Droplet Transport Research Articles

Programmable curvilinear self‐propelling of droplets without preset channels

AbstractCurvilinear self‐propelling of droplets has attracted great interest in the past few decades due to their irreplaceable roles in many areas. Conventional understanding is that a droplet moves only along a preset channel formed by morphology or chemical components. Achieving programmable curvilinear droplet motion independent of a preset channel remains greatly challenging. Here, we report a programmable curvilinear self‐propelling of droplets (circle, divergence, and convergence) based on the collaboration of the curvilinear wetting gradient and the Leidenfrost effect. This design achieves motion trajectory in a well‐controlled manner as well as high velocity and long distance of droplet transport independent of the preset channel. Moreover, the motion behaviors of droplets could be predicted accurately by theoretic simulation. We envision that our unique design could manifest extensive practical applications in fluidic devices, liquid transport, and heat transfer systems.

The collective transportation of many self-propelled liquid metal droplets in Tesla valve

Gallium-based liquid metal droplet is active particle that maintains a liquid phase at room temperature and perform Self-propelled motion in electrolyte solutions. Unlike passive rigid particles, the collective transportation of swarming liquid metal droplets is strongly influenced by the surface tension, the shape deformation, the collision dynamics and the coalescence process of liquid metal droplets. We drive the swarming micron-sized Galinstan droplets to pass a Tesla microfluidic valve by applying external electric field, the diodicity of liquid metal droplet flow in Tesla valve is verified experimentally, which increases with respect to an increasing voltage. Theoretical simulations of flow field within the Tesla microfluidic valve suggested different turbulent flow field with opposite input flows. The coalescence of liquid metal droplets was enhanced when the Tesla valve blocks the flow. The collective transportation of swarming self-propelled liquid metal droplets has promising applications in biomedics, flexible electronics and swarm microrobotic technology.

Sunflower-Inspired Superhydrophobic Surface with Composite Structured Microcone Array for Anisotropy Liquid/Ice Manipulation.

Precisely controlling the directional motion trajectories of droplets on anisotropic 3D functional surfaces has great application potential in self-cleaning, drug delivery, and droplet power generation, but it also faces huge challenges. Herein, inspired by the microcone structure in the heart of sunflowers, a nanoneedle-modified microcone array surface (NMAS)is reported. The surface is created using a combination of nanosecond laser direct engraving and electroforming and is subsequently fluorinated. Through programmable control of the laser spot, the geometric parameters and inclination angle of the microcone can be quickly and finely adjusted, thereby achieving precise control of the droplet bouncing trajectory. The results show that droplets can achieve programmable multiple bouncing behaviors on patterned functional surfaces, including gravity-defying hopping and directional water transport. It is worth noting that this functional surface has delayed freezing and anti-freezing effects. Furthermore, this functional surface has a wide range of potential applications, including surface self-cleaning, droplet capture, and droplet-based chemical microreactions, especially in the field of anti-icing operations. This opens up a new way for the directional transport of droplets on biomimetic functional surfaces.

Enhancing water transportation capacity by asymmetrical patterned surface with super-wettability

Spontaneous and directional water droplet transportation based on patterned surface with super-wettability is crucial for the development of frontier science technology. However, water droplet transportation cannot meet both long distance and fast transportation simultaneously. Here, we overcame this limitation by proposing an asymmetric serial brachistochrone-shaped pattern (ASBP). Water droplet could be transported on the ASBP with a transportation distance of 72.52 mm and a transportation velocity of 158 mm/s after a series of single-factor experiments, orthogonal design optimization, and junction transition optimization. In addition, the water droplet could be transported on a curved ASBP, a super-long ASBP for multi-droplet scenarios, and an ASBP at an inclination angle. Moreover, acidic and alkaline aqueous solution droplet showed similar transportation distance and transportation velocity on the ASBP. Based on the aforementioned superb water transportation capacity, this ASBP can be applied in the fields of fog collection, solution mixing and reaction, and reagent detection. This work has strong implications for promoting the application of patterned surface with super-wettability in the field of high-performance fluid transportation systems.

