Droplet Speed Research Articles

Cilia‐Inspired Functional Channels for High‐Speed and Directional PEMFC Drainage

AbstractProton Exchange Membrane Fuel Cells (PEMFC) consist of channels that require efficient drainage for optimum performance. However, it remains a grand challenge to achieve both directed and high‐speed active drainage in PEMFC channels. This greatly limits current PEMFC performance. Herein, inspired by respiratory cilia, a bioinspired structural design strategy is introduced into the channel surface to significantly enhance its active droplet transport efficiency. Two artificial cilia arrays are fabricated and named as plane‐bottom artificial cilia array (PACA) and concave‐bottom artificial cilia array (CACA) according to their respective shapes. PACA demonstrates stable directional transport. The droplet speed reaches as high as 86 mm s−1 with droplet volumes ranging from 10 to 20 µL. This is surpassed by CACA. Its speed reaches as high as 163 mm s−1 for droplet volumes ranging from 10 to 50 µL. PEMFC experimental evaluation of diameter sizes shows that CACA can be tuned for enhancing its electrochemical performance. CACA‐8 achieves the maximum power density (0.5339 W cm−2) at 1.1864 A cm−2, representing an 18.70% increase compared to conventional PEMFC channels. This work provides a promising strategy for the stable and efficient transport of liquid water in PEMFC.

Measurement of inkjet droplet speed using interference fringe by diffracted light

Inkjet printers are key technologies in manufacturing organic light-emitting diodes and quantum dot light-emitting diode panels, but precise measurement and control of inkjet droplets remains challenging. The international standard, IEC 62899-302-1, uses shadow image-based measurement with high magnification microscopes to observe picoliter-sized droplets. However, high magnification lens results in a shallow depth of field or narrow optimal measurement area, causing the blurring image if the droplet does not pass through the optimal measurement area. To solve this, we propose using the interference image-based measurement with interference fringe patterns by inkjet droplets as a tool to measure the flight speed of droplets. The interference fringe patterns can be obtained simply passing the droplet through within the light beam path, providing approximately 1000× wider measurement area compared to the shadow image-based measurement, making it practical to use in the industry. The flight speed of droplets analyzed with the interference image-based measurement at various frequencies and amplitudes of the inkjet driving voltage were compared with the shadow image-based measurement. The interference image-based measurement showed a coefficient of variation of less than 3%, showing higher repeatability than the shadow image-based measurements.

Marangoni Droplets of Dextran in PEG Solution and Its Motile Change Due to Coil-Globule Transition of Coexisting DNA.

Motile droplets using Marangoni convection are attracting attention for their potential as cell-mimicking small robots. However, the motion of droplets relative to the internal and external environments that generate Marangoni convection has not been quantitatively described. In this study, we used an aqueous two-phase system [poly(ethylene glycol) (PEG) and dextran] in an elongated chamber to generate motile dextran droplets in a constant PEG concentration gradient. We demonstrated that dextran droplets move by Marangoni convection, resulting from the PEG concentration gradient and the active transport of PEG and dextran into and out of the motile dextran droplet. Furthermore, by spontaneously incorporating long DNA into the dextran droplets, we achieved cell-like motility changes controlled by coexisting environment-sensing molecules. The DNA changes its position within the droplet and motile speed in response to external conditions. In the presence of Mg2+, the coil-globule transition of DNA inside the droplet accelerates the motile speed due to the decrease in the droplet's dynamic viscosity. Globule DNA condenses at the rear part of the droplet along the convection, while coil DNA moves away from the droplet's central axis, separating the dipole convections. These results provide a blueprint for designing autonomous small robots using phase-separated droplets, which change the mobility and molecular distribution within the droplet in reaction with the environment. It will also open unexplored areas of self-assembly mechanisms through phase separation under convections, such as intracellular phase separation.

