Motion Of Water Droplets Research Articles

Poly(l-lactic acid) Janus Shape Memory Membranes with Uniform Vertically Penetrative Channels and Slit Pores for Enhanced Fog Collection: Synergistic Effect of Transport through Membranes and Sequential Sliding Behaviors of Water Droplets.

In this work, poly(l-lactic acid) (PLLA) Janus shape memory membranes with uniform vertically penetrative channels (SMEUVs) with slit pores have been fabricated with the help of template-assisting spray-coating and uniaxial tension at high temperature. During fog collection, superhydrophobic and hydrophilic surfaces act as the front (fog-facing) side and back side, respectively, in which the structural characteristics play essential roles. On one hand, the vertically penetrative channels in SMEUVs and the special pore geometry contribute to lower resistance, accelerating the transport of captured water through membranes (from the superhydrophobic side to the hydrophilic side). On the other hand, the movement of water droplets along the back side has been guided by the oriented structures of slit pores, promoting the detachment of droplets from the hydrophilic surface. Their synergistic effect removes captured water in a timely manner and provides fresh sites for the subsequent nucleation of water, enhancing fog collection performance. As a result, the optimal specimen (Janus SMEUVs with a draw ratio of 2.5, placed in the parallel direction) exhibits a much higher water collection rate (∼6×) relative to references (superhydrophobic and hydrophilic membranes). Our results are significant for sustainable development in view of both fog collection in arid regions and the biodegradability of PLLA.

Emerging magnetic-responsive hydrogel actuators with anisotropic shape for intracardiovascular directional motion

A magnetically responsive valve actuator inspired by the starfish shape and fabricated with hydrophilic and hydrophobic surfaces was developed. The surface morphology and wettability of the magnetically responsive colloidal crystal hydrogel actuator were characterized using scanning electron microscopy and contact angle measurements. The actuator, resembling a flower, consists of Poly(HEMA-co-AAm)/Fe3O4/PDMS, exhibiting distinct Janus wettability with hydrophobic Fe3O4/PDMS and hydrophilic Poly(HEMA-co-AAm) surfaces. The directional movement of water droplets on the V-shaped structured film confirmed the anisotropic wetting behavior, influenced by the film’s wettability and structural dimensions. Moreover, the actuator demonstrated responsive angular and positional adjustments under varying magnetic fields, facilitated by embedded Fe3O4 particles aligning with magnetic lines of force. In experimental setups simulating intravascular applications, the actuator exhibited controlled movement within tubes and demonstrated effective adhesion to heart valve injuries under magnetic guidance. Additionally, a layered inverse opal film within the actuator showed promising capabilities for unidirectional fluid permeation, enhancing its functional versatility. These findings underscore the potential of the magnetically responsive colloidal crystal hydrogel actuator for biomedical applications, particularly in minimally invasive cardiac interventions requiring precise actuation and adhesive functionalities.

Efficient Fog-Harvesting Origami Fan.

Fog harvesting represents a promising strategy to address the global freshwater shortage. To enhance the water collection efficiency, diverse geometric structures that can effectively drive water droplet movement are essential. Inspired by the Livistona chinensis leaf, which naturally facilitates directional droplet motion through its unique gradually varying V-groove structure, we have developed a novel origami fan structure for fog harvesting through theoretical analysis. A key feature is that we can modulate the speed of droplet transport by adjusting the opening angle of the V-shaped grooves positioned at the outer circumference. Interestingly, the water collection efficiency exhibits a linear correlation with the opening angle. The highest efficiency of the origami fan can reach 5.75 times that of the control group calculated by the projected area and 3.76 times that of the control group calculated by the real area, showcasing its significant potential for enhancing water collection from fog. The simulations demonstrate that the hollow structure enhances the condensation rate of droplets, the geometric gradient of the gradual-variation V-groove drives the condensed droplets to move rapidly on the surface, and the Janus membrane permits the aggregated droplets to transit to the fan's rear side. The synergistic action of these three components ensures a clean surface for the subsequent water-collecting cycle, contributing to the high fog-harvesting efficiency. Given its simple fabrication and superior water transfer efficiency, the origami fan holds substantial promise for widespread application in the field of droplet manipulation.

