Lateral trapping forces

Fig. 7. Schematic diagram of forces exerted on a cell when using an inverted microscope with (A) epi-illumination (i.e., laser focused through the objective) or (B) transillumination (i.e., laser focused through the condenser). is the axial force, and Fl is the lateral trapping force. Curved arrows represent the laser beam waist and point in the direction of light propagation.

When an atom or molecule approaches a surface, it feels an attractive force. The interaction potential between the atom or molecule and the surface, which depends on the distance between the molecule and the surface and on the lateral position above the surface, detemiines the strength of this force. The incoming molecule feels this potential, and upon adsorption becomes trapped near the minimum m the well. Often the molecule has to overcome an activation barrier, before adsorption can occur. 

Owing to the operation of these ion-dipole forces, a number of water molecules in the immediate vicinity of the ion (the number will be discussed later) may be trapped and oriented in the ionic field. Such water molecules cease to associate with the water molecules that remain part of the network characteristic of water (Section 2.4.3). They are immobilized except insofar as the ion moves, in which case the sheath of immobilized water molecules moves with the ion. The ion and its water sheath then become a single kinetic entity (there is more discussion of this in Section 2.4.3). Thus, the picture (Fig. 2.11) of a hydrated ion is one of an ion enveloped by a solvent sheath of oriented, immobilized water molecules. 

Numerous articles can be found on the use of sorbent materials such as silica, stainless steel beads, and common solid-phase extraction materials for collection of supercritical fluid-extracted analytes.38 2 In this scenario the fluid is usually allowed to expand into a gas, which is forced through a packed bed of solid sorbent material. The analytes are bound or simply deposited and cryogenically trapped onto the sorbent and are later eluted with appropriate solvents. The advantage of this technique is that further cleanup can be performed by carefully selecting the eluting solvent. 

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