Droplet Capture Research Articles

Design and preparation of a simplified microdroplet generation device for nanoliter volume collection and measurement with liquid microjunction–surface sampling probe–mass spectrometry

AbstractGiven recent interest in laboratory automation and miniaturization, the microdroplet research space has expanded across research disciplines and sectors. In turn, the microdroplet field is continually evolving and seeking new methods to generate microdroplets, especially in ways that can be integrated into diverse (microfluidic) workflows. Herein, we present a convenient, low‐cost, and re‐usable microdroplet generation device, termed as the “NanoWand,” which enables microdroplet formation in the nanoliter volume range through modulated surface energy and roughness, that is, an open surface energy trap (oSET), using commercially available and readily assembled coating and substrate materials. A wand‐like shape is excised from a microscope glass cover slip via laser‐micromachining and rendered hydrophobic; a circle is then cut‐out from the hydrophobically modified wand's tip using laser‐micromachining to create the oSET. By adjusting the size of the oSET with laser‐micromachining, the volume of the microdroplet can be similarly controlled. Using liquid microjunction–surface sampling probe–mass spectrometry (LMJ‐SSP‐MS), specific NanoWand droplet capture volumes were estimated to be 117 ± 23.6 nL, 198 ± 30.3 nL, and 277 ± 37.1 nL, corresponding to oSET diameters of 0.75, 1.00, and 1.25 mm, respectively. This simple approach provides a user‐friendly way to form and transfer microdroplets that could be integrated into different liquid handling applications, especially when combined with the LMJ‐SSP and ambient ionization MS as a powerful and rapid analytical tool.

Nanosized caltrops enable selective capture and directional maneuvering of water droplets

AbstractSurface design by tailoring topographical features and interface function groups to modulate dynamic or kinetic behaviors of liquid droplets, has been an increasing hotspot due to its broad spectrum of applications in biochemical diagnosis, microfabrication, and energy conversion systems. Here we report an engineered surface decorated by packed nanosized caltrops resulting from two perfectly articulated oxidation processes, where self-assembled nanoislands generated in the 1st plasma oxidation serve as protective masks in the 2nd chemical oxidation. As caltrops per design can effectively block lateral motion, the present surface can anchor contact lines of advancing water films when being hydrophilic and selectively capture impinging droplets when being hydrophobic. Furthermore, biphilic patterns can be readily obtained by integrating nanocaltrops with other surface asperities, engendering directional droplet maneuvering and designated droplet arraying. This work provides guidelines in designing nanostructures that achieve on-demand manipulation of droplets and flow patterns for multifunctional applications.

Modular arborized fog harvesting device with coordinated mechanism of capture and transport

With global water scarcity a growing problem, fog harvesting technology has emerged as an effective solution. Industrial development and population growth have exacerbated the need for efficient, dismantlable, and assembled fog harvesting devices, as well as the requirement to optimize the droplet capture-transport relationship. In this study, a novel modular bionic 3D tree-like structure for fog harvesting system (3D-TSFHS) was developed to achieve rapid droplet transport by means of Nepenthes-inspired superslip leaves (SSLs). In addition, aluminum (Al) cones with superhydrophilic and superhydrophobic (SHL-SHB) patterns were prepared to enhance the droplet capture efficiency by drawing on the special wettability and structural features of various plants and animals such as cactus, spider silk, desert beetles, lizards and camphor leaves. This artificial design significantly enhances the overall capture and transport relationship. The resulting embedded superslip leaves Al cone fog harvesting (E-SLAC-FH) dramatically improves the fog harvesting efficiency and exhibits excellent durability. Assembling this fog harvesting into a 3D-TSFHS achieves a high fog harvesting efficiency of 0.462 g∙cm−2∙min−1. The system's modular design and its exceptional durability ensure its potential for a wide range of applications in a variety of real-world scenarios.