Facilitating water droplet removal from surfaces using air flow and wettability gradients

A new method is proposed to mitigate ice accretions on wind turbine blades via the creation of a microstructural gradient surface geometry that facilitates spontaneous water droplet motion along the surface. The wettability gradients are formed by laser etching 35 μm wide, 35 μm deep channels into aluminum to form a surface with a gradually increasing solid area fraction. Different design variations are then proposed and systematically evaluated on the merits of their performance. An analytical model is also derived based on a balance of hysteresis and drag forces to predict the critical airspeed necessary for droplet movement as a function of the droplet size and surface contact angle. Experimentation has shown good agreement with the model for both the baseline and fixed-pitch channel surfaces and has also demonstrated that, in certain cases, up to 70 % lower critical airspeeds are needed to initiate droplet motion on these microstructured surfaces.

Fabrication of functional surfaces using layer height method in material extrusion type 3D printing

Various layer stacking methods have been employed in material extrusion type three-dimensional (3D) printing to utilize their potential for addressing the inherent technological limitation of reduced surface quality due to layer stacking. The process involved fabricating a material extrusion type 3D printed mold using diverse layer height-changing methods and subsequently replicating the surface pattern with the polymer. This approach is called the layer height method (LHM) and comprises three distinct variations. The first method involves altering the height of the individual layers to generate diverse surface morphologies and, consequently, varying the range of water contact angles. The second method focuses on rapid layer height changes to facilitate liquid deposition within regions with low contact angles. Finally, a layer height gradient was systematically established within the mold, resulting in a wettability gradient surface capable of controlling the movement of water droplets across the surface. The performance of these functional surfaces was successfully validated using various experimental methods. This study introduces a manufacturing technique based on changing layer heights within the framework of material extrusion-type 3D printing technology. A wide range of intricate and diverse functional surfaces can be produced by extending the manufacturing method proposed in this study to 3D printing technologies beyond the scope of material extrusion type 3D printing.

Unconventional Dually-Mobile Superrepellent Surfaces.

The ability of water droplets to move freely on superrepellent surfaces is a crucial feature that enables effective liquid repellency. Common superrepellent surfaces allow free motion of droplets in the Cassie state, with the liquid resting on the surface textures. However, liquid impalement into the textures generally leads to a wetting transition to the Wenzel state and droplet immobilization on the surface, thereby destroying the liquid repellency. This study reports the creation of a novel type of superrepellent surface through rational structural control combined with liquid-like surface chemistry, which allows for the free movement of water droplets and effective repellency in both the Cassie and Wenzel states. Theoretical guidelines for designing such surfaces are provided, and experimental results are consistent with theoretical analysis. Furthermore, this work demonstrates the enhanced ice resistance of the dually-mobile superrepellent surfaces, along with their distinctive self-cleaning capability to eliminate internal contaminants. This study expands the understanding of superrepellency and offers new possibilities for the development of repellent surfaces with exceptional anti-wetting properties.

Repulsion, Acceleration, and Coalescence between Water Droplets on Superhydrophobic Glass by Triboelectrification.

