Surface forces, localization

An important consequence of the variation in the environment of the molecules in the region of the surface of the fluid, in both pure fluids and soap solutions, is the presence of a macroscopic surface force localized within about one atomic thickness of the surface. For most purposes it is justifiable to consider this as a surface tension, that is a force per unit length, a, in a membrane of negligible thickness at the surface of the fluid. 

Conceptually, predecessors of the scanning force microscope are the surface force apparatus (SFA) [73,74] and the stylus profilometer [75,76]. The SFA enables measurement of normal and friction forces between molecularly smooth surfaces as small as 1 nN as a function of distance with a resolution of 0.1 nm. In addition to the local force measurement, the profilometer provides a topographic map of the surface by scanning the surface with a sharp probe. However, the profilometer is not suitable for structure characterisation because of the large radius of the tip (about 1 pm) and the low sensitivity of the force sensor (in the range of 1(T2 to 1(T5 N). 

Complementary to the SFA experiments, SFM techniques enabled direct, non-destructive and non-contact measurement of forces which can be as small as 1 pN. Compared to other probes such as optical tweezers, surface force balance and osmotic stress [378-380], the scanning force microscope has an advantage due to its ability in local force measurements on heterogeneous and rough surfaces with excellent spatial resolution [381]. Thus, a force-distance dependence measured from a small surface area provides a microscopic basis for understanding the macroscopic interfacial properties. Furthermore, lateral mapping... 

It can be concluded that the direct energetic effect of the surface force field can be estimated to be of the order of one to two monolayers. Due to the local origin of the adsorption force and high flexibility of the siloxane chain, the local motions of PDMS chains at a distance more than 2 nm from the Aerosil surface are about the same as in unfilled PDMS [7, 8,21]. 

Using a surface forces apparatus, Israelachvili determined the force law for two molecularly smooth charged mica surfaces immersed in an aqueous solvent (see Israelachvili and Marra, 1986, and references cited therein). The repulsive hydration force is oscillatory (Fig. 8A). It is understood to reflect the geometry and local structure of the solvent and... 

The properties of the surface layers have a strong effect on the deposition process. The driving force of the electrochemical reaction is the potential difference over the electrochemical double layer. Adsorption of species can change this potential. For example, the additives used in electrodeposition adsorb in the Helmholtz layer. They can change the local potential difference, block active deposition sites, and so on. The thickness of the diffusion layer affects the mass-transfer rate to the electrode. The diffusion layer becomes thinner with increasing flow rate. When the diffusion layer is thicker than the electrode surface profile, local mass-transfer rates are not equal along the electrode surface. This means that under mass-transfer control, metal deposition on electrode surface peaks is faster than in the valleys and a rough deposit will result. 

Local reactions take place, for instance, in unconventional lithography approaches. Here, the surface of a polystyrene-Woc -poly(ter/-butyl acrylate) (PS69o-fr-PtBA12io) block copolymer film comprising reactive ter/-butyl ester moieties at the film surface (skin layer of 8 nm thickness) is locally hydrolyzed by a reactant (trifluoro acetic acid) that is delivered by a soft elastomeric stamp, as shown in Fig. 4.38. Thus, the surface is locally modified to yield poly(acrylic acid), which possess higher friction forces than the unreacted tert-butylester region. 

The second fairly modern group of methods introduces (of numerical reasons) a 3D continuous surface force (CSF) or a 3D continuum surface stress (CSS) acting locally within the whole transition region constituting a meso-scale interface. Notice that since we are primarily interested in the interfacial forces, the latter group of techniques were used approximating the surface effects without actually reconstructing the interface. 

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