Mechanism chaotic movement of Leidenfrost droplets

One of the most effective methods for cooling overheated surfaces is drip irrigation. If the surface temperature exceeds the Leidenfrost temperature, then a vapor film is formed between the droplet and the surface, which leads not only to a decrease in heat transfer intensity but also causes droplet mobility. For a number of applications, the mobility of droplets is an undesirable phenomenon, so the analysis of the factors responsible for their movement is a relevant task. Here we analyze the movement mechanism of the Leidenfrost droplets with variations in the composition and volume of the droplets. The data obtained show that the droplet speed increases with an increase in the droplet volume. However, smaller droplets change direction of motion more often than larger droplets. To substantiate the experimental data, a hypothesis is proposed, according to which the mechanism of movement of Leidenfrost droplets is caused by the reactive force that arises due to the evaporation of liquid. A Leidenfrost droplet changes the direction of its movement due to the deformation of its surface under the influence of gravity and capillary force. To substantiate the experimental data a simple phenomenological model is proposed.

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.

Ultrafast self-transportation and efficient separation of organic droplets on semi-conical asymmetric structure

Manipulating oil droplets in an aqueous solution is highly desirable for organic multiphase liquid separation. Despite substantial works in the realm of organic multiphase liquid manipulation and separation, the ultrafast transportation and efficient and precise separation of these liquids, especially those with varied surface tensions, encounters significant challenges due to little driving forces. To address this issue, a semi-conical structure with pine-needlelike features and incorporated wedge-shaped grooves is fabricated, which could support the ultrafast transport and separation of organic droplets. For pre-wetted superhydrophobic surfaces, organic liquids with lower surface tension can be transported at a speed of 305.6 mm/s. The Laplace pressure difference caused by the conical structure determines the direction of transport of organic liquids, and the capillary force caused by flat wedge-shaped grooves increases the transport speed of organic droplets on the surface. Based on the characteristics of “differential” transportation and directional transportation, the separation of mixed organic liquids is realized. This biomimetic design concept will pave a path for microfluidics, liquid manipulation, and the separation of various organic liquids.

Self-supervised learning of shedding droplet dynamics during steam condensation

Knowledge of condensate shedding droplet dynamics provides important information for the characterization of two-phase heat and mass transfer phenomena. Detecting and segmenting the droplets during shedding requires considerable time and effort if performed manually. Here, we developed a self-supervised deep learning model for segmenting shedding droplets from a variety of dropwise and filmwise condensing surfaces. The model eliminates the need for image annotation by humans in the training step and, therefore, reduces labor significantly. The trained model achieved an average accuracy greater than 0.9 on a new unseen test dataset. After extracting the shedding droplet size and speed, we developed a data-driven model for shedding droplet dynamics based on condensation heat flux and surface properties such as wettability and tube diameter. Our results demonstrate that condensate droplet departure size is both heat flux and tube size dependent and follows different trends based on the condensation mode. The results of this work provide an annotation-free methodology for falling droplet segmentation as well as a statistical understanding of droplet dynamics during condensation.

Droplet manipulation in liquid flow using of magnetic micromotors for drug delivery and microfluidic systems

The task of droplet manipulation is of great practical importance in such areas as biochemistry, drug delivery and microfluidic systems. Here we demonstrate a method for controlling the movement of droplets in an oil-in-water emulsion flow using magnetic nanoparticles (MNPs). A distinctive feature of this method is that the MNPs are not dispersed within droplets but in the continuous phase of the emulsion. Due to this, after the droplet manipulation procedure is completed, the MNPs can be easily removed from the emulsion without destroying the droplets. This method represents an innovative alternative to the previously known methods. The MNPs can be used to change the speed and direction of droplet movement in a liquid flow. If the magnetic field gradient is directed against the liquid flow, and the flow velocity is less than a certain critical value, then with help of a magnetic field it is possible to move droplets against the liquid flow. The ability to control the movement of droplets in a flow largely depends on the surface wettability of the MNPs. Hydrophobic MNPs are adsorbed on the surface of droplets and move with them in a magnetic field. If the magnetic field gradient is directed along the liquid flow, this leads to an increase in the droplet speed. In the case of using hydrophilic MNPs, the influence of the magnetic field on the droplet speed in the flow turns out to be ambiguous. These results can provide a basis for the development of magnetic droplet-based drug delivery systems.

Effect of Surfactants on the Splashing Dynamics of Drops Impacting Smooth Substrates.

