Behavior Of Liquid Droplets Research Articles

Bionic e-skin with precise multi-directional droplet sliding sensing for enhanced robotic perception

Electronic skins with deep and comprehensive liquid information detection are desired to endow intelligent robotic devices with augmented perception and autonomous regulation in common droplet environments. At present, one technical limitation of electronic skins is the inability to perceive the liquid sliding information as realistically as humans and give feedback in time. To this critical challenge, in this work, a self-powered bionic droplet electronic skin is proposed by constructing an ingenious co-layer interlaced electrode network and using an overpass connection method. The bionic skin is used for droplet environment reconnaissance and converts various dynamic droplet sliding behaviors into electrical signals based on triboelectricity. More importantly, the two-dimensional sliding behavior of liquid droplets is comprehensively perceived by the e-skin and visually fed back in real-time on an indicator. Furthermore, the flow direction warning and intelligent closed-loop control of water leakage are also achieved by this e-skin, achieving the effect of human neuromodulation. This strategy compensates for the limitations of e-skin sensing droplets and greatly narrows the gap between artificial e-skins and human skins in perceiving functions.

In Ukrainian

The contact wetting is one of the effective methods of studying the adsorption capacity of sorbents. The purpose of the work was to compare the experimentally obtained data on the adsorption capacity of shungite, obtained by the sessile drop method, and the results of modeling the behavior of liquid droplets on heterogeneous surfaces using the Boltzmann lattice method, and to show the suitability of the simplified version of the LBM method that we applied within the framework of a two-dimensional model for modeling complex cases of contact interaction between liquids and sorbent, when it cannot be carried out by the method of contact wetting. The adsorption properties of shungite with regard to the extraction of various impurities from water-alcohol solutions and the capability of the sorbent to recover were investigated by the method of contact wetting and analyzed by involving the data obtained by the methods of nitrogen adsorption, thermogravimetry and IR spectroscopy. It is shown that the adsorption properties of shungite are due to the presence on its surface of hydroxyl functional groups attached to carbon atoms in phenol or enol form, which give the surface hydrophilic characteristics. These groups play a key role in the adsorption of components from the liquid (aqueous) phase due to the formation of a hydrogen bond during the sorption of components from the liquid phase, and are restored after heating in the temperature range of 80–180 °C with the formation of carbon-containing gases and water. It has been found that silanol groups present in shungite do not participate in sorption. Compared to the original shungite sample, the sample after five cycles of adsorption is characterized by a noticeable effect of mass loss (1.8 %) in the temperature range of 80–180 °С. At the same time, the loss of mass is not significant at temperatures below 100 °С. This suggests that the sorbed substances are in the pores and not on the surface of shungite, and they begin to be removed only after heating above 100 °C. The LBM method was used to study fast-moving processes at the meso-level. A comparative analysis of the experimental data obtained by the method of contact wetting with the results of simulation by the Boltzmann lattice method within the framework of the two-dimensional model was carried out. 2D modeling by the LBM method turned out to be an effective means of studying capillary condensation in mesopores, anticipatory wetting of the solid phase, liquid penetration into a porous medium with different topologies, and the formation of anisotropic droplets and anisotropic bridges. The role of mesopores in the sorption process was analyzed by modeling the behavior of liquid droplets on heterogeneous surfaces and using data on the course of adsorption and capillary processes on the surface of a solid phase with different levels of porosity, roughness, and functional composition.

Impact of Droplets on Surfaces Designed with Wettability-Gradient Properties: Directional Migration, Oblique Rebound, and Reduced Contact Time.

Efficiently regulating the rebound behavior of droplets post-impact is crucial for various fields, mainly including the development of self-cleaning applications, the design of surface functional materials, and the advancement of industrial techniques. By performing molecular dynamics simulations, we investigated the impact and jumping behavior of droplets on heterogeneous substrates with different wetting regions. We found that, during the impacting evolution process, the retracted droplets would move toward regions with stronger wettability due to the unbalanced force caused by the wettability difference, revealing the directional migration ability. The values of the wettability difference strongly affect the degree of oblique rebound and contact time when droplets can jump off the substrate. We then designed the surfaces with a wettability gradient and found that the oblique rebound angle could be well controlled and the contact time further reduced. Our findings may provide valuable insight into the relationship between the wettability gradient and the behavior of liquid droplets on surfaces, with broad implications for various fields such as surface engineering, materials science, microfluidics, etc.