Microfluidics-based condensation bioaerosol sampler for multipoint airborne virus monitoring

To facilitate rapid monitoring of airborne viruses, they must be collected with high efficiency and concentrated in a small volume of a liquid sample. In addition, the development of low-cost miniaturized samplers is essential for multipoint monitoring. Thus, in an attempt to fulfill these requirements, this study developed a microfluidic condensation bioaerosol sampler (MCBS). The developed sampler comprised two parts: a virus growth section and a virus droplet-to-liquid sample conversion section, each of which was fabricated on a chip using microfluidic technology. The condensation nucleus growth technique used in the virus growth section grew nanometer-sized airborne viruses into micro-sized droplets, making it possible to collection of viruses easier and with high efficiency. In addition, the virus droplet-to-liquid sample conversion section controlled the transport of droplets based on electrowetting technology. This enabled the collected airborne viruses to be concentrated in tens of microliters of the liquid sample. To evaluate the performance of both the sections, the virus dropletization, virus collection efficiency, and virus droplet-to-liquid sample conversion efficiency were evaluated through quantitative experiments. H1N1 and HCOV-229E viruses were used to conduct quantitative experiments on MCBS. We could obtain virus liquid samples with at 72.8- and 89.9-times higher concentration through 1:1 evaluation with a commercial sampler. Thus, the developed sampler facilitated efficient collection and concentration of airborne viruses in a compact, cost-effective manner. This is expected to facilitate rapid and accurate multipoint monitoring of viral aerosols.

Biomimetic Fog Collector with Hybrid and Gradient Wettabilities.

Water scarcity is a global problem and collecting water from the air is a viable solution to this crisis. Inspired by Namib Desert beetle, leaf venation and spider silk, we designed an integrated biomimetic system with hybrid wettability and wettability gradient. The hybrid hydrophilic-hydrophobic wettability design that bionomics desert beetle's back can construct a three-dimensional topography with a water layer on the surface, expanding the contact area with the fog flow and thus improving the droplet trapping efficiency. The venation-like structure with wettability gradient not only provides a planned path for water transportation, but also accelerates water removal under the synergistic effect of gravity and wettability driving force, thus further improving the surface regeneration rate. The collector combines droplet capture, coalescence, transportation, separation, and storage capabilities, which provides new ideas for the design of future high-efficiency fog collectors.

Abstract We106: Long Non-Coding RNA LIPTER Regulates Lipid Metabolism of Human Cardiomyocytes and Preserves Cardiac Function

Background: Recently, long non-coding RNAs (lncRNAs) have emerged as novel human-specific (i.e. non-conserved) regulators of cardiovascular development and disease, which possess diverse functions through interacting with other molecules, including DNA, RNA, protein, and lipid. We identified a human lncRNA, LIPTER, which mediates lipid droplet (LD) transport to regulate lipid metabilism of human cardiomyocytes (CMs). Mechanistically, LIPTER binds phospholipids PA and PI4P on the LD membranes and the MYH10 protein, connecting LDs to the cytoskeleton and facilitating LD transport to mitochondria. LIPTER deficiencies led to prominent LD accumulation, mitochondrial dysfunction, and death of human iPS cell-derived CMs. Since downregulation of LIPTER and enahnced LD accumulation are associated with cardiac dysfunction and heart failure in patients with metabolic disorders, such as obesity and diabetes mellitus, we thereby seek to explore the preclinical potential of LIPTER for the therapy of human metabolic disorder associated cardiac dysfunction and heart failure. Hypothesis: We hypothesize that transgenic overexpression of the human lncRNA LIPTER in mouse heart could attenuate metabolic syndrome-associated myocardium lipid accumulation and cardiac dysfunction, as well as prevent isoproterenol-induced heart failure through increasing the fatty acid oxidation (FAO) of mouse heart. Goals: To evaluate the preclinical potential of human lncRNA LIPTER for the therapy of cardiac dysfunction and heart failure by using mouse cardiac dysfunction and heart failure models. Methods: We established a LIPTER transgenic overexpression mouse line (LIPTER Tg ). Both WT and LIPTER Tg mice were fed with high fat diet for 10 months to induce cardiac lipid accumulation and dysfunction. Additionally, heart failure was induced by the chronic isoproterenol administration. After treatment, mice were subjected to histological and molecular analyses. Results: We found LIPTER Tg significantly enhanced FAO of mouse heart, reduced cardiac lipid accumulation and fibrosis, and alleviated cardiac dysfunction of high-fat-diet fed mice, as well as preserved cardiac function of mice post isoproterenol administration. Conclusions: We unveil a crucial role of human lncRNA LIPTER in regulating lipid metabolism of human CMs and testify its clinical potential for treating cardiac dysfunction and heart failure.