Manipulating the motion of water droplets on surfaces, which is crucial for various applications, such as microfluidics and heat transfer, presents considerable challenges, primarily due to the significant influence of capillary forces. This effect becomes more pronounced when droplets are in close proximity, often resulting in undesired coalescence. Triboelectrification, which involves charging pure water droplets, is a promising approach to enhance the ability to manipulate water droplets. For effective triboelectrification, charges must accumulate within the droplets; this ensures efficient and sustained droplet manipulation while minimizing dissipation. Low-friction, superhydrophobic, insulating surfaces are ideal for this purpose. However, few studies have explored the application of insulating superhydrophobic surfaces to manipulate droplet motion. In this study, we investigated the behavior of water droplets on insulating superhydrophobic quartz surfaces after triboelectrification. The droplets acquired significant charge when dripped onto a superhydrophobic glass surface. Consequently, these charged droplets exhibited behaviors such as repulsion and acceleration from one another, uphill movement, and rapid long-distance transport to specific positions. These advancements in droplet manipulation techniques hold promise for diverse fields such as microfluidics and heat exchangers.

The influence of geometric boundary features on droplet wetting and directional motion

Structured surfaces with micro and nanostructures have shown great potential for a wide range of applications, such as self-cleaning, deicing, and drag reduction. Previous research has extensively explored the interactions between droplets and microstructures, particularly in terms of Cassie-Baxter and Wenzel states. However, there is still a notable gap in systematically studying the interactions between droplets and various types of geometrical boundary features. In this study, we employed molecular dynamics simulations to investigate how different geometric boundary features affect the wetting behavior and motion of water droplets. Our detailed analysis led to the derivation of a formula to calculate the critical contact angle for a droplet interacting with any geometric boundary feature from a mechanical perspective. Based on these findings, we successfully controlled motion direction of water droplets along a separation membrane. We observed that due to the influence of geometric boundary features at the holes, droplets exhibited unidirectional motion on the separation membrane. By considering the influence of geometric boundary features, our research provides valuable insights for preventing wetting transition, enhancing the storage of lubricating oil in grooves, facilitating efficient oil–water separation, and regulating fluid flow in microchannels.

Optimization of dynamic control systems using water cycle algorithm

In this paper, path optimization in dynamic control systems was performed using water cycle algorithm. The core principles and assumptions that guide the suggested technique are drawn from nature and are based on observations of the water cycle process and how rivers and streams naturally flow into the sea. The Water Cycle Algorithm has been recognized as a reliable method for dealing with large-scale optimization challenges, largely because of its proficient exploitation and exploration capabilities. This Algorithm does not require the continuity of functions when specifying the issue, unlike many gradient-based approaches. This is achieved through the use of a simple mathematical model that represents the water cycle process. This model is used to simulate the movement of water droplets in a cloud, where each water droplet represents a potential solution to the optimization problem. Although water cycle optimization has been utilized successfully to tackle optimization issues, this is the first time dynamic control optimization has been used. The control problem is switched out for an optimum programming problem in order to find optimal pathways utilizing the Shifted Chebyshev polynomial. The water cycle method was then used to resolve the new issue. The numerical results demonstrate the excellent precision with which this search method can discover solutions.

Analysis of water droplet motion on hydrophobic surfaces with different microstructures

AbstractThe influence of material surface microstructure on interface properties is crucial for the preparation and development of special functional materials. In this paper, polydimethylsiloxane‐based superhydrophobic surfaces of varying roughness and surface energy are prepared using the template method and a spraying technique, and the effect of surface microstructure on lateral friction is studied using a lateral friction detection system. The experimental results indicate that as the surface roughness of a sample decreases, it is observed that the water droplets transition from a Cassie state to a Wenzel state. When the roughness changes from 4.619 μm, reduce to 3.512 μm, the maximum static friction force increases from 33.59 μN increased to 51.52 μN. The lateral sliding friction force ranges from 15.87 ± 1.9 μN increases to 40.91 ± 1.2 μN. For a given surface roughness, when the surface energy is reduced, the water droplets on the surface can be seen to transition from a Wenzel state to a Cassie state, the maximum static friction force increases from 19.32 μN dropped to 9.23 μN, and the lateral sliding friction force increases from 15.26 ± 1.6 μN dropped to 3.47 ± 2.1 μN. However, this behavior is distinct from that observed when changing the roughness of the sample. Studying the influence of the surface microstructure on the lateral friction force observed at a solid–liquid interface can provide important insights into novel geometries that may be used in new, advanced functional surfaces.

