Dynamic Behavior Of Water Droplet Research Articles

Study on dynamics and freezing behaviors of water droplet on superhydrophobic aluminum surface

In this work, a kind of superhydrophobic aluminum surface (treated-Al-SA) is fabricated by chemical etching using hydrochloric acid and hydrogen peroxide, and then treated by stearic acid. The surface of treated-Al-SA has typical hierarchical structure, which hold its durable superhydrophobicity under water droplet impact test, bending test, temperature test, humidity test and corrosive liquids test. Moreover, treated-Al-SA show excellent self-cleaning property at the different tilt angle under room temperature. Results on the dynamic behavior of water droplet is indicated the good anti-adhesion and superhydrophobicity of the treated-Al-SA. The whole freezing process of the water droplets on the surface of untreated-Al and treated-Al-SA is observed. The freezing time of treated-Al-SA is 59 s, which is longer than that of untreated-Al. It is estimated that the different anti-freezing behavior is caused by the thermal conductivity between the water droplet and plate, which is provided an insight into freezing characteristics of water droplet. This study is offered the novel opinion of the superhydrophobic aluminum surface produced by chemical etching, which may help the application of structured aluminum surface.

Direct Visualization of Microscale Dynamics of Water Droplets on under-Oil-Hydrophilic Membranes by Using Synchrotron White-Beam X-ray Microimaging Techniques.

Despite considerable academical and practical interests on separation of water-in-oil emulsion via special wettable membranes, fundamental understanding on microscale dynamics of water droplets on under-oil-hydrophilic membranes (UOHMs) at early stages during separation is still very preliminary due to temporal and spatial resolution of existing visualization techniques. To this end, we here succeed in a direct microscopic visualization of separation processes of water droplets on the UOHMs by employing a high-speed, two-dimensional synchrotron white-beam X-ray microimaging technique. During the separation of water-in-oil emulsion, microscale dynamic behaviors of water droplets on hydrophilic membrane surfaces immersed in the different oil media (i.e., hexane, kerosene, and light and heavy mineral oils) and oil films between water droplets and membrane surfaces are visualized and analyzed.

Dynamic Wetting Behaviors of Water Droplets on Surfaces with Dual Structures at the Nanoscale

Molecular dynamics simulations were carried out to investigate the static and dynamic wetting behaviors of water droplets on surfaces with single and dual structures at the nanoscale. The effects of the surface characteristic energy, pillar height, and applied force on the dynamic wetting behavior were considered in this study. The dual structures were constructed by attaching small minor bump textures to the main pillar structures. The dynamic behaviors of water droplets could be classified into four different groups according to their dynamic shapes. The dynamic wetting behaviors were quantitatively evaluated according to the contact angle hysteresis and the ratio of water molecules penetrating into the gaps between pillars. As a result, the dual structures make the surface more hydrophobic in the dynamic state, as well as the static state, compared to the single structures.

Study of droplet dynamics and condensation heat transfer on superhydrophobic copper surface

Superhydrophobic surface for dropwise condensation is prepared using hotplate solution immersion method on copper substrate. The preprocessed bare copper plate is immersed in a solution consist of 0.004-0.008 M ethanol (CH3-CH2-OH) and tetradecanoic acid (CH3(CH2)12COOH) then heating the plates in the solution at 30-50?C for 1-6 hours. The contact angle of water droplet on the prepared surface is measured using low bond axisymmetric drop shape analysis, which gives the maximum contact angle of 168? and average value of 166? ?2?. The maximum contact angle is obtained by adjusting the composition of the solution, temperature of the solution and immersion time to 0.006 M, 45?C and 4 hours, respectively. The various superhydrophobic surfaces are prepared by changing constituents of solution, hotplate temperature and processing time, respectively. Further dynamic behavior of water droplet on the prepared surfaces like jumping effect and rolling effect is presented in this work. In addition, experimental work is carried out on the prepared surface for dropwise condensation and the obtained results are com-pared with condensation on bare copper plate produces higher heat transfer coefficient.