Scaling laws of droplets on vibrating liquid-infused surfaces

Droplets oscillating on vibrating substrates are very interesting scientifically, with applications such as anti-icing, droplet transportation, and measuring dynamic surface tension. Reported here are the dynamics of droplets with different volumes on a vibrating smooth surface infused with liquid of different viscosities. The movement of the three-phase droplet contact line is used to quantify the droplet dynamics, and it is found that this movement is linearly proportional to the amplitude of the substrate and inversely proportional to the viscosity of the liquid infused therein. When the substrate viscosity is relatively low, the droplet volume also affects the contact-line movement. Scaling laws for the contact-line movement are derived involving the Ohnesorge number and the reciprocal of the capillary number. Also elucidated is the relationship between the resonance frequency and the substrate viscosity, and the characteristic droplet morphology under different substrate viscosities is extracted to describe the contact-line movement. Interestingly, the substrate viscosity is controlled in an innovative way to achieve almost the same contact-line movement on the present surface as on superhydrophobic and hydrophilic surfaces.

Liquid Springs for High‐Speed Contamination‐Free Manipulation of Droplets and Solid Particles

AbstractAccurate and flexible control of droplets is essential in many industrial applications, such as water harvesting, chemical assays, and biological detection. Magnetic force‐based methods have been broadly exploited to fulfill the goals due to the advantage of non‐contact, easy control, and long‐range navigation. Nevertheless, it still suffers from some challenges, such as sample fouling, and the paradox between the droplet efficient motion and the large volume. Here a ferrofluid‐based liquid spring to achieve contamination‐free and fast droplet transportation on non‐wetting solid surfaces is proposed. The liquid spring is based on the actuation of a ferrofluid droplet in an external uniform magnetic field. The actuation enables the liquid spring to propel tiny objects, including the non‐magnetic miscible droplets and water‐repellent solid particles, with adjustable motion velocity. Furthermore, the magnetic force, applied on the ferrofluid via an additional permanent magnet, makes it possible to navigate the liquid spring in a programmable way. With the aid of the liquid spring, the single or multiple droplets/solid particles advancing, on‐demand droplets coalescence, and out‐of‐plane droplet motion are achievable.

Automated droplet manipulation enabled by a machine-vision-assisted acoustic tweezer

Automated droplet manipulation holds great significance for various biochemical applications, including but limited to heavy metal ions detection. Despite notable progress, contactless-acoustic-tweezer-based automated droplet manipulation on superhydrophobic surfaces is rarely reported. In this work, we introduce a machine-vision-assisted acoustic tweezer (MVAAT) for automated and contactless manipulation of droplets on a superhydrophobic surface. The MVAAT generates ultrasound standing waves between an ultrasonic transducer (UST) and a superhydrophobic surface, inducing acoustic radiation force to facilitate contactless droplet manipulation on the superhydrophobic surface. An industrial-camera-based machine vision system is employed for real-time detection and tracking of droplets, providing precise droplet positions for automated droplet transportation and merging via a UST equipped on a Cartesian robot. Experimental results demonstrate that the proposed MVAAT can transport droplets of various volumes on a superhydrophobic surface along planned paths at considerably high velocities exceeding one centimeter per second. Furthermore, with the vision feedback, the MVAAT can reliably and precisely transport a droplet to a target position, even if the droplet fails to follow the UST during its movement. Moreover, automated merging and patterning of multiple droplets are achieved, showcasing the versatility of the MVAAT. Last, the MVAAT is applied for fluorescence-based Cu2+ detection to showcase its potential for practical biochemical analyses. The proposed MVAAT significantly expands the capability of ultrasound for droplet manipulation on superhydrophobic surfaces, promising numerous practical applications in biology and chemistry.