Ore sulfur of Golovnin Volcano, Kunashir Island

Research subject. Hydrothermal deposits of Golovnin Caldera. Aim. To study the epithermal volcanogenic ore formation. Key points. Until now, there has been a consensus on the exogenous sedimentary (colloidal) genesis of sulfur in volcanic lakes. Our observations and microstructure studies indicate the presence of sulfur melt at the bottom of Kipyaschee Lake. Drops of this melt are carried to the surface of the lake as part of a light gray foam. The significant differences of sulfur spherules in the concentration of sulfide mineralization, in its composition, as well as in the presence or absence of numerous opal inclusions are most simply explained by the capture of droplets in various parts of the sulfur melt and their subsequent movement by a gas stream passing through the melt. Elemental sulfur condensate is formed in bottom sediments as a result of forced cooling of endogenous gas flows by lake water. The main condensation of sulfur occurs here (96% or more of the total potential of fluid sulfur). Residual condensation of sulfur occurs in the aquatic environment. Finely dispersed sulfur condensate in a mixture with water is unstable and breaks down over time with the release of hydrogen sulfide and the formation of sulfurous and sulfuric acids. The activity of bottom hydrotherms and coastal unrest prevents the formation of colloidal sulfur sediment at the bottom of lakes. In the crater depressions at the bottom of the lakes of the Golovnin Caldera, sulfidization of its melt occurs simultaneously with the condensation of sulfur itself. Gravitational deposition of sulfides in the sulfur melt leads to their enrichment of the root parts of crater depressions, where pyrite ore bodies are formed in real time. Terrestrial sulfur deposits, together with the modified rocks overlying them, demonstrate the full profile of endogenous apical oxidation under gas-hydrothermal action: sulfur and sulfur-opal rocks up the section are replaced by gypsum-jarosite rocks and, further, by an “iron hat” of limonite-cemented breccias of the dome mantle. Conclusions. Observations, microstructure studies and molecular chemical modeling indicate the endogenous condensate origin of ore sulfur in the Golovnin Caldera and exclude its exogenous sedimentary genesis.

Movement behavior and morphological change of droplets on vertically crossed fibers

Droplet capture and coalescence are critical processes for enhancing emulsion separation efficiency in oily wastewater treatment. In this paper, four kinds of motion behaviors and secondary droplet formation behaviors of oil droplets on vertically crossed fibers in a water environment were investigated using high-speed camera technology. The research shows that during the droplet capture process, droplets on large-diameter horizontal fiber exhibit greater lateral spreading, and smaller vertical elongation. Increasing the diameter of horizontal fiber significantly shortens droplet capture time. The capture time on lipophilic fiber intersection of 190–420 μm is 14 ms less than that of 190–190 μm. During the droplet coalescence process, the liquid bridge on large-diameter horizontal fiber is wider, and the droplet contraction ratio is smaller. The droplet contraction ratio on fiber intersection of 190–420 μm is 0.3 smaller than that of 190–190 μm. Increasing the diameter of horizontal fiber can promote rapid fusion between droplets, but lipophobic fibers hinder the degree of fusion and deformation. Furthermore, the diameter and wettability of horizontal fibers have significant effects on the volume of secondary droplets. The volume of secondary droplets increases with the diameter of lipophilic horizontal fiber in the capture process. The volume of secondary droplets on lipophilic horizontal fiber is approximately three times larger than that on lipophobic horizontal fiber in the coalescence process. These findings contribute to understanding the details of coalescence separation and optimizing the design of fiber fillers.

Fluorescence visualization dynamics of carbonated water injection processes in homogeneous glass micromodels

Understanding the flow behavior of oil–water phases in porous media is crucial for revealing the kinetic mechanisms underlying oil–water movement. In this study, we designed and constructed a fluorescence visualization experimental platform for carbonated water injection (CWI). We evaluated the effects of nine distinct injection rates on oil production and CO2 sequestration capabilities using three types of homogeneous porous structure micromodels. The microscale oil–water seepage behavior at the pore scale was also explored. The experimental results indicate that optimally increasing the injection rate can improve displacement efficiency, whereas excessively high rates may lead to viscous and capillary fingering. The geometry of the micromodels, particularly the hexagonal matrix, demonstrates potential advantages in enhancing oil displacement due to its uniform flow paths. The capillary number (Ca) significantly influences oil droplet capture behavior within micromodels. For instance, micromodel A captures the largest ganglion area at low Ca values, with a noticeable reduction in average ganglion area as Ca increases. Conversely, micromodels B and C exhibit similar trends in ganglion size variation at high Ca values, suggesting a more uniform impact of pore structure on oil droplet capture behavior. At elevated Ca values, capillary forces exert a more uniform and controlled effect on oil droplet capture and release, minimizing size variability. Additionally, the injection rate plays a critical role in CO2 sequestration, with higher rates promoting convective mixing between CO2 and reservoir fluids, thereby enhancing dissolution and storage efficiency. The complexity of the pore structure, characterized by the presence of micropores and macropores, affects the dissolution kinetics and mobility of CO2, influencing sequestration efficiency. These experimental findings provide valuable insights into the dynamics of oil–water motion and the potential for enhanced oil recovery (EOR) and CO2 storage, offering essential guidance for optimizing CWI strategies in the petroleum industry.