Hydrovoltaics brings a new solution in alternative energy: an interview with Prof. Wanlin Guo.

The Water Hub Project at the Three Gorges, also known as the 'Sanxia Project', now provides more than 1 billion kilowatt-hours of electricity daily to 10 provinces in eastern China. Facing rising energy demands and climate change, despite being the largest hydroelectric power station in the world, this project may still not live up to the vision of the inventor of hydroelectric power, Nikola Tesla, when he said: 'Electric power is everywhere present in unlimited quantities and can drive the world's machinery without the need of common fuels.' Hydrovoltaic technology, invented by Prof. Wanlin Guo of Nanjing University of Aeronautics and Astronautics (NUAA), aims to generate electricity through processes such as evaporation and the motion of water droplets on synthetic nanomaterials. National Science Review recently invited Prof. Guo for an in-depth interview to discuss this exciting new technology and how it may represent the next great opportunity to transition from fossil fuels to renewable energy sources.

Numerical Investigation of Supercooled Large Droplets Impingement Characteristics of the Rotating Spinner

Aircraft engine icing caused by supercooled large droplets (SLD) poses a significant threat to flight safety. In this paper, the SLD impingement characteristics of the rotating spinner were investigated using FLUENT UDS and the governing equations for water droplet motion were solved based on the Eulerian method. The droplet breakup was simulated using the number density equation, while the droplet rebound and splashing were simulated using a semiempirical model. The effects of rotational speed, droplet diameter, and inflow velocity on the SLD impingement characteristics of the rotating spinner were studied. Some new valuable insights have been found for the SLD impingement. The results indicated that as the rotational speed increases, the local collection efficiency of the rotating spinner decreases. Higher rotational speed results in reduced droplet impingement angle and stronger droplet rebound and splashing. For the droplets with diameters smaller than 111 μm, the local collection efficiency increases with the increase of the droplet diameter. However, when the droplet diameter exceeds 111 μm, the local collection efficiency decreases near the leading edge of the rotating spinner. Additionally, the local collection efficiency decreases as the inflow velocity increases near the leading edge of the rotating spinner. However, higher inflow velocities lead to larger droplet impingement angles, resulting in higher local collection efficiency near the tail of the rotating spinner. The critical impingement angle increases with the increase of the inflow velocity, leading to a more pronounced rebound and splashing of SLD. The research in this paper provides useful help for ice shape prediction and anti-icing system design of rotating spinner in SLD environment.

Dynamic processes in the liquid crystal-water emulsion under shear flow and electric field

We present the results of experimental investigation of dynamical processes arising in liquid crystal-water emulsion under action of a decay shear flow and ac electric field. The 10 : 1 emulsion, consisting of the well-studied nematic mixture E7 and ionized water was prepared by usage of ultrasonic (29 kHz) mixer. It made possible to get a number of water droplets with the distribution of diameters in the range 2… 20μm. So, the slow decreasing of sizes of these instabilities resulted in corresponding slow motion of the attracted water droplets. The strong electric filled, which stabilized the initial homeotropic orientation of LC suppressed the flow induced instabilities and the slow motion of water droplets mentioned above. It also induced the aggregation of droplets in the vicinity of boundary between regions with strong field and free of field. So, combined action of shear flow and electric field can be effectively used for manipulation of the motion and interaction of water droplets. The obtained results are discussed within the existing theory of liquid crystals.