Synergistically Utilizing a Liquid Bridge and Interconnected Porous Superhydrophilic Structures to Achieve a One-Step Fog Collection Mode.

Conventional fog collection efficiency is subject to the inherent inefficiencies of its three constituent steps: fog capture, coalescence, and transportation. This study presents a liquid bridge synergistic fog collection system (LSFCS) by synergistically utilizing a liquid bridge and interconnected porous superhydrophilic structures (IPHS). The results indicate that the introduction of liquid bridge not only greatly accelerates water droplet transportation, but also facilitates the IPHS in maintaining rough structures that realize stable and efficient fog capture. During fog collection, the lower section of the IPHS is covered by a water layer, however due to the effect of the liquid bridge, the upper section protrudes out, while covered by a connective thin water film that does not obscure the microstructures of the upper section. Under these conditions, a one-step fog collection mode is realized. Once captured by the IPHS, fog droplets immediately coalesce with the water film, and are simultaneously transported into a container under the effect of the liquid bridge. The LSFCS achieves a collection efficiency of 6.5kg m-2 h-1, 2.3 times that of a system without a liquid bridge. This study offers insight on improving fog collection efficiency, and holds promise for condensation water collection or droplet manipulation.

Effect of high-intensity vibration parameter on droplet transport characteristics in gas channel of proton exchange membrane fuel cell

In aerospace applications, proton exchange membrane fuel cells are subject to high-intensity vibration. Vibration has a substantial effect on water transport in fuel cells. However, there is a lack of research on the effects of multi-directional and high-intensity vibration conditions on fuel cells. In this study, the droplet transport characteristics in the gas channel under high-intensity vibration conditions are investigated for the first time. Moreover, the effects of vibration direction and vibration frequency on the droplet dynamic transport process are specifically studied. The droplet transport state, drainage performance, and pressure drop variation in the gas channel under different vibration conditions are analyzed and interpreted. The results show that low-frequency vibration in the longitudinal direction is more conducive to the droplet discharge, and the pressure drop undergoes a periodic change corresponding to the vibration frequency. Vertical vibration promotes droplet detachment from the gas diffusion layer surface, which contributes to the reduction in water coverage ratio on its surface, despite an increase in the channel pressure drop. Under the influence of horizontal vibration, droplets are more susceptible to adsorption on the channel sidewalls and turn into a liquid film, which leads to a longer discharge time and a reduction in pressure drop.

Magnetically-responsive microwall arrays with path-guide for directional transportation of droplets

This research provides a comprehensive exploration of the development and characterization of magnetically responsive microwall arrays (MRMAs), presenting a novel approach to precise droplet manipulation. The proposed fabrication process involves microscale wall arrays created using carbonyl iron particles embedded in polydimethylsiloxane through a replica molding process. The MRMAs demonstrate a unique response to magnetic fields, enabling precise control over droplet movement. Through superhydrophobic coatings and meticulously adjusted magnetic fields, the system facilitates the efficient movement of droplets along predefined routes, achieving outstanding accuracy in droplet directionality and positioning. The experiments highlight the capability of MRMAs to merge differently colored droplets, underscoring their potential in long-distance droplet transportation. The results suggest applications in microfluidic systems, lab-on-a-chip devices, and targeted drug delivery, marking a significant advancement in microfluidic research.