Thermoresponsive nanofiber yarns for water harvesting enhanced by harp system

The escalating global water crisis necessitates innovative approaches to developing sustainable water resources. Fog water collectors with variable surface wettability offer controlled fog harvesting in water-scarce regions. This study develops thermoresponsive fog collecting materials by electrospinning poly(N-isopropylacrylamide)-polyvinylidene fluoride (PNIPAm-PVDF) into yarns that are transformed into harp-like structures for enhanced water harvesting rate. Both meshes and harps using electrospun membranes exhibit the remarkable ability to transition between hydrophilic and hydrophobic wetting states at temperatures below the lower critical solution temperature (LCST). Hydrophilic and hydrophobic surfaces play distinct roles in fog water collection. Hydrophilic surfaces have a high affinity for water and enables droplet capture. Hydrophobic surfaces help the removal of aggregated water droplet and fog water collection. The highest water collection rate obtained with the electrospun PNIPAm-PVDF harp was 1415 ± 7.0 mg·cm−2 h−1. The water harvesting system based on the electrospun PNIPAm-PVDF harps exhibits a 485 % increase in water collection compared to the standard meshes made from the same material, emphasizing their potential for significantly improving the overall rate in fog water harvesting applications.

Influence of imidazole-based ionic liquid surfactants on the wetting effect of long-flame coals

In order to study the wetting effect of C12MImBF4, C12MImBr and C12MImCl on long-flame coal, and their mechanism of action on coal-water bonding, physical experiments, molecular dynamics simulation and DFT theoretical calculation were combined in this work. It is confirmed that the imidazole ionic liquid surfactant optimizes the wetting effect by forming hydrogen bonds, reducing the surface tension of water, and increasing the number of free hydroxyl groups in coal. After the wettability test, the concentration of 4 g/L of the three ionic liquid surfactants had the best wetting effect on long-flame coal, and the contact angle at this concentration was reduced by more than 40 % compared with water within 0.2 s after dripping on the coal. Due to the difference in the anionic head group of the three surfactants, there are differences in the number and strength of hydrogen bonds formed between the three surfactants and water molecules, the results of wettability C12MImBF4 > C12MImCl > C12MImBr were presented. Combined with these characteristics of imidazole ionic liquids on long-flame coal, the reasonable application in the wet dust removal process of actual coal mine production can optimize the efficiency of droplet capture of coal particles, so as to effectively improve the dust removal capacity.

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.

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.

Enhanced interfacial capture with an elliptical cylinder

Interfacial capture of particles and droplets is important in practical applications. Interfacial capture owing to the surface tension force occurs only when the target (particle or droplet) is close to the collector, leading to low efficiency with collector or collector arrays. An elliptical cylinder instead of a common circular cylinder is a method to enhance capture efficiency. We theoretically investigate this situation by modelling the motion of floating particles around a surface-piercing elliptical cylindrical collector at Reynolds numbers of 1–10. Results indicate that, despite a smaller capture width, elliptical cross-sections, especially with smaller aspect ratios, are more efficient in capturing owing to a much smaller contact line perimeter compared with circular cross-sections. The aligned orientation (the major axis is parallel to the flow) is better than the orthogonal orientation (the major axis is perpendicular to the flow) owing to a larger capture width. A larger collector is beneficial to capillary capture but a very large size is adverse. Smaller but heavier floating particles for smaller capillary numbers can be easily captured at lower Reynolds numbers. The aligned orientation has a greater advantage over the circular cross-section and the orthogonal orientation in the capillary capture. This provides a good method for enhancing the capture efficiency of floating particles in applications.

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.

Droplet Bottom Expansion and Its Wettability Control Mechanism Based on Macroscopic Defects.

