Field-Induced Surface Phenomena

Furthermore, the radicals formed upon field-induced hydrogen abstraction can lead to polymerization products on the emitter surface. The mechanism of this field polymerization helped to elucidate the phenomenon of activation of field emitters, i.e., the growth of microneedles on the emitter surface under the conditions of field ionization of certain polar organic compounds. [59]... 

A very useful type of phenomenon in the study of colloidal particles is the electrokinetic phenomenon that results from the movement of a solid phase with surface charge relative to an electrolyte-containing liquid phase. An applied electric field induces movement or, conversely, movement induces an electric field. The phenomena can be divided into two types ... 

We will now consider a second example which illustrates the electrostatic induction phenomenon. First of all, let us suppose that a conductive body of arbitrary shape is situated within the region of influence of an electric field Eq as shown in Fig. 1.9. Under the action of the field, the positive and negative charges residing inside the conductor move in opposite directions. As consequence of this movement, electric charges accumulate on both sides of the conductor. In so doing, they create a secondary electric field, which is directed in opposite direction to the primary field inside the conductor. The induced surface charges distribute themselves in such a way that the total electric field inside the conductor disappears, that is ... 

In the absence of walls the dynamic behavior may be described, as in nematics, by adding a viscous torque to the elastic and electric ones. For the field-off state, the distortion decays with the nematic time constant (see Eq. (34), where K should be substituted for 22)- The time of the response to an electric field can only be calculated for the simplest case when jU=0, Te=7 / e sin 2 E -EI), where E is defined by Eq. (69). Thus, the response of the smectic C phase is sin 2 times slower than that of the nematic phase. However, in experiments the same substance often responds faster in the smectic C phase than in the nematic one [159-161]. This may be due to the smaller value of Yi when the motion of the director is confined by the cone surface. The same phenomenon has been observed for the ferroelectric smectic C phase [162]. The domain-wall motion makes the dynamics of switching more complicated the field-induced wall velocity has been calculated by Schiller et al. [70]. 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

The concept of a pore potential is generally accepted in gas adsorption theory to account for capillary condensation at pressures well below the expected values. Gregg and Sing ° described the intensification of the attractive forces acting on adsorbate molecules by overlapping fields from the pore wall. Adamson has pointed out that evidence exists for changes induced in liquids by capillary walls over distances in the order of a micron. The Polanyi potential theory postulates that molecules can fall into the potential field at the surface of a solid, a phenomenon which would be greatly enhanced in a narrow pore. 

In addition to the field enhancement, the increases of the radiative decay rate of the molecule also lead to the fluorescence enhancement. This happens when molecules are S -20nm away from metal nanoparticies aggregated on surfaces [19-21]. Lakowicz and coworkers have characterized this phenomena by using silver island films deposited on the internal surface of two quartz plates which sandwich a bulk fluorophore solution [20]. The fluorophores are physically placed close to silver islands so that there are a range of distances between the fluorophore and metal. The fluorescence enhancement is accompanied by decreased lifetimes and increased photostability. This phenomenon shows that the silver island increases the radiative decay rate of the fluorophore and therefore induces the fluorescence enhancement. 

---