Capturing aerosol droplets with fibers

Twisted fibers enable drop flow control and enhance fog capture

Drop-fiber interactions are fundamental to the operation of technologies such as atmospheric fog capture, oil filtration, refrigeration, and dehumidification. We demonstrate that by twisting together two fibers, a sliding drop's flow path can be controlled by tuning the ratio between its size and the twist wavelength. We find both experimentally and numerically that twisted fiber systems are able to asymmetrically stabilize drops, both enhancing drop transport speeds and creating a rich array of new flow patterns. We show that the passive flow control generated by twisting fibers allows for woven nets that can be "programmed" with junctions that predetermine drop interactions and can be anticlogging. Furthermore, it is shown that twisted fiber structures are significantly more effective at capturing atmospheric fog compared to straight fibers.

Geometry for low-inertia aerosol capture: Lessons from fog-basking beetles

Water in the form of windborne fog droplets supports life in many coastal arid regions, where natural selection has driven nontrivial physical adaptation toward its separation and collection. For two species of Namib desert beetle whose body geometry makes for a poor filter, subtle modifications in shape and texture have been previously associated with improved performance by facilitating water drainage from its collecting surface. However, little is known about the relevance of these modifications to the flow physics that underlies droplets' impaction in the first place. We find, through coupled experiments and simulations, that such alterations can produce large relative gains in water collection by encouraging droplets to "slip" toward targets at the millimetric scale, and by disrupting boundary and lubrication layer effects at the microscopic scale. Our results offer a lesson in biological fog collection and design principles for controlling particle separation beyond the specific case of fog-basking beetles.

Effect of Droplet-Laden Fibers on Aerodynamics of Fog Collection on Vertical Fiber Arrays

Growing population, along with rapid urbanization, has led to severe water scarcity, necessitating development of novel techniques to mitigate this looming problem. Fog contains water in the form of liquid droplets suspended in air, which can be collected on a porous structure placed in the path of the fog flow. We first develop an artificial fog-generating system using the thermodynamic principle of mixing of air streams followed by condensation, which closely mimics the liquid water content and droplet size distribution of natural fog. We then investigate how collected fog droplets growing on fiber surfaces alter the aerodynamics of fog flow across vertical fiber arrays, called harps, thus affecting their fog collection efficiency. As deposited droplets grow on the fiber surface, they increase the area occluded by droplet-laden fibers, thus increasing the effective shade coefficient (SCact), which increases with time from its initial geometric value (SCgeo), eventually reaching a quasi-steady state, as droplet shedding due to gravity and droplet growth due to fog collection balance each other. We find that this difference in the SCgeo and SCact is governed by local fiber geometry and its physico-chemical morphology; the process dynamics is captured by a nondimensional number, SC*, which increases with the length scale corresponding to the critical volume of droplet shedding relative to the fiber diameter, V*. Thus, there is a significantly greater increase in the effective shade coefficient for thin fibers having larger values of V* as compared to fibers with larger diameters which have lower V* values. On hydrophobic fibers, the quasi-steady state is achieved faster, and the time-averaged SCact is lower as compared to hydrophilic fibers due to the lower critical volume of droplet shedding. The shape of droplets growing on harp fibers affects the aerodynamics of fog flow, its inertial capture mechanism, and efficiency, which can guide design considerations for fog harps toward achieving optimal fog collection performance.

Dynamics of fog droplets on a harp wire

Fog harps effectively drain small droplets, which prevents clogging and results in more water harvested from fog compared to mesh nets. However, the dynamics of fog droplets coalescing and sliding down a vertical wire remain poorly understood. Here, we develop an analytical model that captures the physics of fog droplets draining down a single vertical wire. The driving forces are gravity and the surface energy released from coalescence events, whereas the dominant resisting forces are revealed to be inertia, contact angle hysteresis, and local viscous dissipation within the droplet's receding wedge. The average sliding velocity of fog droplets on a Teflon-coated wire was only half that of an uncoated stainless steel wire, due to non-coalescence events exclusive to the hydrophobic wire disrupting the momentum of droplet sliding.