Biomimetic surfaces with special wettability have received much attention due to their promising prospects in droplet manipulation. Although some progress has been made, the manipulation of droplets by macroscopic defects of the millimeter structure and the wetting-state transition mechanism have rarely been reported. Herein, inspired by lotus leaves and desert beetles, biomimetic surfaces with macroscopic defects are prepared by laser processing and chemical modification. Various functions of droplet manipulation are achieved by controlling the millimeter-scale macroscopic defects, such as droplet capture, motion trajectory changing, and liquid well. And a droplet bottom expansion phenomenon is proposed: wetting-state transition in superhydrophobic regions around defects. The "edge failure effect" is proposed to explain the force analysis of droplet capture and the droplet bottom expansion to distinguish it from the adhesion phenomenon presented by the droplet sliding. 53.28° is defined as the expanded saturated angle of the as-prepared surface, which is used to distinguish whether the defect could cause the droplet bottom expansion. An enhanced edge failure effect experiment is designed to make the droplet bottom expansion more intuitive. This work provides a mechanistic explanation of the surfaces that utilize macroscopic defects for droplet manipulation. It can be applied to the monitoring of droplet storage limits, providing a perspective on the design and optimization of superhydrophobic surfaces with droplet manipulation.

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.

Droplet array with microfluidic concentration gradient (DA-MCG) for 2-dimensional reaction condition screening

Droplet microfluidic technology can use each microdroplet as a microreactor, which has the advantages of low reagent dosage, less cross contamination, and fast reaction time. Combining concentration gradient generation with droplet formation and capture to form a two-dimensional reaction condition screening platform (including reactant concentration and reaction time) is expected to broaden the application range of microfluidic screening. In this work, a microfluidic chip that can dynamically generate and capture microdroplets and form static microdroplet array was designed and fabricated. An optimized Christmas tree structure by adjusting the horizontal channel length ratio was used to generate a chemical concentration gradient while obtaining a uniform outlet flow rate, forming a droplet array with different concentrations. The performance of droplet array with microfluidic concentration gradient (DA-MCG) was verified using sodium fluorescein as a model reagent. The chromogenic reaction of NaOH and phenolphthalein, and luminol chemiluminescence reaction were used to verify the two-dimensional screening ability of DA-MCG. The results indicated that the DA-MCG has the potential to be applied in the field of multi-dimensional drug screening.

Improved Fog Collection on a Hybrid Surface with Acylated Cellulose Coating.

Fog collection serves as an efficient method to alleviate water scarcity in foggy, water-stressed regions. Recent research has focused on constructing a hybrid surface to enhance fog collection efficiency, with one approach being the prevention of liquid film formation at hydrophilic sites. Inspired by the desert beetle, a coating (10-MCC) made by partially acylating microcrystalline cellulose (MCC) exhibits hydrophilic sites alongside a hydrophobic skeleton enabling rapid droplet capture despite its overall hydrophobicity. The captured droplets quickly coalesce into a large droplet driven by the wetting gradient created by the hydrophobic backbone and hydrophilic sites. To achieve greater fog collection efficiency, a hydrophobic-superhydrophobic hybrid surface is formed by combining a coating of 10-MCC with a superhydrophobic surface. The construction of superhydrophobic surfaces typically involves creating a rough surface with a distinctive structure produced by the anodization technique and modifying it with stearic acid. The superhydrophobic surface exhibits excellent corrosion resistance and mechanical stability. Moreover, the hybrid surface shows high efficiency in fog collection, with a tested maximum efficiency of approximately 1.5092 g/cm2/h, 1.77 times that of the original Al sheets. The results demonstrate a remarkable enhancement in fog collection capacity. Furthermore, this work serves as an inspiration for the low-cost and innovative design of engineered surfaces for efficient fog collection.

Multifunctional integrated pattern for enhancing fog harvesting water unidirectional transport in a heterogeneous pattern

Solid surfaces with improved wettability as well as geometric structures can enhance capture and droplet removal, thereby improving fog harvesting. We fabricated Al wires by combining superhydrophilic (SHL), superhydrophobic (SHB), and oil-infused SHB (SHBO) surfaces into a pattern whose fog-harvesting efficiency could be measured. The SHL-SHBO-SHL pattern showed the highest promise of water droplet capture and mobility on a solid surface with 42% efficiency compared to the 34% efficiency of Bare. In order to identify the optimal efficiency features, two boundary conditions (boundary I: from SHL to SHBO and boundary II: from SHBO to SHL) were introduced, and the impact of the hydrophilic area was examined. Boundary I boosts capture efficiency whereas boundary II increases drain efficiency. Understanding the forces operating at the wettability gradient surface, as well as incorporating the area ratio of SHL and SHBO via wettability combinations, are key to designing effective fog harvesting systems.

