We study experimentally the behaviour of a drop deposited on a conical fibre. It is shown that for wetting liquids, such a drop spontaneously moves towards the region of lower curvature. The driving force is measured and shown to be a gradient of Laplace pressure, which allows us to characterize the dynamics of these self-propelling drops. We conclude by discussing the efficiency of this device for drying a solid initially coated with a liquid film.

Drops on a conical wire

Inspired by the efficient fog collection on cactus spines, conical copper wires with gradient wettability are fabricated through gradient electrochemical corrosion and subsequent gradient chemical modification. These dual-gradient copper wires' fog-collection ability is demonstrated to be higher than that of conical copper wires with pure hydrophobic surfaces or pure hydrophilic surfaces, and the underlying mechanism is also analyzed.

Bioinspired Conical Copper Wire with Gradient Wettability for Continuous and Efficient Fog Collection

Carbon nanotubes (CNTs) were grafted on IM7 carbon fibres using a chemical vapour deposition method. The overall grafting process resulted in a threefold increase of the BET surface area compared to the original primary carbon fibres (0.57 m 2 /g). At the same time, there was a degradation of fibre tensile strength by around 15% (depending on gauge length), due to the dissolution of iron catalyst into the carbon; the modulus was not significantly affected. The wetting behaviour between fibres and poly(methyl methacrylate) (PMMA) was directly quantified using contact angle measurements for drop-on-fibre systems and indicated good wettability. Single fibre fragmentation tests were conducted on hierarchical fibre/PMMA model composites, demonstrating a significant (26%) improvement of the apparent interfacial shear strength (IFSS) over the baseline composites. The result is associated with improved stress transfer between the carbon fibres and surrounding matrix, through the grafted CNT layer. The improved IFSS was found to correlate directly with a reduced contact angle between fibre and matrix.

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Carbon nanotube grafted carbon fibres: A study of wetting and fibre fragmentation

Particulate emissions from low-temperature biomass burning are dominated by organic matter. Here, we show that such emissions have a liquid, bead-like appearance when collected on fibrous filters, and the number of these beads are far less than expected for solid spherical particles. These shapes are in line with published drop-on-fiber theories for liquids entrained on filaments. A smoldering pine sample is yellowish, with organic carbon over 99% of the total carbon, and chars substantially in thermal-optical analysis (TOA), indicating that such liquid organic particles could affect both absorption measurements and TOA of such samples. Similar colored samples collected in the field from rice-straw burning and cook stove emissions also show a similar liquid appearance.

Yellow Beads and Missing Particles: Trouble Ahead for Filter-Based Absorption Measurements

Drops on cylindrical fibers are a familiar sight, for instance in the form of dew drops on spider webs. They can exist in two competing morphologies, a cylindrically symmetric barrel state completely engulfing the fiber and an asymmetric clamshell state, in which the drop touches the fiber only sideways. Despite their omnipresence and their practical relevance, e.g. for the adherence of drops to fibers in separation technology and filter materials, the physical mechanisms governing the stability of the two morphologies remained elusive. Using electrowetting-functionalized fibers we can tune the wettability of the fibers and thereby reversibly switch between the two states. This allows determination of the stability limits of both morphologies as a function of the two relevant control parameters, namely the contact angle and the liquid volume. While clamshells are found to prevail for large contact angles and small volumes, and barrels prevail for small angles and large volumes, there is also a wide range of intermediate parameter values, for which both morphologies are mechanically stable. Mapping out the energy landscape of the system by numerical minimization of the free energy we find that the barrel state is easily deformed by non-axisymmetric perturbations. Such perturbations facilitate the transition to the clamshell state and thereby the removal of drops from the fibers. From a general perspective, the demonstration of electrowetting-based reversible switching of liquid morphologies on fibers opens up opportunities for designing functional textiles and porous materials for various applications in detergency, filtering, and controlled absorption and release of liquids.

Drops on functional fibers: from barrels to clamshells and back

The shape of a small liquid drop on a small diameter fiber may be either an axisymmetric barrel shape or it may be a non-axisymmetric clam-shell shape. Experiments show that when the reduced volume, given by the volume of droplet divided by the fiber radius, is large the barrel shape is the preferred conformation, but that as the volume reduces a transition to a clam-shell (pearl) shape occurs. The volume at which this stability transition occurs depends upon the equilibrium contact angle. In this work we review the known solution to Laplace's equation for the barrel shape and consider the link between the profile, the inflexion in the profile and the stability of the droplet. No known solution of Laplace's equation exists for the clam-shell shape droplet. We therefore consider a finite element approach to determining the possible shapes of a droplet on a fiber and give numerical results for the clam-shell profile. The surface free energies for the two types of droplet conformation on a fiber are computed for several droplet volumes and equilibrium contact angles and the implications of this for droplet stability are discussed.

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The shape and stability of small liquid drops on fibers

A technique for the quantitative investigation of the wetting dynamics and droplet deposition or removal on fibres is discussed. The method presented uses light from a laser diode, which is guided down a textile fibre. Light transmission losses are dependent on the type and extent of droplet or particle coverage of the fibre surface. Changes in light transmission can be induced by deposition of droplets of the dispersed phase of an emulsion, by the removal of particles or droplets from the fibre surface, or from changes in the refractive index of the liquid in which the fibre is immersed. These changes in light transmission intensity are monitored in real-time using a lock-in amplifier principle. Experiments on coiled PMMA optical fibers of diameters 250 gm and for straight polyester fibres of diameters 80 pm and 800 pm immersed in a range of oils and in an emulsion system of decane dispersed in 15 mM dodecyltrimethylammonium bromide (DTAB) are presented. For the first time, we show that the light-guiding technique can be applied to straight lengths of textile-type fibers and to smaller fiber diameters of around 50-80 mum. The results show that changes in the wetting of a fiber surface by liquids and droplets of oils can be monitored in real-time. These results are discussed in terms of droplet shape, extent of coverage and droplet kinetics.

Brendan Carroll

Papers

The shape and stability of small liquid drops on fibers

Measurement of droplet deposition on fibers