Martin E. R. Shanahan

Stick-slip of evaporating droplets: substrate hydrophobicity and nanoparticle concentration.

The dynamics of the three-phase contact line for water and ethanol is experimentally investigated using substrates of various hydrophobicities. Different evolutions of the droplet profile (contact line, R, and contact angle, θ) are found to be dependent on the hydrophobicity of the substrate. A simple theoretical approach based on the unbalanced Young force is used to explain the depinning of the contact line on hydrophilic surfaces or the monotonic slip on hydrophobic substrates. The second part of the article involves the addition of different quantities of titanium oxide nanoparticles to water, and a comparison of the evaporative behavior of these novel fluids with the base liquid (water) on substrates varying in hydrophobicity (i.e., silicon, Cytop, and PTFE) is presented. The observed stick-slip behavior is found to be dependent on the nanoparticle concentration. The evaporation rate is closely related to the dynamics of the contact line. These findings may have an important impact when considering the evaporation of droplets on different substrates and/or those containing nanoparticles.

Wetting and Evaporation of Binary Mixture Drops

Experimental results on the wetting behavior of water, methanol, and binary mixture sessile drops on a smooth, polymer-coated substrate are reported. The wetting behavior of evaporating water/methanol drops was also studied in a water-saturated environment. Drop parameters (contact angle, shape, and volume) were monitored in time. The effects of the initial relative concentrations on subsequent evaporation and wetting dynamics were investigated. Physical mechanisms responsible for the various types of wetting behavior during different stages are proposed and discussed. Competition between evaporation and hydrodynamic flow are evoked. Using an environment saturated with water vapor allowed further exploration of the controlling mechanisms and underlying processes. Wetting stages attributed to differential evaporation of methanol were identified. Methanol, the more volatile component, evaporates predominantly in the initial stage. The data, however, suggest that a small proportion of methanol remained in the drop after the first stage of evaporation. This residual methanol within the drop seems to influence subsequent wetting behavior strongly.

Effect of Adhesive Compliance in the Assessment of Soft Adhesives with the Wedge Test

Wedge tests are usually analysed assuming that the free, unbonded members may be treated as encastré cantilever beams. However, if the adhesive layer is sufficiently flexible (e. g., due to low elastic modulus), then significant strain in the bonded region may occur and lead to modification of the behaviour outside this region. Using in conjunction a sensitive strain gauge method on asymmetric wedge tests and a mathematical analysis developed from the work of Winkler, we conclude that the standard, simple beam theory approach significantly overestimates crack length for a supple adhesive layer. The present contribution mainly considers strain effects in the intact, bonded zone, rather than fracture per se. However, it is concluded that, if in fracture tests, the incorrect values of crack length obtained from the encastré beam assumption are employed to calculate fracture energy using the simpler model, there will be some self-compensation and little error in estimates of the latter will result (at least in the cases presently studied).

Dependence of Volatile Droplet Lifetime on the Hydrophobicity of the Substrate

In this Article, we demonstrate the dependence of the lifetime of a volatile droplet on the hydrophobicity of the substrate. Ethanol droplets placed on the molecularly smooth surfaces of three polymers, applied to substrates by spin-coating, showed distinct types of behavior depending on the hydrophobicity of the latter. High contact angles, θ, lead to fairly regular recession of the triple line during liquid evaporation at essentially constant θ, whereas low contact angle caused pinning, θ decreasing with time. The latter case leads to shorter drop lifetimes.

Inertial to Viscoelastic Transition in Early Drop Spreading on Soft Surfaces

It has been known for many years that a spreading liquid droplet can be appreciably slowed on a soft, viscoelastic substrate by the appearance of a wetting ridge or protuberance of the solid near the triple phase contact line because of capillary forces. Viscoelastic dissipation in the solid surface can outweigh that of liquid viscosity and, therefore, dominate wetting dynamics. In this paper, we show that a short, rapid spreading stage exists after initial contact. The requisite balance determining the speed of motion is between capillary forces and inertial effects. As spreading proceeds, however, inertia lessens and the lower spreading speed allow for viscoelastic effects in the solid to increase. The transition between early inertial and viscoelastic regimes is studied with high-speed photography and explained by a simple theory.

