Wetting behaviour

Simons, S.J.R. and Fairbrother, R.J., 2000. Direct observation of liquid binder-particle interactions the role of wetting behaviour in agglomerate growth. Powder Technology, 110, 44-58. 

Figure 3. Wetting behaviour of the melt on the crystal, (a) rounded part (the dashed white line corresponds to the position of the external solid-liquid-vapor line seen dmingthe ejq)eriment). (b) 111) facet.

This dramatic macroscopic difference in wetting behaviour is caused by only a thin molecnlar layer on the snrface of glass and clearly demonstrates the importance of surface properties. The same type of... 

Clean a soda glass plate by washing in 10% NaOH (care) followed by rinsing in double-distilled water. Blow dry with Ni and observe the behaviour of a droplet of clean water on the plate. Blow dry again, place droplets of the various CTAB solutions used on the clean plate and observe the wetting behaviour with CTAB concentration. 

Due to the chain architecture and the large size of the macromolecules, the wetting behaviour of polymer liquids can be different from that of simple liquids. The effect becomes particularly strong when the dimension of the liquid phase, e.g. film thickness and droplet diameter, approaches the dimension of the polymer coil. In addition to the spreading coefficient and the surface pressure effects, entropic elasticity of the polymer chain provides a strong contribution to the free energy for a constant volume V0=Ad ... 

The dendrimer droplets were used as a tool to examine surface heterogeneity on the nanoscale [319]. Figure 32 shows two pairs of SFM images of the droplets deposited on mica (a,b) and a copolymer film (c,d), respectively. On mica, the hydroxyl terminated dendrimer showed the autophobic or pseudo-partial wetting behaviour as the droplets sit on a thin film of the same material [312,320]. 

Modern coating technologies require increasingly thinner polymer films. This requirement is opposed by the surface pressure and the chain elasticity. Below a certain equilibrium thickness, the film is either metastable or even unstable and tends to break into droplets regardless of the chemical structure of the substrate [321, 322]. Anomalous wetting behaviour was observed for amphiphilic polymer films whose stability is controlled by the orientation of the surface active moieties [323,324]. All these phenomena belong to the dewetting problem. 

The spreading coefficient, and therefore, wetting properties of the substrate can be varied by chemical modification of the substrate surface or the liquid itself. From SFM observations, the wetting behaviour of PS on SiOx substrates was changed by partial sulfonation [331]. Unlike the unmodified PS, which readily... 

Fig. 34. SFM amplitude images of the oligo(hexafluoropropene) substituted PMA films on mica after annealing at room temperature for 50 h [324]. Depending on the number ol HFPO units, polymers exhibit either a autophobic wetting behaviour for HFP05 or b stable coverage with a 10 nm thick film for HFP03...

Wagner, R., Richter, L., Weissmuller, J., Reiners, J., Klein, K.D., Schaefer, D. and Stadtmuller, S. (1997) Silicon-modified carbohydrate surfactants 4. The impact of substructures on the wetting behaviour of siloxanyl-modified carbohydrate surfactants on low-energy surfaces. Appl. Organometallic Chem., 11(7), 617-32. 

Z. Florvolgyi, M. Mate, A. Daniel, and J. Szalma, Wetting behaviour of silanized glass microspheres at water-air interfaces a Wilhelmy film balance study, Colloids Surf. A 156, 501-508 (1999). 

In this work, ordered arrays of core-shell particles were used as model surfaces to study the water wetting behaviour of these surfaces. Two factors were varied in the wetting experiments (i) the shell chemistry and hence the solid surface tension of the organic shell, and (ii) the height roughness from sub- xm up to xm roughness values whereas the Wenzel roughness factor was kept constant. 

Consider a flat, undeformable, perfectly smooth and chemically homogeneous solid surface in contact with a non-reactive liquid in the presence of a vapour phase. If the liquid does not completely cover the solid, the liquid surface will intersect the solid surface at a contact angle 9. The equilibrium value of 9, used to define the wetting behaviour of the liquid, obeys the classical equation of Young (1805) ... 

Not all aspects of wetting behaviour are sensitive to the precise values of small contact angles. For example, the rise up a vertical capillary is a function of the cosine of the contact angle (equation (1.55)), and therefore a decrease in 6 from 10° to 0° causes an increase in penetration of only 1.5%. 

Figure5.4.1) Contact angle versus time for a eutectic (Ag-Cu) dropon polycrystalline W at 900°C in a high vacuum. Before the experiment, the W substrate was heat-treated in high vacuum at 1100°C for 2 h. Despite this treatment, the surface remained oxidised and a slow spreading, controlled by W deoxidation, was observed. 2) The same without prior heat treatment of W. In this case segregation of O at the W surface, by fast grain-boundary diffusion, prevents deoxidation of the substrate, resulting in non-wetting behaviour. From Lorrain (1996).

The behaviour described in the preceding Sections has been that observed during rigorously controlled laboratory studies using carefully cleaned materials in chemically inert environments. In practice, it may not be possible or economic to adopt such procedures, but nevertheless observations of the wetting behaviour in technical conditions can be interpreted provided it is realised that most solid surfaces of technical materials are covered by oxide films. Thus consideration must be given to how metal-metal contact can be achieved as well as to the implications of such contact for wetting behaviour. 

Literature data for the wetting behaviour of low or moderate melting point metals on AIN in a high vacuum are reported on Table 7.7. Large non-wetting contact angles are observed, usually in the range 130-150° (it is noteworthy that... 

Figure7.31.The influence ofstoichiometry ofTiC. yon the wetting behaviour ofCuat 1100 1150°C.

The ferrous metals Fe, Co and Ni and also Pd and Pt dissolve relatively large amounts of C but do not form stable carbides (Fe carbides, for instance Fe3C, are metastable). Their wetting behaviour on C substrates is very complex and not yet understood due to the multiplicity of elementary phenomena acting simultaneously and to serious experimental difficulties. Experimental results suggest that metals presaturated with C do not wet graphite (0 90°) while pure metals wet it well (0 90°). 

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