Dynamic behavior of water droplets impacting on the superhydrophobic surface: Both experimental study and molecular dynamics simulation study

Dynamic behavior of water droplets is an important feature in the study of wettability of solid surfaces. Here, molecular dynamics (MD) simulation was used to fabricate a three-dimensional physical model of a nanodroplet on a superhydrophobic nano-pillared surface that has same contact angle and sliding angle as the fabricated superhydrophobic surface with dandelion-like structures. Accordingly, we fabricated the relationship between the macro-scale experimental work and the nanoscale MD simulation work according to Weber number to study dynamics of water droplets impacting on the superhydrophobic surface. Based on experimental and MD simulation results, we find that dynamics of water droplets impacting on the superhydrophobic surface can be subdivided into spreading section and retraction section. We also used experiments and MD simulations to investigate the relationship between contact time and impact velocity, and found that the impact velocity has no influence on the contact time. Furthermore, the relationship between contact time, as well as restitution coefficient, and impact velocity (or Weber number) was characterized. All the trends we see from MD simulations are consistent with experiments. Therefore, the combined experiment and simulation provide an integrated study of the dynamics of droplets impacting on solid surfaces.

Dynamic behavior of water droplets on wetted superhydrophobic surfaces under a high AC electric field

In this paper, the dynamic behavior of water droplets was investigated on wetted hydrophobic and surperhydrophobic surfaces under a high AC electric field. The droplets on superhydrophobic surface coalesced and slid off the inclined surface before the flashover occurred, while the droplets on hydrophobic surface stayed on the surface and could not roll off easily even under the breakdown field strength. Due to the behaviors of water droplets on the surfaces under the high AC electric field, the flashover voltage of the superhydrophobic surface was increased by 35.8 percent. A two-dimensional simulation model was developed to investigate the dynamic behavior of water droplets on surperhydrophobic surface. The results showed that, the adjacent water droplets were elongated and a liquid bridge was formed between the droplets under electric field. The coalescence of water droplets was impelled by the surface tension force and electric force.

Effect of the oxidation surface on dynamic behavior of water droplet impacting on the molten tin

Abstract A series of experiments were carried out on water droplet impacting on the molten tin surface using a high speed camera. The falling height of water droplet ranges from 10 to 90 cm, and the oxidation layer thickness effect was taken into account by varying the oxidation time of the molten tin. The results show the Weber number and the oxidation time have significant effect on the droplet impact behavior. Three typical phenomena can be observed: Droplet breakup, Bubble and Crown. For the lower Weber number, more than one phenomenon occurs even for the same Weber number and oxidation time. For the Weber number higher than 800, only crown phenomenon appears. The formation of the oxidation layer causes the probability of vapor explosion to decrease, but the intensity of vapor explosion increases with the oxidation time. With the increase in the Weber number and the oxidation time, the maximum crown width and height increase, which implies that the vapor explosion is more violent The correlations of the maximum crown height and width as a function of the Weber number were proposed for specified oxidation time.

Nano-sized amorphous carbon covered surface formed by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs)

In this paper, a nano-sized amorphous carbon covered surface was fabricated by an additive manufacturing process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles (NPs) in ambient condition. This technique synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting. Additionally, the polymer in the solvent was decomposed by the thermal energy, which resulted in the formation of nano-sized amorphous carbon. Consequently, two layers of copper and carbon were fabricated on the substrate of stainless steel which was illustrated by the cross-section SEM. The upper layer of carbon possesses super-hydrophobic and high-light absorptance properties. The reaction products were characterized by the analyze of XRD, XPS and EDS. The contact angle and dynamic behavior of water droplet on the fabricated surface were carried out to demonstrate the super-hydrophobicity. The study provides an easy and flexible method for the fabrication of the functional surface by laser sintering in air ambient.