Experimental study on dynamic wetting behavior of liquid droplets on the aluminum surfaces excited by ultrasonic waves

In this paper, an anti-frosting strategy utilizing ultrasonic excitation to drive droplets away from cold surfaces was proposed. Based on this strategy, experimental studies were conducted to investigate the dynamic wetting behavior of droplets on aluminum surfaces under ultrasonic excitation. Specifically, the vibration characteristics of the plate and the properties of the droplets were revealed under the influence of ultrasonic waves. The results indicate that the droplet will undergo three stages of spreading-movement-contraction under ultrasonic excitation and the maximum contact angle hysteresis and maximum diameter spreading rate exhibit a positive correlation with droplet volume and ultrasonic power. It is also found that ultrasonic excitation can significantly reduce the droplet contact angle, which can be reduced by up to 70°. The contact angle hysteresis, diameter spreading rate and movement velocity show almost the same trend with time under ultrasonic action, that is, increasing first and then decreasing. Notably, the contact angle hysteresis and the diameter spreading rate reached a peak later than the movement velocity, which is attributed to the droplet's deformation relies on the full propagation of the ultrasonic vibration inside the droplet, resulting in hysteresis. Finally, it is found that the droplet-wetting state is permanently changed under ultrasonic action and the final contact angle is greatly reduced compared to the initial value even after stopping ultrasonic excitation.

The effect of Aceria litchii (Keifer) infestation on the surface properties of litchi leaf hosts.

The wettability of target crop surfaces affects pesticide wetting and deposition. The structure and properties of the leaf surface of litchi leaves undergo severe changes after infestation by Aceria litchii (Keifer). The objective of this study was to systematically investigate the surface texture and wettability of litchi leaves infested. Firstly, the investigation focused on the surface structure and physicochemical properties of litchi leaves infested with Aceria litchii. Subsequently, different levels of Contact Angle (CA) were measured individually on the infested litchi leaves. Lastly, Surface Free Energy (SFE) and its polar and dispersive components were calculated using the Owens-Wendt-Rabel-Kaelble (OWRK) method. The outcomes revealed distinctive 3D surface structures of the erineum at various stages of mycorrhizal growth. At stage NO. 1, the height of the fungus displayed a peaked appearance, with the skewness value indicating a surface characterized by more crests. In contrast, at stages NO. 2 and NO. 3, the surface appeared relatively flat. Furthermore, post-infestation of litchi leaves, the CA of droplets on the abaxial surface of diseased leaves exhibited an increase, while the SFE value on the abaxial surface of leaves decreased significantly, in contrast to the abaxial surface of healthy leaves. The infestation behavior of Aceria litchii changed the surface structure and chemistry of litchi leaves, which directly affected the CA value of foliar liquids and the SFE value of leaves, changing the surface wettability of litchi leaves from hydrophobic to superhydrophobic. This study provides useful information for improving the wetting and deposition behavior of liquid droplets on the surface of infested leaves. © 2024 Society of Chemical Industry.

Molecular dynamics study on wetting behavior of aluminum plates with different surface energies by microstructure difference

Wettability is the main factor that affects the self-cleaning and corrosion resistance of metal surfaces. The main factor that affects wettability is the change of surface microstructure and surface energy of metal material. When modifying metals, it is necessary to master their wetting behavior to obtain the required performance surface. In this paper, the wetting behavior of liquid droplets on smooth, square, papillary and nail-shaped surfaces with different surface energies was studied systematically by molecular dynamics simulation. The results show that the solid–liquid interaction factor is negatively correlated with the surface contact angle of aluminum plate. When the solid–liquid interaction factor is greater than the droplet potential energy parameter, the nail-shaped surface has the largest solid–liquid contact area and droplet centroid displacement rate due to its large roughness, and the surface hydrophilicity is the best; When the solid–liquid interaction factor is close to the droplet potential energy parameter, the nail-shaped surface has the smallest solid–liquid contact area and droplet centroid displacement rate due to its large roughness, and realizes the transition from neutral to hydrophobic; When the solid–liquid interaction factor is less than the droplet potential energy parameter, the papillary surface has the smallest solid–liquid contact area and droplet centroid displacement rate due to its small solid–liquid area fraction, thus realizing the transition from hydrophobicity to superhydrophobicity. The simulation results of this study are consistent with the classical theoretical calculation and experimental results, which reveals the interaction between metal surface wettability and surface energy and surface morphology.