Directional motion of water droplets enhances anti-icing failure of structural superhydrophobic surfaces

Preventing ice formation on solid surfaces is a crucial concern in various applications, such as aircraft de-icing and protecting electrical transmission cables. One approach to combat ice or frost formation involves the fabrication of superhydrophobic surfaces with micro- or nanostructures. However, some research suggests that the effectiveness of these superhydrophobic surfaces in preventing ice formation may be compromised under conditions of high humidity and low temperatures. Many researchers attribute the resistance of the wetting transition from the Cassie-Baxter state to the Wenzel state on superhydrophobic surfaces to Laplace pressure difference. However, according to molecular dynamics simulations and physical experiments, we find that the impact of Laplace pressure difference depends on the relative size of water droplets and air pockets on the structure. If the size of water droplets is smaller than the characteristic length of the air pockets, directional motion due to Laplace pressure difference and van der Waals force leads to water droplet aggregation and accelerates the ice formation and failure of anti-icing measures. Therefore, our research offers significant insights into the mechanical mechanisms that contribute to the failure of anti-icing techniques on structural superhydrophobic surfaces. This understanding is critical for the design of effective anti-icing or icephobic surfaces.

Visualization of Charge Dynamics when Water Droplets Bounce on a Hydrophobic Surface.

Visualizing the motion of water droplets and understanding their electrification behavior holds significance for applications related to droplet transport, self-cleaning, and anti-icing/deicing and for providing a comprehensive explanation of the solid-liquid triboelectrification mechanism. Here, by constructing microcolumnar structures on the polytetrafluoroethylene surface, a water droplet-based single electrode triboelectric nanogenerator was fabricated for visualizing charge dynamics when a water droplet bounces on a hydrophobic surface. The motion state of the water droplet is closely linked to its electrification behavior through the integration of a high-speed camera and an ammeter. The electrification behavior stemming from the bounce of the water droplet is dynamically captured in real-time. The results show that the magnitude and polarity of the electrical signal have strong dependence on the motion state of the water droplet. For instance, when a water droplet approaches or moves away from the substrate in a single direction, a unipolar electrical signal is generated. However, when the water droplet reaches its limit in the initial motion direction, it signifies a static equilibrium state, resulting in the electrical signal being at zero. Furthermore, we examine the impact of factors such as impact speed, drop contact area, contact line spreading/retraction speed, and impact angle on electrification. Finally, based on the close relationship between poly(ethylene oxide) (PEO) droplet bounce dynamics and electrical signals, the bouncing details of PEO droplets with different concentrations are tracked by electrical signals. This study digitally presents the whole process of droplet bounce in situ and provides a means for monitoring and tracking droplet movement.

Computational Investigation of the Water Droplet Effects on Shapes of Ice on Airfoils

The paper presents the results of studying the effects of droplet diameters on the NACA0012 airfoil ice accretion, which have been obtained in the 3D numerical simulation of icing. To simulate the motion of water droplets as a multiphase medium, the Eulerian approach is used, which assumes that water droplets have spherical shapes, do not undergo deformation and breakup, do not interact with each other, and that coalescence/fragmentation of droplets does not take place. Both monodisperse (of the same size) and polydisperse (of various sizes) droplets are considered; they are represented by the spectral Langmuir distributions. These spectral distributions take into account the polydisperse nature of droplets and provide a higher efficiency in predicting ice shapes. The obtained ice shapes on an airfoil are compared with the available experimental and calculated data. It should be noted according to the simulation results that the use of the standard size of droplet diameter equal to 20 μm does not allow for obtaining correct shapes of ice on the leading edge of the wing profile not at all temperature regimes. For temperatures from −20 °C to −10 °C, there is a noticeable difference compared to the experimental data. At the same time, for this temperature range, the use of the Langmuir spectral distribution of droplet diameters relative to 15 μm provides a better agreement of the formed ice forms with the experiment.