Preparation and droplet impact dynamics of superhydrophobic nano-aluminum films on metal substrates

A simple electrophoretic deposition method followed by stearic acid modification was developed to prepare superhydrophobic multiscale nano-aluminum films on a series of metal substrates such as nickel, copper, titanium and stainless steel. The surface morphology and chemical compositions were characterized by a field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscope, Fourier-transform infrared spectrophotometer and white light interferometer. The resultant nano-aluminum films exhibited excellent superhydrophobicity with a water contact angle of 168.5° and a water sliding angle of 2°. Furthermore, the obtained nano-Al films showed the desirable stability and durability after the related testing. In addition, the impact dynamic behaviors of water droplet on the superhydrophobic nano-Al films were described via the corresponding parameters, such as Weber number (We), spreading factor (β), contact time (tc), etc. Based on these characteristics, the superhydrophobic nano-Al films presented excellent water-repellent property. The EPD process is an efficient method to prepare superhydrophobic nano-Al films on common engineering metal materials, which can be applied to various industrial or manufacturing fields, including oil–water separation, anti-icing, self-cleaning, anti-corrosion, microelectronic fabrication and so forth.

Dynamic behavior of water droplets and flashover characteristics on a superhydrophobic silicone rubber surface

In this paper, a superhydrophobic surface is used to increase the flashover voltage when water droplets are present on a silicone rubber surface. The dynamic behavior of a water droplet and the associated flashover characteristics are studied on common and superhydrophobic silicone rubber surfaces under a high DC voltage. On common silicone rubber, the droplet elongates and the flashover voltage decreases with increasing droplet volume and conductivity. In contrast, the droplet slides off the superhydrophobic surface, leading to an increased flashover voltage. This droplet sliding is due to the low adhesion of the superhydrophobic surface and a sufficiently high electrostatic force provided by the DC voltage. Experimental results show that a superhydrophobic surface is effective at inhibiting flashover.

A study about the influence of single-scale and dual-scale structures on surface wettability

Surface morphology is known as the key factor to obtain lyophobic surface. This paper illustrates two superhydrophobic surfaces with different surface morphologies including single-scale nanorod structure and dual-scale flower-like structure. We compared contact angles of many liquids and dynamic behavior of water droplet impinging on the nanorod structured surface to those of on the flower-like structured surface. It was found that both the single-scale nanorod structure and the dual-scale flower-like structure can achieve superhydrophobicity. However, the dual-scale flower-like structure is superior to the single-scale nanorod structure in terms of repelling the droplet with low surface tension. In addition, we investigated stability and corrosion resistance of these two superhydrophobic surfaces, showing that both the single-scale nanorod and the dual-scale flower-like structured superhydrophobic surfaces had excellent long-term stability and thermal stability. Nevertheless, the dual-scale flower-like structured superhydrophobic surface was more stable under outside vibration and had better corrosion resistance than the single-scale nanorod structured superhydrophobic surface.

Controllable Dianthus caryophyllus-like superhydrophilic/superhydrophobic hierarchical structure based on self-congregated nanowires for corrosion inhibition and biofouling mitigation

Here we present our findings of superhydrophilic (SHPL)/superhydrophobic (SHPB) Dianthus caryophyllus-like hierarchical structure for corrosion inhibition and biofouling mitigation through a relatively high-speed hard anodisation (HA) technique. The surface morphologies, chemical compositions were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectrophotometer (FTIR). The static and dynamic behaviors of water droplets and water stream demonstrate the extremely low contact angle of SHPL surface and the low adhesive force of SHPB surface. Electrochemical impedance spectra (EIS) in 3.5wt% NaCl solution and settlement experiments in Chlorella vulgaris-inoculated culture medium revealed that the SHPL surface cannot efficiently protect the underlying substrates from corrosion and Chlorella vulgaris cell adhesion, whereas the SHPB surface exhibited noticeable corrosion inhibition and biofouling mitigation. In addition, the synthetic SHPB surface presents good self-cleaning ability to contamination and thermal stability to hot water droplets. We believe that our findings of the anodized SHPL/SHPB Dianthus caryophyllus-on-nanowires structure are helpful in understanding the difference between superhydrophilicity and superhydrophobicity, which is beneficial for designing new bio-inspired interfacial materials with super-wettability for potential marine applications.