Hydrodynamic behavior of liquid flow in a rotating packed bed

Rotating Packed Bed (RPB) technology has emerged as a promising technology for reducing carbon footprint of various petrochemical processes by leveraging the HiGee field-induced mass transfer enhancement and allow for better capture of greenhouse gases. Despite its potential, there are still significant challenges that hinder the effective utilization of RPB technology in process intensification. Specifically, the hydraulics of liquid and gas phases within RPB packings present a significant challenge. In this study, 2D – VOF approach is employed to study the behavior of liquid droplets impacting on single rotating wire and on a whole rotating wire mesh within a Rotating Packed Bed (RPB). A comprehensive computational fluid dynamics (CFD) model is developed to analyze the influence of rotating speed, liquid velocity, and droplet diameter on the dynamics of liquid droplets. The boundaries of splashing – no splashing and dripping – capture breakup modes were validated for a single rotating wire and identified for wire mesh in RPBs by constructing the corresponding regime maps. This is achieved thanks to a new data processing algorithm that applies the principle of neighborhood aggregation. The proposed data processing algorithm helps reconstruct the regime maps for the various breakup phenomena within RPBs. The results show that the splashing mechanism reproduces higher-velocity detached daughter droplets, while the momentum-induced dripping mechanism results in low velocity daughter droplets. The results also reveal that the deposited liquid film with the wires has a key role in enhancing the splashing phenomena in RPBs. The radial distribution of diameter and velocity of liquid droplets in RPB is also analyzed in light of the droplet impact dynamics. The increase in the average liquid velocity with radial direction was attributed to the increase of splashed drops, which led to a bigger number of high-velocity droplets. The conclusions of this study can be used to help improve the dynamics of the two-phase flow within RPBs through a better packing design and flow conditions.

Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?

In the last ten years, advances in experimental techniques have enabled remarkable discoveries of how the dynamics of thin gas films can profoundly influence the behavior of liquid droplets. Drops impacting onto solids can skate on a film of air so that they bounce off solids. For drop–drop collisions, this effect, which prevents coalescence, has been long recognized. Notably, the precise physical mechanisms governing these phenomena have been a topic of intense debate, leading to a synergistic interplay of experimental, theoretical, and computational approaches. This review attempts to synthesize our knowledge of when and how drops bounce, with a focus on ( a) the unconventional microscale and nanoscale physics required to predict transitions to/from merging and ( b) the development of computational models. This naturally leads to the exploration of an array of other topics, such as the Leidenfrost effect and dynamic wetting, in which gas films also play a prominent role.

Thermal-hydraulic performance enhancement of fin-and-tube heat exchangers using carbon nanotube coatings under dry and wet conditions

Carbon nanotube coatings have been widely adopted in various heat transfer fields to improve their heat transfer performance. However, comprehensive studies on the thermal–hydraulic performance of fin-and-tube heat exchangers with carbon nanotube coatings are limited. In this study, the heat transfer characteristics of fin-and-tube heat exchangers with uncoated, hydrophilic, hydrophobic, and carbon nanotube coatings were measured under dry and wet conditions. The Colburn j-factor, friction factor, and thermal–hydraulic performance (jf–1/3) of fin-and-tube heat exchangers with various coatings were analyzed and compared under dry and wet conditions. The carbon nanotube-coated heat exchanger showed the best performance in terms of heat transfer and frictional pressure drop under both dry and wet conditions among the tested fin-and-tube heat exchangers. This is because the unique microscopic surface structure of the carbon nanotube coating increased the heat transfer coefficient, whereas the superlubricity of the carbon nanotube coating decreased the frictional pressure drop. The behavior of liquid droplets on the surfaces of various fin-and-tube heat exchangers was investigated in detail under dry and wet conditions. In addition, the thermal–hydraulic performance (jf–1/3) of the carbon nanotube-coated heat exchangers was 8.9% and 6.1% on average higher than that of the uncoated heat exchanger under dry and wet conditions, respectively. The results of this study can provide helpful information for developing fin-and-tube heat exchangers with carbon nanotube coatings to achieve substantial energy savings.