We present the results of a systematic study elucidating the role that dynamic surface tension has on the spreading and splashing dynamics of surfactant-laden droplets during the impact on hydrophobic substrates. Using four different surfactants at various concentrations, we generated a range of solutions whose dynamic surface tension were characterized to submillisecond timescales using maximum bubble-pressure tensiometry. Impact dynamics of these solutions were observed by high-speed imaging with subsequent quantitative image processing to determine the impact parameters (droplet size and speed) and dynamic wetting properties (dynamic contact angle). Droplets were slowly formed by dripping to allow the surfactants to achieve equilibrium at the free surface prior to impact. Our results indicate that while only the fastest surfactants appreciably affect the maximum spreading diameter, the droplet morphology during the initial stages of spreading is different to water for all surfactant solutions studied. Moreover, we show that surfactant-laden droplets splash more easily than pure liquid (water). Based on the association of the splashing ratio to our tensiometry measurements, we are able to predict the effective surface tension acting during splashing. These results suggest that droplet splashing characteristics are primarily defined by the stretching of the equilibrated droplet free surface.

Effect of Dropping Speed of Reducing Agent on the Preparation of LA/Ag Phase-Change Nanocapsules

Lauric Acid (LA) phase-change nanocapsules prepared with silver as the shell exhibit excellent energy storage capacity and high thermal conductivity. Still, their functionality could be improved by ensuring uniform morphologies, even the size and thickness of silver particles. In this study, the LA/Ag nanocapsules were prepared under different reductant drop speeds. By adjusting the droplet speed of the reducing agent, the concentration of silver in the solution can be controlled, which affects the nucleation and growth rate of silver particles, thereby influencing the deposition of silver particles on the surface of the core material. The characterization results indicate the successful preparation of high sphericity and uniform-sized LA/Ag nanocapsules. The average diameter of capsules was 117–140 nm, the latent heat was 43.69–47.78 J/g, and the encapsulation efficiency was 80.69–82.53%. As the droplet speed increased, the thickness of the silver shell increased while the encapsulation efficiency decreased. The highest encapsulation efficiency was achieved when the reducing agent dropping speed was 0.03 mL/s.

Flow-Driven Self-Propulsion of Oil Droplet on a Surfactant Solution Surface, as Observed by Time-Resolved Interfacial Tension and Surface Flow Speed Measurements.

The imbalanced force of the interfacial tension applied to an object has often been taken into account in the analysis of the motion mechanism of self-propelled systems. However, heterogeneous distributions of the interfacial tension also cause Marangoni flows, and these flows also contribute to the self-propulsion through the viscous force. The contribution of such flows has not been observed directly, while the interfacial tension difference has been measured in some systems. In this study, simultaneous measurements of the interfacial tension and surface flow speed of the unidirectional self-propelled motion of a butyl salicylate (BS) droplet in a circular channel on a sodium dodecyl sulfate (SDS) aqueous solution were achieved by the quasi-elastic laser scattering method. The droplet position was also recorded by observing its fluorescence excited by a UV light. The BS droplet speed dependence of the interfacial tension and surface flow speed were measured by varying the initial BS concentration codissolved in the SDS aqueous solution. As a result, a periodic decrease of the interfacial tension and a periodic increase of the speed of both forward and backward flows were observed when the droplet passed the sampling position of the time-resolved measurements. When they were converted to the distribution in space of the droplet position, no droplet speed dependence of the interfacial tension difference between the front and rear of the droplet was observed. On the other hand, the speed of both forward and backward flows increased as the droplet speed increased. By analysis of the above results with a simplified model, it was clarified that the forward flow driven by the interfacial tension gradient at the droplet front is actually important in the mechanism of the unidirectional self-propelled motion of a droplet.

