their distribution, slowing the oil-water interface movement and impeding separation. Furthermore, high pressure enhanced emulsion stability, with over 80% of emulsions remaining unseparated at pressures exceeding 60.0 bar. These findings underscore the necessity of post-treatment strategies for emulsions in high-pressure separation systems.