Static and Dynamic Hydrophobic Properties of Honeycomb Structured Films via Breath Figure Method

This work reported the static and dynamic hydrophobic properties of the honeycomb porous and peeled films with pincushion structures by changing the concentrations of the copolymer solution. Honeycomb porous films were fabricated via breath figure technology employing a pentablock copolymer containing poly(ethylene glycol)/PEG, poly(methyl methacrylate)/PMMA, and poly(trifluoroethyl methacrylate)/PTFEMA. The relationship between the copolymer concentrations and the pore size (D) and rim width (W) was examined. Moreover, we studied the hydrophobic properties from different surfaces of flat, porous, and pincushion structures, and the contact angles, W/D ratios, and fraction of air on the porous and pincushion films were obtained. It was noteworthy that the W/D ratios and surface hydrophobic properties had a notable correlation. Finally, the dynamic behaviors of water droplets impacting on these porous and pincushion films were examined using a high speed camera. It examined the influence of the fraction of ...

Fabrication of hierarchical structures for stable superhydrophobicity on metallic planar and cylindrical inner surfaces

Recently, the construction of stable superhydrophobicity on metallic wetting surfaces has gained increasing attention due to its potential wide applications. In this paper, we propose an economic fabricating method, which not only is suitable for metallic planar surfaces, but also could be applied onto cylindrical inner surfaces. It mainly involves two steps: etching micro-concaves by a movable mask electrochemical micromachining (EMM) technique and fabricating nanopillars of ZnO by a hydrothermal method. Then the influences of surface morphology on the static and dynamic behaviors of water droplets are investigated. The energy loss during impact on the surfaces is quantified in terms of the restitution coefficient for droplets bouncing off the surfaces. For hierarchical structures with excellent superhydrophobicity (contact angle ≈180° and sliding angle ≤1°), the droplet bounces off the surface several times, superior to the droplet's response on single nanopillars (contact angle ≈165.8° and sliding angle ≈6.29°) where droplet bounces off only for limited a number of times, and even far better than the dynamics of a liquid droplet impinging on microstructures (contact angle ≈132.1° and sliding angle >90°) where droplet does not rebound and remains pinned. The highest elasticity is obtained on the hierarchical surface, where the restitution coefficient can be as large as 0.94. The fabricating method is then applied onto the cylindrical inner surface and the wetting behavior is confirmed to be consistent with the planar surface. This method, which can be generalized to any kind of solid electroconductive metal or other surfaces with different shapes, could find wide practical applications in self-cleaning surfaces, chemical industry, microfluidic devices, mechanical engineering and aviation.

Static and dynamic behavior of water droplet on solid surfaces with pillar-type nanostructures from molecular dynamics simulation

Abstract Predicting the droplet state accurately is significant in manufacturing rough surfaces with superhydrophobic properties. We use molecular dynamics simulation to investigate the static behavior of the water droplet on solid surfaces featuring pillared structures. Results show that the droplet in either the Wenzel state or the Cassie state depends on the height of the pillars and the pillar surface fraction. We then apply a vibration to the smooth or rough surface and evaluate the dynamic behavior of the water droplet. In general, the smooth surface becomes more hydrophobic due to the presence of vibration and the water droplet on the pillared surface is able to transform from the Wenzel state to the Cassie state even though the former is originally favored. In addition, the vibration must meet certain conditions to achieve this transition and the droplet will separate from solid surface while continuing to enhance the vibration.

Effect of nanoparticle enwrapping on the static and dynamic behavior of water droplet

We studied the static and dynamic properties of water droplet and the droplet coated with silica nanoparticles. The water droplet changes from wetting on the quartz substrate to superhydrophobic state with the enwrapping of particles. The impact dynamics of the droplets impinging on the superhydrophobic surface has been monitored by a high speed camera. The coated layer on the droplet surface damps the violent oscillation and accelerates its decay, shortens the contact time and oscillation period. The results suggest that the adsorption of nanoparticles on droplet surface increases the effective surface tension of droplet. The viscoelastic properties of the enwrapping particle layer play an important role in the decay of droplet oscillation.