Numerical study of the Marangoni effect induced by soluble surfactants and solute based on rising droplets

In this study, we present a numerical investigation into the phenomenon of rising droplets in immiscible fluids, focusing on the Marangoni effect induced by both solute and a combination of solute and soluble surfactants. We meticulously examine the interfacial behaviors of pure solute droplets and mixed droplets, with a particular interest on the intricate interplay among interfacial concentration, interfacial tension, Marangoni stress, and Marangoni convection. Our investigation provides insight into the influence of key physicochemical parameters, such as viscosity, diffusion coefficient, partition coefficient, and interfacial tension gradient, on the Marangoni instability. Furthermore, we conduct a comprehensive parametric exploration of the impact of dimensionless numbers such as the Langmuir number (La), the Damkohler number (Da), the Peclet number (Pe), and the elasticity number β on the stabilizing efficacy of surfactants. The research findings underscore the effectiveness of our numerical method in capturing the distinctive two-step acceleration characteristics of pure solute droplets and the stabilizing effect of surfactants on mixed droplets. Notably, our study reveals that the Marangoni instability may manifest even when the viscosity and diffusivity ratios of the two-phase fluids are closely matched. Partition coefficients below unity exhibit only a marginal influence on the re-acceleration time of the droplets. Systems characterized by extremely low interfacial tension gradients tend to exhibit no Marangoni instability. Moreover, an increase in La enhances the stability of mixed droplets, while a significant threshold is identified for Da to affect the stability of mixed droplets. The ascent speed of mixed droplets displays pronounced variation across varying Pe magnitudes. Finally, in scenarios involving a wide-ranging variation in β, mixed droplets transition between the states of pure solute droplets and rigid spheres, revealing a distinct-state transition point.

Droplet Motion Driven by Liquid Dielectrophoresis in the Low-Frequency Range.

Electrohydrodynamic wetting manipulation plays a major role in modern microfluidic technologies such as lab-on-a-chip applications and digital microfluidics. Liquid dielectrophoresis (LDEP) is a common driving mechanism, which induces hydrodynamic motion in liquids by the application of nonhomogeneous electrical fields. Among strategies to analyze droplet movement, systematic research on the influence of different frequencies under AC voltage is missing. In this paper, we therefore present a first study covering the motion characteristics of LDEP-driven droplets of the dielectric liquids ethylene glycol and glycerol carbonate in the driving voltage frequency range from 50 Hz to 1600 Hz. A correlation between the switching speed of LDEP-actuated droplets in a planar electrode configuration and the frequency of the applied voltage is shown. Hereby, motion times of different-sized droplets could be reduced by up to a factor of 5.3. A possible excitation of the droplets within their range of eigenfrequencies is investigated using numerical calculations. The featured fluidic device is designed using larger-sized electrodes rather than typical finger or strip electrodes, which are commonly employed in LDEP devices. The influence of the electrode shape is considered simulatively by studying the electric field gradients.

Measuring diameters and velocities of artificial raindrops with a neuromorphic event camera

Abstract. Hydrometers that measure size and velocity distributions of precipitation are needed for research and corrections of rainfall estimates from weather radars and microwave links. Existing optical disdrometers measure droplet size distributions, but underestimate small raindrops and are impractical for widespread always-on IoT deployment. We study the feasibility of measuring droplet size and velocity using a neuromorphic event camera. These dynamic vision sensors asynchronously output a sparse stream of pixel brightness changes. Droplets falling through the plane of focus of a steeply down-looking camera create events generated by the motion of the droplet across the field of view. Droplet size and speed are inferred from the hourglass-shaped stream of events. Using an improved hard disk arm actuator to reliably generate artificial raindrops with a range of small sizes, our experiments show maximum errors of 7 % (mean absolute percentage error) for droplet sizes from 0.3 to 2.5 mm and speeds from 1.3 to 8.0 m s−1. Measurements with the same setup from a commercial PARSIVEL disdrometer show similar results. Both devices slightly overestimate the small droplet volume with a volume overestimation of 25 % from the event camera measurements and 50 % from the PARSIVEL instrument. Each droplet requires processing of 5000 to 50 000 brightness change events, potentially enabling low-power always-on disdrometers that consume power proportional to the rainfall rate. Data and code are available at the paper website https://sites.google.com/view/dvs-disdrometer/home (Micev et al., 2023).

Flow and clogging of capillary droplets.

