Controlling Wettability

In industrial applications, it is often necessary to control wettability. A windscreen or the cabin of a plane should be non-wettable, in order to avoid the formation of a film of frost, whereas a plastic greenhouse should be wettable by water, in order to avoid droplets forming by condensation and scattering the light. 

In the same way, a non-wettable plastic can be made wettable by evaporating a thin metallic film onto its surface, or by corona discharge, in which the plastic is bombarded with ions in order to activate certain surface groups. 

Zisman s Rule. Having treated a surface, wettability can be characterised by measuring the contact angle for each member of the alkane series. The curve cos E = /(7n) (see Fig. 1.7) is used to define a critical surface tension qc  

This rule, due to Zisman, is a useful way of characterising a surface treatment. It can be used to predict wettability by simple liquids, subject only to van der Waals forces, but becomes inaccurate for polar liquids. 

Breath Patterns. The hydrophobicity of a surface can be checked simply by breath patterns (see Fig. 1.8). If we breathe on dirty glass, water condensation forms microdroplets. However, rubbing the glass with a piece of potato, and 

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One of the most difficult parenteral dosage forms to formulate is a suspension. It requires a delicate balance of variables to formulate a product that is easily resuspended and can be ejected through an 18-to 21-gauge needle through its shelf life. To achieve these properties it is necessary to select and carefully maintain particle size distribution, zeta potential, and rheological properties, as well as the manufacturing steps that control wettability and surface tension. The requirements for, limitations in, and differences between the design of injectable suspensions and other suspensions have been previously summarized [17b, 18,19]. 

Instead of considering wettability from the above traditional perspective, let us look at it from the opposite side What if we can control wettability of a surface at microscopic length scales, perhaps actively New developments in surface chemistry and fabrication of self-assembled monolayers (SAMs see Chapter 7) have opened up precisely such a possibility. How can we take advantage of such opportunities ... 

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. 

Variation of the head group of the monolayer makes it possible to control wettability etc., and also allows the introduction of different chemical moieties with specific properties such as nonspecific binding of proteins to surfaces. For example the introduction of oligoethylene glycol functionality to the end of the alkyl chain results in protein-resistant properties (37). Thus instead of synthesizing different thiols/silanes with different head groups, it is more convenient to use a number of standard SAMs and subsequently perform reactions on SAMs to modify the surface chemistry. Performing reactions on... 

In this context, nanoporous carbons are extremely interesting materials which can be used either as electrodes of supercapacitors or hydrogen reservoir. They are commercially available at a low cost and under various forms (powder, fibers, foams, fabrics, composites) [3]. They can be obtained with well-developed and controlled porosity [4,5] and with a rich surface functionality [6,7], As far as electrochemistry applications are concerned, very important advantages of carbons are a high electrical conductivity, a good chemical stability in various electrolytic media and the possibility to control wettability by the nature of the surface functionality. When they are not playing the role of active material for the storage process, carbons may be also useful as additive in a composite to improve its physical properties. Particularly carbon nanotubes are able to improve the electrical conductivity and mechanical properties of electrodes [8],... 

Moreover, due to the controllable wettability by counteranion exchange [56, 57], SWNT-IL-Br, which was both hydrophobic and hydrophilic, could assemble at the water/oil interface in a controllable manner, i.e., from monolayer to multilayers [58]. Thus, it enabled a novel SWNT-sandwiched water/oil interface for the electron transfer, one of the most fundamental chemical processes... 

Reversibly Photo-Responsive Polymer Surfaces for Controlled Wettability... 

Based on colloidal monolayers of polystyrene spheres, we have prepared various two-dimensional nano-structured arrays by solution routes and electrodeposition. Many ordered structured arrays generated using these methods are of surface roughness on the nano- and micro-scales, and could be superhydrophobic or superhydrophilic. The nano-devices based on such nano-structured arrays would be waterproof and selfcleaning, in addition to their special device functions. In this article, taking silica, ZnO and gold as examples of the insulators, semiconductors and metals, respectively, we report some of our recent results to demonstrate controlled wettability and superhydrophobicity of two-dimensional ordered nano-stmctured arrays with centimeter square-size based on colloidal monolayers. 

In summary, our results have demonstrated that a colloidal monolayer can be used as a flexible template to create ordered nano-structured arrays. Combining it with other techniques, a series of ordered nano-structured arrays with centimeter size could be easily fabricated on many different substrates. These nano-structured arrays are of surface roughness on the nano- and micro-scales, or similar to the microstructure of lotus leaves. Our results have demonstrated that such micro/nano-structured arrays, not only of insulators and semiconductors but also of metals, display morphology-dependent wettability. Significant enhancement of both hydrophobicity and hydrophilicity can be achieved by fabricating special surface micro/nano-structure on any material. This means that we can also realize tunable and controllable wettability of surface of any material by designing the proper surface structure. From this study, it can thus be expected that the nano-devices based on our nano-structured arrays would be waterproof and self-cleaning, in addition to their special device functions. 

This target is usually achieved with a compromise between pore size and controlled wettability. 

The behavior of polymers at a solid substrate is closely related to the behavior of maaomolecules in thin films. Either such a thin film can be fabricated by confining a polymer layer between two solid substrates or one can consider a polymer film that wets a solid substrate in contact with air orvacuum. As the liquid-vapor interface that constitutes the free surface of the supported film resembles the interface between a polymer liquid and a hard, nonattractive solid substrate at the coexistence pressure, both situations are qualitatively similar. The former situation is often employed in computer simulations, whereas the latter setup is of great praaical interest owing to applications of thin polymer films as protective coating layers that control wettability, adhesion, or friction. 

Figure 2. Hypothetical ultrathin membrane constructed from stacked p-sheets of a periodic polypeptide. Large and small circles represent steric requirements of amino acid side chains. Variation in side chain size creates local vacancies - or pores - in the structure functional groups X decorate the membrane surface and control wettability.

It is clear that there is a bright future for responsive polymer brashes in the field of biomedical applications. They provide a unique control over surfaces that is difficult to match by any other surface coating. They have been shown be able to control wettability, cell adhesion, bacterial adhesion, and protein adsorption, which provide many opportunities for functional coatings of medical implants, protein carriers, biosensors. 

Wang, S.T., Song, Y.L, and Jiang, L. (2007) Photoresponsive surfaces with controllable wettability. J. Photochem. Photobiol. C-Photochem. Rev., 8, 18-29. 

Wu H, Zhang R, Sun Y, Lin D, Sun Z, Pan W, Downs P (2008) Biomimetic nanofiber patterns with controlled wettability. Soft Matter 4(12) 2429-2433. doi 10.1039/B805570J, 10.1039/B805570J blank Link to landing page via DOI... 

Wang, S., Feng, X., Yao, J., Jiang, L. (2006). Controlling wettability and photochromism in a dual-responsive tungsten oxide film. Angewandte Chemie International Edition, 45, 1264-1267. http //dx.doi.Org/10.1002/anie.200502061. 

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