Synergistically biomimetic platform that enables droplets to be self-propelled

Droplet transport still faces numerous challenges, such as a limited transport distance, large volume loss, and liquid contamination. Inspired by the principle of ‘synergistic biomimetics’, we propose a design for a platform that enables droplets to be self-propelled. The orchid leaf-like three-dimensional driving structure provides driving forces for the liquid droplets, whereas the lotus leaf-like superhydrophobic surface prevents liquid adhesion, and the bamboo-like nodes enable long-distance transport. During droplet transport, no external energy input is required, no fluid adhesion or residue is induced, and no contamination or mass loss of the fluid is caused. We explore the influence of various types and parameters of wedge structures on droplet transportation, the deceleration of droplet speed at nodal points, and the distribution of internal pressure. The results indicate that the transport platform exhibits insensitivity to pH value and temperature. It allows droplets to be transported with varying curvatures in a spatial environment, making it applicable in tasks like target collection, as well as load, fused, anti-gravity, and long-distance transport. The maximum droplet transport speed reached (58 ± 5) mm·s−1, whereas the transport distance extended to (136 ± 4) mm. The developed platform holds significant application prospects in the fields of biomedicine and chemistry, such as high-throughput screening of drugs, genomic bioanalysis, microfluidic chip technology for drug delivery, and analysis of biological samples.

3D Bionic Water Harvesting System for Efficient Fog Capturing and Transporting

AbstractFog harvest has emerged as a direct and efficient water harvesting technology to relieve the intense pressure of freshwater scarcity worldwide. With the vagaries of climate and increasing amount of energy consumption, high‐efficiency fog harvest devices focus on the fast water droplet capture and transportation are highly desired. In this study, a novel harp structure is developed using cross‐twisted copper filaments arranged in a spatial triangular pattern to enhance water droplet capture and transportation. Inspired by the natural differences in Laplace pressure observed in cactus and spider silks, this design accelerates the movement of water bridges. Besides, drawing on the fruit waxes on the surface of hogweed and blueberries, a paraffin wax coating is applied on the copper sheet frame to create a solid slip frame, improving the synergy between filament capture and frame transportation. The monolithic fog collector (MFC) thus achieves a significant increase in fog harvest efficiency and demonstrates excellent durability. Integration of the MFCs into a 3D high‐efficiency fog harvest system results in a harvest rate of 0.5027 g cm−2 min−1, showing promise for practical applications due to its durability, simplicity, and environmental friendliness.

Janus Flexible Device with Microcone Channels for Sampling and Analysis of Biological Microfluidics.

Controlling the spontaneous directional transport of droplets plays an important role in the application of microchemical reactions and microdroplet detection. Although the relevant technologies have been widely studied, the existing spontaneous droplet transport strategies still face problems of complex structure, single function, and poor flexibility. Inspired by the spontaneous droplet transport strategy in nature, an asymmetric wettability surface with microcone channels (AWS-MC) is prepared on a flexible fabric by combining surface modification and femtosecond laser manufacturing technology. On this surface, the capillary force and Laplace pressure induced by the wettability gradient and the geometric structure gradient drive the droplet transport from the hydrophobic surface to the hydrophilic surface. Notably, droplets in adjacent hydrophilic regions do not exchange substances even if the gap in the hydrophilic region is only 1 mm, which provides an ideal platform for numerous detections by a single drop. The droplet transport strategy does not require external energy and can adapt to the manipulation of various droplet types. Application of this surface in the blood of organisms is demonstrated. This work provides an effective method for microdroplet-directed self-transport and microdroplet detection.

Janus membrane with heterogeneous wettability top surface for fog harvesting

To alleviate the global water scarcity crisis, fog harvesting has seen considerable development as a promising strategy. However, the current main challenge lies in achieving efficient fog deposition, timely transport of water droplets, and storage. Here, inspired by clusters of cactus trichomes and sponges, a Janus membrane with heterogeneous wettability top surface (JM-HWTS) is proposed. The JM-HWTS was obtained by spraying paraffin wax particles (hydrophobic) on the copper mesh top surface (superhydrophilic). The relationship between the loading amount of paraffin particles and the surface morphology, water permeability, and one-way penetration ability of the sample was clarified. Compared to fog collection surfaces with single hydrophilic or hydrophobic properties, the proposed JM-HWTS system significantly increased fog collection efficiency. For JM-HWTS, the fog will first deposit into water droplets on the hydrophobic front side, then grow up, and finally be sucked into the hydrophilic back side. This unique cyclic fog collection method is the key to efficient fog collection. The JM-HWTS exhibits excellent water-collection efficiency, enabling efficient capture of fog and rapid directional delivery of droplets. This work will significantly complement the fog collection and water droplet storage fields.