Parametric analysis of acoustically levitated droplet for potential microgravity application

Acoustic levitation is a versatile technique for the non-contact handling of both solid and liquid samples, yet its potential remains underutilized in container-less processing and lab-on-a-drop application. This study demonstrates the efficient use of acoustically levitated droplets as microgravity simulators on earth similar to clinostat. Using a simple and affordable non-resonant single-axis acoustic levitator called TinyLev, we first investigate the impact of applied voltage and droplet volume on the shape and rotational motion of levitated water droplets. Furthermore, we examine the rotational motion of suspended microparticles within the droplets. Our findings reveal the feasibility of using the levitated droplets as a miniaturized microgravity simulators on earth.

Optimization of Welded Beam Design Problem Using Water Evaporation Optimization Algorithm

This paper introduces a novel approach to tackle the Welded Beam Design Problem through the application of the Water Evaporation Optimization Algorithm (WEOA), a nature-inspired metaheuristic. The problem involves finding the optimal dimensions of a welded beam that can support a given load while minimizing its weight. The Water Evaporation Optimization Algorithm draws inspiration from the evaporation process and water droplet movement in nature. The design is formulated as an optimization challenge with beam dimensions as variables and incorporate constraints such as allowable stress and geometric limitations. The fitness function is tailored to evaluate each candidate solution based on load-bearing capacity and weight. To demonstrate the efficacy of the proposed method, extensive experimental evaluations are conducted. Comparisons with traditional optimization techniques highlight the WEOA's superior convergence and global search capabilities. Real-world case studies further illustrate the practical applicability of the optimized welded beam designs, showcasing their cost-effectiveness and high-performance characteristics. The results underscore the potential of the Water Evaporation Optimization Algorithm as a robust and efficient tool for tackling the welded beam design problem. The approach provides engineers with valuable support in achieving optimized beam designs, leading to improved structural performance and material utilization.

Parameter Optimization of Support Vector Machine using River Formation Dynamic on Brain Tumor Classification

Brain tumor is a condition that can interfece with brain function due to abnormal cell growth in the brain. MRI is used as a diagnostic tool when a patient has a brain tumor. The images obtained through MRI will be analyzed by the doctor to determine the type of tumor. Therefore, it is necessary to have a system that can classify tumor types based on MRI images. The image will be extracted using the HOG method, then classified using SVM. Certain measures can be taken to improve SVM performance, such as optimizing its parameters. This research develops a system that uses a novel combination, the SVM with the River Formation Dynamic (RFD) algorithm. RFD is being used to optimize parameters of SVM (C and gamma). The basic idea of RFD is to imitate the movement of water droplets flowing from high to low areas. This research compares the accuracy produced by SVM with the accuracy produced by SVM-RFD. The result is that SVM-RFD provides the better accuracy than only using SVM. The accuracy result by SVM on the MRI dataset is 74.37%. When we compared it with SVM-RFD, the accuracy increased by 13.19% to 87.56%. Further work will be carried out on the implementation of RFD on other SVM parameters to find other parameter combinations that can improve the accuracy of SVM.

Prediction of Contact Angle for Oriented Hydrophobic Surface and Experimental Verification by Micro-Milling

The rectangular microgroove surfaces have obvious anisotropy, which can control the movement of water droplets in parallel and vertical directions. Based on such a property, anisotropic functional surfaces are expected to have potential applications in the fields of droplet-oriented delivery and microfluidics. Micro-milling can accurately adjust the dimension of microstructures, which is convenient to explore the optimal micro-structural parameters. In this study, the non-composite and composite state prediction models of contact angle on the oriented hydrophobic surface were established based on minimum Gibbs free energy, and the effect of micro-structural dimension parameters on contact angle was investigated. Then, the rectangular microgroove structure on 316 L stainless steel was prepared using micro-milling. The composite state prediction model of contact angle was found to be more consistent with the actual situation, and reducing the width of the convex platform was beneficial to increasing the contact angle. In particular, the contact angle in the parallel direction reached 146.5° when the width of the convex platform was 60 μm, and the accuracy of the prediction model was 98.4%. The proposed prediction models of contact angle provide a theoretical basis for designing and preparing oriented hydrophobic surfaces.