Colloid adhesion force

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

Vakarelski et al. [88] also investigated the adhesive forces between a colloid particle and a flat surface in solution. In their case they investigated a sihca sphere and a mica surface in chloride solutions of monovalent cations CsCl, KCl, NaCl, and LiCl. The pH was kept at 5.6 for all the experiments. To obtain the adhesive force in the presence of an electrostatic interaction, they summed the repulsive force and the pull-off force (coined foe by the authors ) to obtain a value for the adhesive force that is independent of the electrostatic component. 

Some of the disadvantages were overcome by the use of the colloidal probe technique to measure adhesion forces (review Ref. [216]). The colloidal probe technique offers the advantage that the same particle can be used for a series of experiments and its surface can be examined afterwards. The accessible range of particle size is typically limited to a range between 1 /zm and 50 pm. The tedious sample preparation, limits the number of different particles used within one study, for practical reasons. Therefore the colloidal probe and centrifugal methods complement each other. 

The class of methods used for preparing colloidal dispersions in which precipitation from either solution or chemical reaction is used to create colloidal species. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. See also Dispersion Methods. An empirical or qualitative term referring to the relative ease with which a material can be deformed or made to flow. It is a reflection of the cohesive and adhesive forces in a mixture or dispersion. See also Atterberg Limits. 

Silica gel could be prepared via the gelation of silica sol. The process for the formation of water-containing uniform gel from spherical silical colloidal particles is very fast. It is known that there is adhesive force on the surface of spherical silica collodial particles, which could lead to the aggregation of these particles. This process could be described as below. 

In the discussion so far it has been assumed that the only interaction between liquid and substrate is by adhesion forces. This means that with porous substrates capillary forces should be absent. This situation can be obtained by filling the substrate pores with another liquid having a surface tension comparable to that of the colloidal solution or by making the substrate hydrophobic (in case of an aqueous solution). 

Recently, Bowen et al. [27,28] and Hilal and Bowen [29] and Hilal et al. [30] applied the APM technique to study adhesion at the membrane sinface. The measurement of interaction forces between a colloid probe and a membrane smface allows quantification of the electrostatic double layer interactions when the colloid probe approaches the membrane surface, and of the adhesion force (van der Waals interaction force) when the colloid probe is withdrawn after it has been in contact with the membrane surface. Quantification of the interaction forces involved in fouling and chemical cleaning of fouled membranes is very important in order to imderstand the mechanism of fouling and to develop a favorable membrane for water treatment. 

To measure the adhesion forces between colloids (fouling) and the surface layer, or between the surface layer and the substrate, AFM is seen as reliable and simple, as discussed earlier. Bowen et ah [36] introduced a new technique for the direct measurement of the force of adhesion of a single particle by AFM. AFM, in conjunction with the colloid probe technique, allows a direct quantification of membrane fouling through the measurement of pull-off (detachment) forces, when the probe is retracted from the surface after contact has been made. 

The school of Bowen [36] used the AFM technique for direct measurement of the adhesion force between a colloidal probe and membrane surfaces. Colloidal probes were prepared by attaching an 11 pm polystyrene sphere with epoxy resin to a V-shaped AFM tipless cantilever (Fig. 7.2). The AFM allowed the measurement of the force between the colloidal probe and a membrane sample as a function of the displacement of the sample, where a piezoelectric crystal varied the sample displacement. A laser beam reflected from the back of the cantilever fell on a split photodiode that detected small changes in the deflection of the cantilever. To convert the deflection to a force, it was necessary to know the spring constant of the cantilever and to define the zero of the force. The spring constant specified by the manufacturer was 0.4 N m The zero of the force was chosen where the deflection was independent of the piezo position (where the coUoidal probe and the membrane surface were far apart.)... 