Elastic Textile Threads for Fog Harvesting

The potential applications of textile materials in fog harvesting have long been demonstrated. This work designed novel fog harvesters according to the distinct features of elastic textile threads (ETTs) to enhance droplet capture, large-droplet growth, and droplet pouring and improve fog harvesting efficiency. We prepared m@ETTs (modified ETTs) using three novel chemical and physical methods. First, we prepared spandex elastic threads with a non-uniform rough surface containing silica nanoparticles and titanium particles through the sol-gel triethoxymethylsilane method. Second, we prepared a rubber/polyester thread with a rough surface by breaking the thread shell with toluene solution, creating knots on the surface of the rubber core. Third, we prepared a polyurethane thread with a bumpy superhydrophobic surface by spraying a tetrafluoroethylene adhesive and silica nanoparticles on the thread. Furthermore, we connected ETTs to an automatic stretching-recovery system to obtain auto-ETTs as another group of harvesters. We obtained auto-i@ETTs by introducing elastic bumps/knots onto the auto-ETT surface. The fog harvesting efficiencies of m@ETTs were approximately 60-120% greater than those of the ETTs. The water harvesting rate of the auto-i@ETT was 2.5 times that of the ETT, with the highest water harvesting rate of auto-i@ETT reaching 3.35 g/h/cm². Moreover, several novel principles of droplet behavior and thread elasticity were revealed. The elastic elongation level of the ETTs was proportional to their water harvesting efficiency. The stretching-recovery state of the elastic thread did not influence the water contact angle but affected the droplet state on the thread surface. The temporary slack/stick state of adjacent elastic threads on auto-ETTs contributed to droplet convergence and pouring. Overall, this novel approach demonstrates the significant potential of elastic threads in fog harvesting applications.

Multiple droplets on a conical fiber: formation, motion, and droplet mergers

Small droplets on slender conical fibers spontaneously move along the fiber due to capillary action. The droplet motion depends on the geometry of the cone, the surface wettability, the surface tension, the viscosity, and the droplet size. Here we study with experiments and numerical simulations, the formation, spontaneous motion, and the eventual merger, of multiple droplets on slender conical fibers as they interact with each other. The droplet size and their spacing on the fibre is controlled by the Plateau-Rayleigh instability after dip-coating the conical fiber. Once these droplets are formed on the fiber, they spontaneously start to move. Since droplets of different size move with different speeds, they effectively coarsen the droplet patterning by merging on the fiber. The droplet merging process affects locally the droplet speed and alters the spatiotemporal film deposition on the fiber.

Three-Dimensional Multilayer Vertical Filament Meshes for Enhancing Efficiency in Fog Water Harvesting

Novel types of vertical filament mesh (VFM) fog harvesters, 3D VFM fog harvesters, and multilayer 3D VFM fog harvesters were developed by mimicking the water-harvesting nature of desert beetles and the spider silks from fog. Four different types of polymer filaments with different hydrophilic-hydrophobic properties were used. The polymer filaments were modified with the polyurethane-sodium alginate (PU-SA) mixture solution, and a simple spraying method was used to form alternating 3D PU-SA microbumps. Polymer VFMs exhibited a higher fog-harvesting efficiency than the vertical metal meshes. Moreover, the hydrophobic VFM was more efficient in fog harvesting than the hydrophilic VFM. Notably, the fog-harvesting efficiency of all VFMs increased by 30-80% after spraying with the mixed PU-SA solution to form a 3D geometric surface structure (3D PU-SA microbumps), which mimicked the desert beetle back surface. This modification caused the fog-harvesting efficiency of PTFE 3D VFM to be thrice higher than that of Fe VFM. This increase was attributed to the improved synergistic effects of fog capturing, droplet growing, and droplet shedding. The multilayer VFMs were more efficient in fog harvesting than the single-layer VFMs because of a larger droplet capture area. The fog-harvesting efficiency of two-layer and four-layer polymer VFMs was approximately 35% and about 45% higher than that of the single-layer polymer VFMs, respectively. The four-layer PTFE 3D VFM with the type B PU-SA bump surface (bump/PU-SA) had the highest efficiency of 287.6 mL/m²/h. Besides the high fog-harvesting efficiency, the proposed polymer VFMs are highly stable, cost-effective, rust-free, and easy to install in practical applications. These advantages are ascribed to the elasticity of the polymer filaments. This work provides new ideas and methods for developing high-performance fog harvesters such as the 3D VFM.