Oil Droplet Capture and Ingestion by Filter-Feeding Sabellid and Serpulid Polychaetes

Benthic filter-feeders form an essential role in marine food chains as they constitute the bridge between the microscopic primary producers and the consumers. Although filter-feeders mainly feed on solid particles, they also capture and ingest oil droplets. Usually, these microdroplets come from the decomposition of animals or algae or from petroleum oils that enter water via spills and leakages. Here, we used videography, TRITC fluorescence microscopy, and fluid mechanics to study the capture mechanisms of canola, fish, and four petroleum motor oil droplets by the filter feeding sabellid and a serpulid polychaetes. Schizobranchia insignis, Eudistylia vancouveri, Myxicola infundibulum and Serpula columbiana actively feed on waste motor oil droplets in seawater. A further experiment found that S. insignis fed on all types of oil droplets, demonstrating no selectivity based on type. The oil droplet capture mechanism of S. insignis were direct interception and sieving, like that of solid particles. The size range of droplets ingested was 10 to 300 µm in diameter, but these ranges differed depending on the density and viscosity of the oils. Higher density and viscous oils were captured at smaller droplet sizes. These results are the first to characterize the mechanics of oil droplet capture, transport and ingestion by benthic ciliary filter feeders, and contribute to understanding the behavior of animals in response to oil emulsions, and how oils enter marine food webs.

Efficient electro-demulsification of O/W emulsions and simultaneous oil removal enabled by a multiscale porous biocarbon electrode

Emulsion wastewater contain substantial amounts of oil and various additives, which pose threats to the environment and human health. Demulsification is a crucial pretreatment stage for wastewater. This study aims to identify a novel electro-demulsification method with high oil removal efficiency and low energy consumption. Modified carbonized birch wood with a unique isotropic multiscale pore structure is used as a self-standing electrode to treat a toluene oil-in-water (O/W) emulsion. The electrode must have a highly porous structure to facilitate efficient water diffusion and oil adsorption. It must also have high electronic conductivity to expedite polarized molecular electrophoresis to realize penetration into the pores and, subsequently, demulsification. Guided by an applied electric field force, polarized O/W droplets are drawn toward the electrode, revealing electrical characteristics distinct from those of polarized organic molecules. This electric field force augments the capture and adhesion of droplets by the electric double layer at the electrode interface. Consequently, adsorbed droplets in close proximity to the electrode rupture due to the combined influence of the electric field force and the electrostatic effects stemming from the electrode's multiscale porous structure. This synergistic action enables demulsification to occur efficiently at low energy consumption levels. This study has revealed that electro-demulsification can effectively treat toluene emulsions stabilized by various surfactants and microemulsion containing toluene. Therefore, this electro-demulsification technology can be further developed for various types of water pollution.

Continuous Reactive-Roll-to-Roll Growth of Carbon Nanotubes for Fog Water Harvesting Applications

A simple method is presented for the continuous generation of carbon nanotube forests stably anchored on stainless-steel surfaces using a reactive-roll-to-roll (RR2R) configuration. No addition of catalyst nanoparticles is required for the CNT-forest generation; the stainless-steel substrate itself is tuned to generate the catalytic growth sites. The process enables very large surfaces covered with CNT forests to have individual CNT roots anchored to the metallic ground through primary bonds. Fog water harvesting is demonstrated and tested as one potential application using long CNT-covered wires. The RR2R is performed in the gas phase; no solution processing of CNT suspensions is used, contrary to usual R2R CNT-based technologies. Full or partial CNT-forest coverage provides tuning of the ratio and shape of hydrophobic and hydrophilic zones on the surface. This enables the optimization of fog water harvesters for droplet capture through the hydrophobic CNT forest and water removal from the hydrophilic SS surface. Water recovery tests using small harp-type harvesters with CNT-forest generate water capture of up to 2.2 g/cm2·h under ultrasound-generated fog flow. The strong CNT root anchoring on the stainless-steel surfaces provides opportunities for (i) robustness and easy transport of the composite structure and (ii) chemical functionalization and/or nanoparticle decoration of the structures, and it opens the road for a series of applications on large-scale surfaces, including fog harvesting.