Multi-biomimetic Double Interlaced Wetting Janus Surface for Efficient Fog Collection.

Various methods to solve water scarcity have attracted increasing attention. However, most existing water harvesting schemes have a high demand for preparation methods and costs. Here, a multi-biomimetic double interlaced wetting Janus surface (DIWJS) was prepared by laser for effective fog collection. The as-prepared surfaces are composed of superhydrophilic points/hydrophobic substrates on the A-side and superhydrophilic stripes/hydrophobic substrates on the B-side. The interlaced wettability and superhydrophilic points on the A side are conducive to capture and permeation of droplets. The superhydrophilic stripes and interlaced wettability on the B-side are conducive to transportation and shedding of droplets. Therefore, the overall fog collection process is accelerated. The proposal of smart farm model validates broad application prospects of DIWJS. This work provides an advanced and multi-biomimetic surface and provides important insights for green, low-cost, and versatile strategies to solve water scarcity issues.

Statistical analysis of infectious disease transmission risk based on exhaled respiratory droplet trajectory distribution

In the present work, the risk of infectious disease transmission is evaluated based on a statistical analysis of respiratory droplet trajectory distribution. An analytical model recently developed by the authors allows the prediction of the trajectory and evaporation rate of exhaled droplets. The model is used to collect data from a sampling set of more than twenty thousand droplets distributed over a range of diameters from 0.1 μm to 1 mm for different respiratory scenarios. The analytical tool implements the governing equations of droplet transport, evaporation, energy balance, and chemical composition. It also features a two-dimensional unsteady empirical model of respiratory cloud including momentum dissipation and buoyancy. A discrete random walk approach to simulate the droplet turbulent dispersion, and the randomization of the droplet release within the exhalation period and the mouth cross section area complete the model enabling statistical analyses to be rightly performed. With the due boundary conditions, different types of respiratory events can be modeled easily. With additional information on the exhaled droplet size distribution and viral content, spatial maps of virus concentration are derived and associated with the risk of infectious disease transmission being able to discriminate between various transmission routes such as fomite, airborne, or direct inhalation. Different scenarios are presented including mouth breathing, nose breathing, speaking, coughing, and sneezing. The fluid dynamic behavior of respiratory droplets is explored on a size basis, and the role of ventilation discussed. Risk evaluation provides useful information for a knowledgeable discussion on the prevention needs and means from case to case.

Quantitative liquid storage by billiards‐like droplet collision on surfaces with patterned wettability

AbstractThere has been significant interest in researching droplet transport behavior on composite wetting surfaces. However, current research is primarily focused on modifying individual droplets and lacks an in‐depth investigation into high‐precision droplet storage. This study introduces a “billiard ball” droplet transport and storage platform (TSP) with differentiated areas. Within this platform, the volume of droplets stored in the area reaches a consistent threshold through droplet “scrambling,” inspired by the water‐gathering behavior of spiders. The TSP involves connecting two regions of different sizes using a three‐dimensional stepped wedge angle structure. However, this connection is not seamless, leaving a 2‐mm gap between the regions. This gap is intentionally designed to enable continuous droplet transfer while preventing any static migration. Through systematic experimental and simulation analysis, we investigated the influence of superhydrophilic pattern structures and parameters on quantitative droplet storage. We established a functional relationship between the pattern area and the stored volume, and analyzed the intrinsic mechanism of droplet collision separation. This enabled us to achieve on‐demand quantitative droplet storage and autonomize the storage process. The “billiard ball” droplet transport–storage platform proposed in this study holds promising applications in the fields of biomedical and organic chemistry.