Figures 7.3 and 7.4 show the plots of normahzed force (force/particle radius) versus piezo displacement as a colloid probe was retracted from the membrane surface for ES 404 and XP 117, respectively. In making such measurements, the colloid probe first had to be brought into (momentary) contact with the membrane surface. It was reported that the loading forces for such contact have an influence on the measured adhesive force [22,37]. The loading forces for the curves shown are 223.5 nN (Fig. 7.3)...

Bowen et al. [39] measured directly the adhesion (interaction) of cellobiose and cellulose with two polymeric UF membranes of similar MWCO, but of different materials. As probes, they used silica spheres (diameter 5-8 im) the surfaces of which were modified by static adsorption of cellobiose. They also used pure cellulose probes. Membrane ES 404 was made of poly(ether sulfone) alone, and EM 006 was made of a poly(ether sulfone)-polyacrylate blend, chosen specifically to increase the hydrophihc properties and decrease the fouling properties of the membrane. Study of ES 404 and EM 006 had shown that the interaction of cellobiose or of colloidal cellulose with the membranes was such that ES 404 always had the greater adhesion and greater fouling tendency. However, if the membrane was first fouled with cellobiose, the colloidal cellulose adhesion force was increased significantly, and the differences between the membranes diminished. Bowen et al. suggested that in the future, it would be possible to use the techniques developed to allow prior assessment of the fouUng propensity of process streams with different types of membranes. 

Hilal et al. [30] used AFM, in conjunction with a colloid probe, coated colloid probe, and cell probe techniques, to measure directly the adhesive force between two different UF membranes and a polystyrene sphere (diameter 11 jim), protein bovine serum albumin (BSA), and a yeast cell. These two membranes were ES 404 and XP 117 mentioned above (Table 7.1). The experiments were performed in 10 M NaCl solution. It was reported that the adhesive force of the polystyrene, the protein, and the cell system on the ES 404 membrane was greater than that on the XP 117 membrane. The relatively high affinity of protein for synthetic membrane surfaces was also observed. 

Polymers in solution have an enormous effect on the adhesion between surfaces. Such polymers are used as lubricants, as thickeners, as colloid stabilisers, as binders, glues, and inks. Also they are ubiquitous in biological systems. Their practical significance is large but understanding their effects remains to be explored to a great extent. This section describes some experimental observations of the adhesion forces and draws a schematic theoretical picture of the effects. 

In a recent study Raj et al. presented the first direct study of adhesion forces, by colloidal force microscopy, between smooth PLA films representing the polymer matrix, and a microbead of cellulose that mimics the cellulose material in flax fibers [65]. Normalized adhesion force measurements demonstrated the importance of capillary forces when experiments were carried out under ambient conditions. Experiments, conducted under dry air allowed for the deduction of the contribution of pure van der Waals forces, and the results, through the calculation of the Hamaker constant, show that these forces, for the PLA/cellulose/air system, were lower than those obtained for the cellulose/cellulose/air system and hence underlined the importance of optimizing the interface among these materials. The study demonstrated the capacity of AFM to probe direct interactions in complex systems by adjusting the nature of the surface and... 

FIGURE 1.102 Effect of the pretreatment procedure of nanosilica A-300 on the shape of (a) radial function of variation in the free energy of adsorbed water and (b) adhesion forces for aqueous suspensions of nanosilica. (Adapted from Colloids Surf. B Biointerfaces, 8, Turov and Barvinchenko, Structurally ordered surface layers of water at the Si02/ice interface and influence of adsorbed molecules of protein hydrolysate on them, 125-132, 1997. Copyright 1997, with permission from Elsevier.)... 

Experimental results obtained by Zhao et al. [84], showed that adhesive force and friction force of DEC films with microgrooves reduced effectively with increase of groove area density and incorporation of thin ILs films. The lowered adhesion and friction force were attributed to two factors including the reduced area of contact between DLC films and colloidal tip, and incorporation of thin ILs films to avoid direct contact between DLC films and colloidal tip and to facilitate sliding of colloidal tip on DLC films. 

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