Liquid harvesting and transport on multiscaled curvatures

Various creatures, such as spider silk and cacti, have harnessed their surface structures to collect fog for survival. These surfaces typically stay dry and have a large contact hysteresis enabling them to move a condensed water droplet, resulting in an intermittent transport state and a relatively reduced speed. In contrast to these creatures, here we demonstrate that Nepenthes alata offers a remarkably integrated system on its peristome surface to harvest water continuously in a humid environment. Multicurvature structures are equipped on the peristome to collect and transport water continuously in three steps: nucleation of droplets on the ratchet teeth, self-pumping of water collection that steadily increases by the concavity, and transport of the acquired water to overflow the whole arch channel of the peristome. The water-wetted peristome surface can further enhance the water transport speed by ∼300 times. The biomimetic design expands the application fields in water and organic fogs gathering to the evaporation tower, laboratory, kitchen, and chemical industry.

Nonlinear theory of wetting on deformable substrates

The spreading of a liquid over a solid material is a key process in a wide range of applications. While this phenomenon is well understood when the solid is undeformable, its "soft" counterpart is still misunderstood and no consensus has been reached with regard to the physical mechanisms ruling the spreading of liquid drops over soft deformable materials. In this work we provide a theoretical framework, based on the nonlinear theory of discontinuities, to describe the behavior of a triple line on a soft material. We show that the contact line motion is opposed both by nonlinear localized capillary and visco-elastic forces. We give an explicit analytic formula relating the dynamic contact angle of a moving drop to its velocity for arbitrary rheology. We then specialize this formula to the experimentally relevant case of elastomers with the Chasset-Thirion (power-law) type of rheologies. The theoretical prediction is in very good agreement with experimental data, without any adjustable parameters. We then show that the nonlinear force balance presented in this work can also be used to recover classical models of wetting. Finally we provide predictions for the dynamic behavior of the yet largely unexplored case of a viscous drop spreading over a soft visco-elastic material and predict the emergence of a new form of apparent hysteresis.

Optimal Design of Multilayer Fog Collectors

The growing concerns over desertification have spurred research into technologies aimed at acquiring water from nontraditional sources such as dew, fog, and water vapor. Some of the most promising developments have focused on improving designs to collect water from fog. However, the absence of a shared framework to predict, measure, and compare the water collection efficiencies of new prototypes is becoming a major obstacle to progress in the field. We address this problem by providing a general theory to design efficient fog collectors as well as a concrete experimental protocol to furnish our theory with all the necessary parameters to quantify the effective water collection efficiency. We show in particular that multilayer collectors are required for high fog collection efficiency and that all efficient designs are found within a narrow range of mesh porosity. We support our conclusions with measurements on simple multilayer harp collectors.

Beetle-like droplet-jumping superamphiphobic coatings for enhancing fog collection of sheet arrays

Fog collection from atmosphere is an effective way to solve the water resource crisis in arid or semi-arid areas. Inspired by the bumpy surface of the desert beetle, this work provides a beetle-like superamphiphobic coating by adding silicon carbide particles to nano-SiO2 superamphiphobic coating in proportion, which shows excellent superamphiphobic performance, high nucleation rate, efficient drop removal efficiency and recommendable fog collection effect. In this work, drop removal is facilitated by the collisions of water droplets between the array sheets, and when the as-prepared samples are placed parallel to each other and with a space of ∼2 mm, the jumping drop collisions between two sample surfaces could promote the departure of droplets, and the water collection rate of the collision surface increased by ∼217% compared to that of the non-collision surface, which provides a new idea to promote water droplet removal. This work findings are instrumental in water collection and have wide application prospects in desalination, heat transfer, anti-fogging and other fields.