Capillary droplets form due to surface tension when two immiscible fluids are mixed. We describe the motion of gravity-driven capillary droplets flowing through narrow constrictions and obstacle arrays in both simulations and experiments. Our new capillary deformable particle model recapitulates the shape and velocity of single oil droplets in water as they pass through narrow constrictions in microfluidic chambers. Using this experimentally validated model, we simulate the flow and clogging of single capillary droplets in narrow channels and obstacle arrays and find several important results. First, the capillary droplet speed profile is nonmonotonic as the droplet exits the narrow orifice, and we can tune the droplet properties so that the speed overshoots the terminal speed far from the constriction. Second, in obstacle arrays, we find that extremely deformable droplets can wrap around obstacles, which leads to decreased average droplet speed in the continuous flow regime and increased probability for clogging in the regime where permanent clogs form. Third, the wrapping mechanism causes the clogging probability in obstacle arrays to become nonmonotonic with surface tension Γ. At large Γ, the droplets are nearly rigid and the clogging probability is large since the droplets can not squeeze through the gaps between obstacles. With decreasing Γ, the clogging probability decreases as the droplets become more deformable. However, in the small-Γ limit, the clogging probability increases since the droplets are extremely deformable and wrap around the obstacles. The results from these studies are important for developing a predictive understanding of capillary droplet flows through complex and confined geometries.

ЗАСТОСУВАННЯ СУЧАСНИХ РОБОЧИХ ОРГАНІВ ДОЩУВАЛЬНИХ МАШИН ЯК ЗАСІБ ПОКРАЩЕННЯ ЯКОСТІ ДОЩУ ТА МІНІМІЗАЦІЇ ЙОГО ВПЛИВУ НА ФІЗИЧНІ ВЛАСТИВОСТІ ТА СТРУКТУРУ ҐРУНТУ

The article presents the results of research on the impact of qualitative and structural characteristics of artificial rain created by sprinklers and ways and means of minimizing its negative impact on the soil by using modern low-pressure sprinklers. The purpose of the research is to study the design features of different types of sprinklers, and the practice and effectiveness of their use in the conditions of irrigated agriculture in Ukraine. Methods and materials. The study of structural and technological features of the working bodies of sprinklers, the practice of their application, and the determination of the main indicators of the quality of the generated rain were carried out in the economic conditions of their operation during the tests. When assessing the quality of work, the intensity of rain, the average diameter of the drops created, the coefficient of effective watering along the sprinkler belt of the tested machines, the specific power of rain, and the comparison of these indicators when using different types of sprinklers were determined according to the standardized methods of the relevant regulatory documents: SOU 74.3-37-152, DSTU ISO 11545. Results. Modern sprinklers are equipped with highly efficient sprinklers with a large capture area and low energy impact on the soil, taking into account all components of the impact of the quality and structure of artificial rain. At low-pressure values, such sprinklers provide a low intensity of rain due to a large spray area, good uniformity, and optimal droplet sizes. The use of these working bodies provided a solution to the problem of minimizing the negative impact of irrigation on the ecology of the environment. The high efficiency of the use of modern sprinklers was confirmed by tests of machines equipped with similar working bodies, according to the results of which a decrease in the intensity of rain and its energy impact on the soil was noted due to the low-working pressure, large coverage area, a nominal spectrum of droplet size and speed of their fall. Conclusions. The use of low-pressure sprinklers makes it possible to reduce the energetic impact of rain on the soil, which ensures the preservation of its structure and reduction of erosion processes, ensures high uniformity of irrigation at low pressure in the irrigation network, reduces moisture loss from the influence of air temperature and wind, and irrigation costs due to the reduction of pressure in irrigation network. Keywords. sprinklers, intensity of rain, size of drops, sprinklers, uniformity of watering, water erosion of the soil, pressure.

Условия закипания капель на поверхности тонкой плёнки оксида цинка при импульсном лазерном осаждении

The pulsed laser deposition (PLD) method is widely used in the production of different thin films. PLD has several advantages: flexible adjustment of parameters and controllability of processes and ease of synthesis of materials. The deposition of zinc oxide droplets during the synthesis of zinc oxide thin films by PLD is considered and the temperature and initial speed of droplets, which can boil on the thin film surface, are determined. The study of the process of the deposition of droplets from a ZnO target was carried out during the growth of thin films according to the traditional scheme for the PLD method. The process of the deposition of ZnO droplets during the PLD of ZnO films under vacuum conditions is considered. For the first time, it has been revealed that ZnO drops are boiling on the thin ZnO film surface. For the first time, the temperature and speed of solid and liquid drops of ZnO, at which they can boil when they hit the substrate, have been calculated taking into account the processes of heat loss through radiation and the heat of phase transition. Thus, for the first time, it has been established experimentally and confirmed in calculations that at the moment of collision some drops of ZnO are boiling. The role of the heat of phase transition and heat loss due to radiation during the flight of droplets during the PLD of ZnO thin films in a vacuum has been clarified. The temperature of a ZnO drop after a collision with a substrate was calculated in a wide range of initial speed and temperature taking into account heat losses due to radiation and heating from impact.