Transport behavior of liquid droplets based on functional liquid crystal surfaces

Transport behavior of liquid droplets based on functional liquid crystal surfaces

Deformation behavior of liquid droplet in shock-induced atomization

The purpose of this study is to clarify the deformation behavior characteristics of a single droplet in the high Weber number region corresponding to catastrophic breakup, depending on the Weber number, and show the relationship between the spread of fragments and their initial deformation behavior. Accordingly, we investigated waves generated from the droplet surface by photographing the deformation behavior of the droplet with high spatio-temporal resolution and the correlations between the Weber number and height and thickness of the parent droplet, center-of-mass movement, and movement of the leading and trailing edges. The results show no significant relationship between the spread of the fragment and its initial deformation behavior. We also measured the wavelength of the wave pattern at the upstream interface of the droplet during a catastrophic breakup. As a result, a relationship between the wavelength of the upstream interface of the droplet and the Weber number was found.

Laser Fabrication of Titanium Alloy-Based Photothermal Responsive Slippery Surface

In recent years, biomimetic materials inspired from natural organisms have attracted great attention due to their promising functionalities and cutting-edge applications, emerging as an important research topic. For example, how to reduce the reflectivity of the solid surface and increase the absorption of the substrate surface is essential for developing light response smart surface. Suitable solutions to this issue can be found in natural creatures; however, it is technologically challenging. In this work, inspired from butterfly wings, we proposed a laser processing technology to prepare micro nanostructured titanium alloy surfaces with anti-reflection properties. The reflectivity is significantly suppressed, and thus, the light absorption is improved. Consequently, the anti-reflection titanium alloy surface can be further employed as a photothermal substrate for developing light-responsive slippery surface. The sliding behavior of liquid droplets on the smart slippery surface can be well controlled via light irradiation. This method facilitates the preparation of low-reflection and high-absorption metallic surfaces towards bionic applications.

2Pb2-2 Analysis of behavior of liquid droplets on vertically oscillating substrates

2Pb2-2 Analysis of behavior of liquid droplets on vertically oscillating substrates

Deformation Behavior of Liquid Droplet in Shock-Wave-Induced Atomization

Deformation Behavior of Liquid Droplet in Shock-Wave-Induced Atomization

Dynamic behavior of droplet transport on realistic gas diffusion layer with inertial effect via a unified lattice Boltzmann method

The dynamic behavior of liquid droplets on a reconstructed real gas diffusion layer (GDL) surface with the inertial effect produced by the three dimensional (3D) flow channel is investigated using an improved pseudopotential multiphase model within the unified lattice Boltzmann model (ULBM) framework, which can realize thermodynamic consistency and tunable surface tension. The microstructure of the GDL (Toray-090) including carbon fibers and polytetrafluoroethylene (PTFE) is reconstructed by a stochastic and mixed-wettability model. The critical force formulation for the Cassie-Wenzel transition of a droplet on GDL surface is derived. The effects of inertia and contact angles on the liquid droplet transport process on a reconstructed real GDL surface with a 3D flow channel are investigated. The results show the normalized center-of-mass coordinate X may enter the channel wall area or fluctuate around the initial position. With increased inertia applied on the droplet, the normalized center-of-mass coordinate Y grows faster and the normalized center-of-mass coordinate Z decreases. It is found by the ULBM for the first time that the liquid droplet is pushed back into the GDL by inertial effect. With the increase of inertia and the decrease of contact angle of GDL, both the droplet penetration depth in GDL and the droplet invasion fraction increase. The droplet invasion fraction in GDL is up to 30%.

Effects of superheat degree and wettability on droplet evaporation time near Leidenfrost point through Lattice Boltzmann simulation

The evaporation of droplet on a superheated surface is a very important and common phenomenon in industrial production, such as spray cooling and metallurgical treatment. In this paper, a two-dimensional numerical simulation based on Lattice Boltzmann Method has been adopted to analyze the evaporation behavior of liquid droplet on an over-heated surface. The effects of superheat degree (Ja number) and surface wettability on droplet evaporation are analyzed respectively. During the evaporation process, the evaporation pressure at the bottom of the droplet exhibits periodic oscillation corresponding to bouncing movement. The oscillation gradually decays in the case of low Ja numbers, but for high Ja numbers, the oscillation doesn't decay. Leidenfrost vapor layer is more likely to be generated on a hydrophobic surface. Meanwhile, the comprehensive influence of wettability and Ja number on droplet evaporation time was studied systematically. On hydrophilic surfaces, there is a critical Ja value, around which the evaporation time exhibits a U-type tendency. However, the evaporation time continues to decrease as Ja number increases on hydrophobic surfaces.