On the effect of pH on spreading of surfactant solutions on hydrophobic surfaces

Surfactants are invaluable in a number of agricultural applications in products such as pesticides and herbicides. In these products, surfactants are very often used in conjunction with acidifiers in order to improve their half-life. In this paper, we investigate how the change in pH affects surfactant wetting and spreading. We compare the performance of a conventional surfactant, Triton X-100, with that of a trisiloxane superspreader, Silwet L-77, on a number of polymer coated surfaces exhibiting various degrees of hydrophobicity. Silwet L-77 in water based solutions showed very good wetting capability on all surfaces. However, its wetting ability was drastically reduced with the addition of acetic acid. On the other hand, Triton X-100 was not affected by the addition of acid and exhibited the same spreading behaviour as in water-based solutions.

Contact Angle Equilibrium on Thin Elastic Solids

Abstract A theoretical analysis has been carried out on the system consisting of an axisymmetric sessile drop resting on a thin elastic solid in the presence of gravity. The solid is treated in one case as a thin plate and in the other case as a membrane. The consequences of the variational treatment employed are equations relating to contact angle equilibrium, drop and solid profiles. It is shown that contact angles are not intrinsic surface properties of the phases involved but invoke equally such characteristics as bulk properties of the solid and physical dimensions when the solid in question is deformable.

Evaporation of nanofluid droplets with applied DC potential.

A considerable growth of interest in electrowetting (EW) has stemmed from the potential exploitation of this technique in numerous industrial and biological applications, such as microfluidics, lab-on-a-chip, electronic paper, and bioanalytical techniques. The application of EW to droplets of liquids containing nanoparticles (nanofluids) is a new area of interest. Understanding the effects of electrowetting at the fundamental level and being able to manipulate deposits from nanofluid droplets represents huge potential. In this work, we study the complete evaporation of nanofluid droplets under DC conditions. Different evolutions of contact angle and contact radius, as well as deposit patterns, are revealed. When a DC potential is applied, continuous and smoother receding of the contact line during the drying out of TiO2 nanofluids and more uniform patterning of the deposit are observed, in contrast to the typical stick-slip behavior and rings stains. Furthermore, the mechanisms for nanoparticle interactions with the applied DC potential differ from those proposed for the EW of droplets under AC conditions. The more uniform patterns of particle deposits resulting from DC potential are a consequence of a shorter timescale for electrophoretic mobility than advection transport driven by evaporation.

Spreading and Wetting Behaviour of Trisiloxanes

Wetting and spreading processes which involve surfactant solutions are widely used in numerous industrial and practical applications nowadays. The performance of different non-ionic surfactants may vary significantly and so far superspreader solutions show the most promising spreading ability. The addition of trisiloxane surfactants to water was proven to enhance wetting, even on hydrophobic surfaces, on which conventional surfactants seem to have little or no effect. Although these extraordinary surfactants have been extensively studied over recent years, complete understanding of their underlying mechanisms and a suitable mathematical model are still lacking. Here we present a possible explanation for the impressive performance of trisiloxane, which is compared to wetting enhancement of a conventional surfactant. Additionally, we will explain why the hydrophobicity of the surface is a crucial factor for the spreading phenomenon. Light will be also shed on the effect of the pH of the solution to which surfactants are added. Finally, we will investigate long-term effects of the water environment on trisiloxane wetting ability and discuss if ageing may significantly affect their performance.

Surface modification of halogenated polymers: 1. Polytetrafluoroethylene

Solutions of solvated electrons in the presence of magnesium offer many advantages for the surface treatment of PTFE when compared to the classical solutions of solvated electrons in the presence of alkalis: the polymer remains white instead of black, its surface is not destroyed and presents a controlled hydrophilic character.