Creeping thermocapillary motion of a Newtonian droplet suspended in a viscoelastic fluid

In this work we consider theoretically the problem of a Newtonian droplet moving in an otherwise quiescent infinite viscoelastic fluid under the influence of an externally applied temperature gradient. The outer fluid is modelled by the Oldroyd-B equation, and the problem is solved for small Weissenberg and Capillary numbers in terms of a double perturbation expansion. We assume microgravity conditions and neglect the convective transport of energy and momentum. We derive expressions for the droplet migration speed and its shape in terms of the properties of both fluids. In the absence of shape deformation, the droplet speed decreases monotonically for sufficiently viscous inner fluids, while for fluids with a smaller inner-to-outer viscosity ratio, the droplet speed first increases and then decreases as a function of the Weissenberg number. For small but finite values of the Capillary number, the droplet speed behaves monotonically as a function of the applied temperature gradient for a fixed ratio of the Capillary and Weissenberg numbers. We demonstrate that this behaviour is related to the polymeric stresses deforming the droplet in the direction of its migration, while the associated changes in its speed are Newtonian in nature, being related to a change in the droplet’s hydrodynamic resistance and its internal temperature distribution. When compared to the results of numerical simulations, our theory exhibits a good predictive power for sufficiently small values of the Capillary and Weissenberg numbers.

Performance Improvement of an STS304-Based Dispensing Needle via Electrochemical Etching.

In this study, we explored the formation of micro-/nanosized porous structures on the surface of a needle composed of STS304 and examined the effect of conventional needles and needles capable of liquid ejection. Aqua regia, composed of HCl and HNO3, was electrochemically etched to form appropriately sized micro-/nanoporous structures. We observed that when dispensing liquids with low surface tension, they do not immediately fall downward but instead spread over the exterior surface of the needle before falling. We found that the extent of spreading on the surface is influenced by an etched porous structure. Furthermore, to analyze the effect of surface tension differences, we dispensed liquids with varying surface tensions using etched needles. Through the analysis, it was confirmed that, despite the low surface tension, the ejected droplet volume and speed could be stably maintained on the etched needle. This indicates that the spreading phenomenon of the liquid on the needle surface just before ejection can be controlled by the micro/nanoporous structure. We anticipate that these characteristics of etched needles could be utilized in industries where precision dispensing of low-surface-tension liquids is essential.

Oblique bouncing of a droplet from a non-slip boundary: computational realization and application of self-spin droplets

Recent studies have demonstrated the significance of droplet spinning motion on droplet collision outcomes and raised the question of generating spinning droplets in an experiment. The present work proposed the idea of making an initially non-spinning droplet self-spin by its oblique impact on a non-slip boundary. We computationally demonstrated the feasibility of the idea and exploited its application in the binary collision of spinning droplets. Specifically, a parametric study was conducted to investigate the effects of impact velocity, impact angle, and liquid viscosity on the spinning droplet. The results showed that a larger impact velocity or a larger liquid viscosity causes an increased angular speed of the spinning droplet, however increasing impact angle leads to a nonmonotonic variation of the angular speed. The oblique-impact-induced droplet spin is attributed to the asymmetric gas film flow, asymmetric lubrication pressure, and shear stress within the gas film region, leading to the earlier bouncing motion on one side to rotate the droplet. In addition, the synergetic effects of the droplet stretching length and the intensity of asymmetric lubrication pressure account for the variation of droplet spin angular speed and droplet separation. As a preliminary application of the proposed idea, the feasible technical approach of realizing the binary collision of spinning droplets was computationally realized.