Recurrent Plot Analysis of Leakage Current in Dynamic Drop Test for Hydrophobicity Evaluation of Silicone Rubber Insulator

In order to reduce electrical accidents caused by aging of polymer insulators, hydrophobicity evaluation is one of the essential methods to monitor the practical performance of insulators. This paper presents recurrent plot (RP) analysis of leakage current (LC) to evaluate the hydrophobicity of the silicone rubber insulator. Experiments were carried out according to the dynamic drop test method. Dynamic behaviors of water droplets and the induced discharge phenomena were captured by using a high-speed camera fixed vertically to the specimen surface. LC was measured and extended to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$m$</tex></formula> -dimensional phase space by using a phase-space reconstructed method. The RP of LC was obtained to reflect the underlying mechanism of surface discharges in relation to the insulator hydrophobicity. The results indicate that the RP technique can give a visual method for the hydrophobicity evaluation. The increasing tendency in RP indicators can reveal the decrease of hydrophobicity.

Droplet impact on regular micro-grooved surfaces

We have investigated experimentally the process of a droplet impact on a regular micro-grooved surface. The target surfaces are patterned such that micro-scale spokes radiate from the center, concentric circles, and parallel lines on the polishing copper plate, using Quasi-LIGA molding technology. The dynamic behavior of water droplets impacting on these structured surfaces is examined using a high-speed camera, including the drop impact processes, the maximum spreading diameters, and the lengths and numbers of fingers at different values of Weber number. Experimental results validate that the spreading processes are arrested on all target surfaces at low velocity. Also, the experimental results at higher impact velocity demonstrate that the spreading process is conducted on the surface parallel to the micro-grooves, but is arrested in the direction perpendicular to the micro-grooves. Besides, the lengths of fingers increase observably, even when they are ejected out as tiny droplets along the groove direction, at the same time the drop recoil velocity is reduced by micro-grooves which are parallel to the spreading direction, but not by micro-grooves which are vertical to the spreading direction.

Dynamic Behavior of Water Droplets on Solid Surfaces with Pillar-Type Nanostructures

In the present study, we investigated the static and dynamic behavior of water droplets on solid surfaces featuring pillar-type nanostructures by using molecular dynamics simulations. We carried out the computation in two stages. As a result of the first computational stage, an initial water cube reached an equilibrium state at which the water droplet showed different shapes depending on the height and the lateral and gap dimensions of the pillars. In the second computational stage, we applied a constant body force to the static water droplet obtained from the first computational stage and evaluated the dynamic behavior of the water droplet as it slid along the pillar-type surface. The dynamic behavior of the water droplet, which could be classified into three different groups, depended on the static state of the water droplet, the pillar characteristics (e.g., height and the lateral and gap dimensions of the pillars), and the magnitude of the applied body force. We obtained the advancing and receding contact angles and the corresponding contact angle hysteresis of the water droplets, which helped classify the water droplets into the three different groups.

A study on the dynamic behaviors of water droplets impacting nanostructured surfaces

We have investigated the influence of impact velocity and intrinsic surface wettability of nanostructures on the impact dynamic behaviors of water droplets on nanostructure surfaces. Nanowires array surfaces with tunable wettabilities ranging from superhydrophilic to superhydrophobic were fabricated by the deposition of surface modifiers differing in alkyl chain length. The transition criteria of rebound/wetting state and rebound/splashing state based on the relationship between the Webber (We) number and the surface free energy were determined. We have confirmed that the critical We number that determines the transition of the rebound/wetting increased as surface energy decreased. Additionally, the We number at which fragmentation occurred on our superhydrophobic surface was relatively low compared to previously reported values.