Progress in reflood thermal hydraulics studies in the past 40 years

Accurate simulation of reflood transients of a reactor core is one of the top priorities for nuclear reactor thermal–hydraulic and safety analysis. An in-depth understanding of the two-phase flow and heat transfer process during reflooding can provide extremely useful information on the design and analysis of the various safety systems as well as on the development of numerical analysis tools. In the current paper, the status of the reflood transients in the past several decades is overviewed and discussed in detail, in the hope to provide a roadmap for future relevant research. Following an introduction of the current topic, the phenomenological aspects related to reflood are discussed first. Then, studies on the two-phase flow thermal–hydraulic non-equilibrium are reviewed and evaluated critically. The flow channel pressure drop as well as the spacer grid effect constitute important parts of reflood thermal–hydraulic and safety analysis, and various studies on these topics are discussed. Another important consideration during reflood transients is the liquid droplet behavior since it affects the rod bundle thermal response. In addition, the current paper also identifies the several limitations and the need for future research work on reflood thermal hydraulics.

Dynamic behavior of liquid droplets with enzyme compartmentalization triggered by sequential glycolytic enzyme reactions.

Dynamic droplet formation via liquid-liquid phase separation (LLPS) is believed to be involved in the regulation of various biological processes. Here, a model LLPS system coupled with a sequential glycolytic enzymatic reaction was developed to reproduce the dynamic control of liquid droplets; (i) the droplets, which consist of poly-L-lysine and nucleotides, compartmentalize two different enzymes (hexokinase and glucose-6-phosphate dehydrogenase) individually, accelerating the overall reaction, and (ii) each enzymatic reaction triggers the formation, dissolution and long-term retention of the droplets by converting the scaffold nucleotides. This model system will offer a new aspect of enzymes associated with LLPS in living cells.

Gas distribution and droplet removal of metal foam flow field for proton exchange membrane fuel cells

Recently, porous metal foam has gained much attention as an alternative gas distributor of proton exchange membrane fuel cells. However, the gas distribution in the intricate porous flow field is different from the conventional flow channels and the liquid droplet behavior remains unclear. Thus, this study numerically investigated the two-phase mass transport capacities of the metal foam flow field. The metal foam morphology is reconstructed based on X-ray computational tomography technique and the two-phase interface is capture by volume of fluid method. A divergent gas transport mode is observed, which promotes the uniformity and convection of gas reactant flow with a much lower permeability than conventional flow channels. The heterogeneity of metal foam pore distribution should be minimized to reduce the pore-scale weak flow area. In addition, the air drag force on liquid droplet grows with droplet diameter in a similar way to that of a flow channel, but the resistance for liquid removal is no longer the shear force by wall surface but the adhesion by ligament surface. The hydrophobicity of ligaments is found necessary to reduce liquid retention phenomenon. In addition, the variation of gas velocity exhibits a stronger influence than droplet diameter on liquid removal, indicating that the metal foam flow field is applicable to high-current-density operation conditions for proton exchange membrane fuel cells.

Understanding wetting dynamics and stability of aqueous droplet over superhydrophilic spot surrounded by superhydrophobic surface

Patterned superhydrophilic-superhydrophobic (SHL – SHB) surfaces have shown promise in droplet-based biochemical assays. However, fundamental understanding of the behavior of liquid droplets on such patterned surfaces has not received much attention. Here, we report wetting dynamics and stability of an aqueous droplet placed over a superhydrophilic spot (θ~0°) surrounded by a superhydrophobic surface (θ~160°). We study the shape evolution (contact angle (θ) and contact line diameter (dc)) of an aqueous droplet placed over a horizontal SHL – SHB surface with its volume (Vd), using experiments and analytical modeling. The results showed that depending upon the Bond number (Bo) and spot diameter (ds), three different regimes: spherical cap with fixed dc and varying θ (Regime I), oblate spheroid with fixed dc and varying θ (Regime II), and oblate spheroid with varying dc and fixed θ (Regime III), are observed. The transition from Regime I to Regime II occurs for Bo~1 whereas that from Regime II to Regime III occurs at Bocr~0.33ds1.30. Analysis of the present case wherein the contact line lies at the boundary of SHL – SHB surfaces, revealed anomaly with respect to the statements of Wenzel, Cassie-Baxter and McCarthy. Further, the stability of a droplet placed over the superhydrophilic spot on an SHL – SHB angular surface is studied using experiments and analytical modeling, which showed that the competition between contact line pinning force (Fp) and gravitational force (Fg) governs its stability. The stable and unstable regimes are identified based on the Bond number (Bo) and spot diameter (ds) and the critical Bond number for stable – unstable transition depends on spot diameter as Bocr~0.5ds-0.93.