Controlling Wetting Properties

As we already said, the immobilization of particles on the surface is driven by the combined effects of capillary forces, both that drive meniscus deformation (and consequently particle confinement in the trapping structure) and that are exerted on the particles during meniscus break-up. Consequently, the assembly process must be sensitive to both the wetting properties of the colloidal suspension and the shape and dimension of the trapping structures. 

We will first consider the influence of the wetting properties of the colloidal suspension relative to a given substrate. The role of surface tension on the yield of the assembly process can be easily investigated by adding a well-defined concentration of surfactants in the initial suspension. As an example, it was shown that the assembly of 500 nm PS particles on a PDMS substrate was optimum for a contact angle ranging from 30° and 60°.  

This result was demonstrated by monitoring the influence of adding a sodium dodecyl sulfate (SDS)/TritonX45 surfactant mixture, while keeping all other experimental parameters constant. Increasing the surfactant concentration resulted in a decrease of the contact angle. For contact angle values below 15°-20°, the process switches from capillary to convective assembly below 20°. Indeed, 

The question of pattern shape and its influence on the wetting properties and trapping efficiency is more complex. In the case of a high-aspect-ratio structure, wetting properties are affected by capillary pressure, which can lead, in some extreme cases, to the conversion of capillary assembly into convective assembly. This case will be discussed in section 15.4.4. 

The applicability of these models remains limited. Indeed, they are only devoted to the calculation of static contact angles. They do not consider friction forces that will affect the dynamic dewetting process. In addition, the impact of the accumulation of particles on 

---

Yu, K., Han, Y. A stable PEO-tethered PDMS surface having controllable wetting property by a swelling-deswelling process. Soft Matter 2006, 2, 705. 

Xu, J., Luo, G., Li, S., Chen, G. (2006). Shear force induced monodisperse droplet formation in a microfluidic device by controlling wetting properties. Lab on a Chip, 6, 131-136. 

The investigation of functional and responsive surfaces with controlled wetting properties [1 ] has attracted the interest of the scientific conununity due to their wide range of potential applications, including controllable drug delivery [5, 6], microfluidic devices [7] and self-cleaning surfaces [8], Tailoring the wettability of a surface has been attempted by various means of surface modification. Deposition... 

A variety of additives are used to control the properties of wetting and dispersion of pigments, flow, Hthography, and mb-off of inks. These additives belong to classes of materials such as surfactants, bentonite clays, alkyds, functional resins, polymers, etc. 

The theory of viscoelastic braking in liquid spreading exposes the various possibilities that may exist for controlling wetting or dewetting speeds by changing solid rather than liquid properties. Applications may exist in the fields of contact lenses, printing, and vehicle tire adhesion. 

Fairweather et al. [204] developed a microfluidic device and method to measure the capillary pressure as a function of fhe liquid water saturation for porous media wifh heferogeneous wetting properties during liquid and gas intrusions. In addition to being able to produce plots of capillary pressure as a function of liquid wafer safuration, their technique also allowed them to investigate both hydrophilic and hydrophobic pore volumes. This method is still in its early stages because the compression pressure and the temperatures were not controlled however, it can become a potential characterization technique that would permit further understanding of mass transport within the DL. 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Native starches are used as disintegrants, diluents, and wet binders. However, their poor flow and high lubricant sensitivity make them less favorable in direct compression. Different chemical, mechanical, and physical modifications of native starches have been used to improve both their direct compression and controlled-release properties (Sanghvi, 1993 van Aerde and Remon, 1988). Schinzinger and Schmidt (2005) used potato starch as an excipient and compared its granulating behavior with a-lactose-monohydrate and di-calcium phosphate anhydrous in a laboratory fluidized bed granulator using statistical methods. 

Dissolution of a drug substance is controlled by several physicochemical properties, including solubility, surface area, and wetting properties. For insoluble compounds, dissolution is often the rate-limiting step in the absorption process. Knowledge ofthe dissolution rate of a drug substance is therefore very useful for formulation development. The appropriate dissolution experiments can help to identify factors that contribute to bioavailability problems, and also assist in the selection of the appropriate crystal form and/or salt form. Dissolution tests are also used for other purposes such as quality control and assisting with the determination of bioequivalence (Dressman et al., 1998). 

The use of a phase-change media circumvents many of these problems. A material slightly above its liquid/solid-phase transition temperature may be ejected from a jet-printing nozzle the droplet solidifies quickly upon contact with a cooler surface. The feature size will then depend more on the cooling rate and less on the material s wetting properties, because a frozen droplet cannot spread. In this situation, the substrate temperature controls the printed feature size for materials having excellent wetting properties. 

The high volatility of benzene is the controlling physical property in the environmental transport and partitioning of this chemical. Benzene is considered to be highly volatile with a vapor pressure of 95.2 mm Hg at 25 °C. Benzene is slightly soluble in water, with a solubility of 1,780 mg/L at 25 °C, and the Henry s law constant for benzene (5.5 10"3 atm-m3/mole at 20 °C) indicates that benzene partitions readily to the atmosphere from surface water (Mackay and Leinonen 1975). Mackay and Leinonen (1975) estimated a volatilization half-life for benzene of 4.81 hours for a 1-meter-deep body of water at 25 °C. Even though benzene is only slightly soluble in water, some minor removal from the atmosphere via wet deposition may occur. A substantial portion of any benzene in rainwater that is deposited to soil or water will be returned to the atmosphere via volatilization. 

The fundamental theoretical questions underlying the wetting of a solid surface by a liquid drop have been described and discussed. These theoretical principles can be directly applied to practice along two main lines (a) characterization of solid surfaces in terms of their surface tension and (b) designing processes based on controlling wettability properties. The following points summarize current understanding for each of these two directions. 

Capillary sealing effects are controlled by wetting phenomena which, for hydrocarbons in general, are poorly constrained. In real sub-surface situations, the assumption of a water-wet seal is reasonable for an initially hydrocarbon-free seal. This may be less likely in dynamic situations where capillary seals may leak periodically in the presence of active charge. The wetting properties of seals may change through time an initially water-wet seal may evolve into a hydrocarbon-wet seal, due to the adsorption of a variety of compounds from crude oil, such as asphaltenes (Anderson, 1986). This may ultimately result in a top seal which has no capillary seal capacity and leaks via two-phase flow. 

The epoxy resin solution is typically a halogenated epoxy resin dissolved in a volatile solvent such as acetone. Various proprietary resin parameters control such properties as cure speed, wetting of the glass, and cured state Tg. 

Sulphosuccinic acid diesters play a role above all in American polymerization formulations. They are rarely used as principal emulsifiers, but rather to control secondary properties, for example, for the production of highly concentrated low viscosity acrylate dispersions. The branched sodium di-2-ethyl hexyl sulphosuccinate is widely used, combining favourable emulsifier properties with excellent wetting power. Dicyclohexyl sulphosuccinate has a particularly high CMC and a particularly high surface tension [48]. 

---