Surface force principal methods

There are many pieces of evidence that convince us that matter is made up of atoms. Some of the most compelling evidence comes from scanning probe microscopy. This technique employs a microscopic rip, which responds to a surface to reveal its architecture. The principal methods of scanning probe microscopy are scanning tunneling microscopy (STM) and atomic force microscopy (AFM). 

Since surface forces depend on the magnimde of the area, the drops tend to be as spherical as possible. Distortions due to gravitational forces depend on the volume of the drop. In principle, it is however possible to determine the surface tension by measurement of the shape of the drop, when gravitational and surface tension forces are comparable. Two principally different methods must be taken into account. There are methods based on the shape of a static drop lying on a solid surface or a bubble adhering underneath a solid plate, and dynamic methods, based on continuously forming and falling drops. It should be noted that all the principles described here for drops are valid also for bubbles. 

Atomic force microscopy (AFM) was first applied to investigate the polymer surfaces in 1988 shortly after its invention [23]. Today, studies by AFM range from simple visualization of morphology to more advanced examination of polymer structure and properties at the nanometer scale. AFM gives three-dimensional pictures of the surfaces, while other methods, SEM and TEM, do not. AFM is frequently applied to polymer surfaces, principally to reveal morphology, nanostructure, chain packing, conformation, pore size, and pore size distribution at the surface. 

The principal methods for the immobilization of chemical receptors are (1) physical adsorption to a solid surface, (2) chemical adsorption (covalent attachment) to the surface, (3) affinity binding to physically or chemically boimd species, and (4) entrapment within a matrix. Since physical adsorption relies on relatively weak forces (van der Waals, ionic, solvation, donor/acceptor), molecules placed in this way may detach over time and/or exhibit nommiform biological activity becanse of a distribution of surface orientations/conformations. However, this method is clearly the simplest of the four and therefore often finds use. An example is the popular enzyme-linked immunosorbent assay (ELISA) used in medical diagnostics. 

Before we start a discussion on the thermodynamics of a contact between particles, it is worthwhile to briefly address the phenomena taking place at the three-phase contact line, and in particular, wetting and capillary forces acting within a liquid meniscus. We will also briefly summarize the principal methods of surface tension measurement. 

A Models to describe microparticles with a core/shell structure. Diametrical compression has been used to measure the mechanical response of many biological materials. A particular application has been cells, which may be considered to have a core/shell structure. However, until recently testing did not fully integrate experimental results and appropriate numerical models. Initial attempts to extract elastic modulus data from compression testing were based on measuring the contact area between the surface and the cell, the applied force and the principal radii of curvature at the point of contact (Cole, 1932 Hiramoto, 1963). From this it was possible to obtain elastic modulus and surface tension data. The major difficulty with this method was obtaining accurate measurements of the contact area. 

The reasons behind the specific choice of apparatus geometry can best be shown by a brief review of prior work. The earliest canal type surface viscometer was introduced by Dervician and Joly (8). In this apparatus, an insoluble monolayer is floated on a substrate fluid in a straight channel. The film is forced to flow through the channel by movement of a floating barrier. This motion is resisted principally by surface viscosity. Thus, the small force required to propel the film at a given speed may be measured and used to determine the surface viscosity of the film. A relatively complete theoretical treatment has been provided by Harkins and Kirkwood (5) for insoluble films with Newtonian surface viscosity in deep channels. Actual measurements are typically made in shallow channels, however, which are formed by floating the channel boundaries on the liquid surface. This method is not applicable to soluble surface films, which tend to diffuse through the substrate fluid and pass behind the barrier. Nevertheless, the most accurate values of surface viscosity available have been produced by this approach. 

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