Probe surface, electric potential

The force harmonics F and F2 are both proportional to the capacitance gradient SCfSz. More importantly, the first harmonic of the electric force Fi is directly proportional to the surface potential Vs- At a given position, this surface potential can be measured directly by varying Vdc, so that the first harmonic of the electric force Ft is equal to zero. In this way, the DC voltage, necessary to compensate the first harmonic, is equal to the surface electric potential Vs- This is the principle of the Kelvin Probe Microscopy. Furthermore, by dividing the first harmonic Fi by the second harmonic F2... 

The Scanning Kelvin Probe is a state-of-the-art device that measures surface electrical potential without actually contacting the sample. Its operation can be grossly summarized as follows (Figure 7.42) A probe tip is positioned close to the skin, creating a type of capacitor the probe tip acts as a plate, whereas the skin acts as the contralateral plate and the potential difference between the two (Vs) generates a charge on the probe tip the probe tip oscillates to vary the distance from the skin (do = tip-to-sample distance,... 

Slow dyes that respond via a redistribution across the entire membrane (sometimes called Nemstain dyes) do so because of a change in the transmembrane electrical potential. As such, they can only be used as probes of the transmembrane potential and not as probes of the surface potential or the dipole potential. Dyes whose electric field sensing mechanism involves a movement between the aqueous medium and its adjacent membrane interface on one side of the membrane can, in principle, respond to changes in both the transmembrane electrical potential and the surface potential. Fast dyes that remain totally in the membrane phase (e.g., styrylpyridinium, annellated hemicyanine, and 3-hydroxyflavone dyes) respond to their local electric field strength, whatever its origin. Therefore, these dyes can, in principle, be used as probes of the transmembrane electrical potential, the surface potential, or the dipole potential. 

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

A comparison between the two experiments was made in an attempt to elucidate whether the observed change in ion current with contamination layer can be explained by the variation of electric potential on the probe surface or not. 

After creating a surface which yields SERS signals, by the proper mechanical, chemical, and electrochemical treatment, one still has the electric potential of the electrode as an independent parameter. This capability is a unique feature of electrochemical systems (and less so of colloids). It gives another very powerful handle to affect and probe the metal-electrolyte interface. 

A further spatially resolved method, also based on work function contrast, is scanning Kelvin probe microscopy (SKPM). As an extended version of atomic force microscopy (AFM), additional information on the local surface potential is revealed by a second feedback circuit. The method delivers information depending on the value (p (p(x) + A x). Here, A(zS(x) is the difference in work function between the sample and the AFM tip and cp(x) is the local electric potential [12]. (p x) itself gives information on additional surface charges due to... 

Infrared methods have been used to study adsorbed species (reactants, intermediates, products), to examine species produced in the thin layer of solution between electrode and window, and to probe the electrical double layer. These approaches have been especially useful with species that have a high infrared absorption coefficients, like CO and CN . In favorable cases, information about the orientation of an adsorbed molecule and the potential dependence of adsorption can be obtained. For example, the SNIFTIRS spectrum obtained with a 0.5 mM aqueous solution of p-difluorobenzene in 0.1 M HCIO4 at a Pt electrode is shown in Figure 17.2.6 (62). The spectrum results from both dissolved (positive AR/R-values) and surface adsorbed (negative AR/R-values) p-difluorobenzene. 

The field signature method (FSM) is a relatively new mefliod developed by CorrOcean ASA from the principle commonly used in electrical resistance (ER) probes. An electric current is sent through the part of flic installation being tested (often a pipeline), and the potential differences between a considerable number of points on the external surface are measured. The potential differences depend on reduction of the material thickness and how this reduction is distributed along the pipe wall [9.12]. 

Perturbations of the medium adjacent to the device surface result in variations in the phase, amplitude, and velocity of the surface acoustic wave. Specifically, these properties will be affected by changes in the density, viscosity, or elastic properties of the medium in contact with the surface. Since the acoustic wave has an electric potential wave associated with it as well, the SAW can also be used to probe the dielectric and conductive properties of this surface medium. By far, the largest number of chemical sensor applications of SAW devices take advantage of the mass sensitivity of SAW oscillators. 

The ability of XPS to detect surface potential changes provides a much needed tool for electrochemistry to simultaneously obtain information about the local chemistry and the electrical potential across interfaces and components of electrochemical devices. The great advantage of XPS is also that it is a local, noncontact probe for surface potentials, with a spatial resolution that is determined by either the beam spot of the incident X-rays or the resolution of a two-dimensional electron detector (for a detailed discussion of the latter case, see Ref. [71]). 

Important measurands for the characterisation of the EDL are the surface charge density and the electrokinetic potential or zeta-potential. The zeta-potential is the electric potential at a h3q)othetical shear plane, which separates the mobile solvent from solvent molecules that adhere to the particle surface. The zeta-potential can he probed by imposing a relative motion between bulk solvent and particle (Delgado et al. 2007). 

Moreover, if the surface potential or surface charge of the colloid has been determined and the solution is of defined ionic content, it is possible to calculate the potential or charge of the surface under investigation by matching the experimentally obtained curves to theoretical calculations based on electrical double layer theory. In the example shown, the best-fit membrane surface charge was —0.00114 Cm and the best-fit membrane surface potential was —64 mV. Furthermore, an important advantage of the colloid probe technique is that it allows exploration of variations in surface electrical interactions at different points on the membrane surface, as the following section shows. 

An electrochemical probe can be used to measure the shear forces at the surface of a membrane in the presence of bubbles. The system consists of a cathode that is mounted flush to the outside of a Teflon tube as a test fiber and a reference anode. The cathode is called the shear probe and is generally made of platinum wire (0.5 mm diameter) which protrudes onto a test fiber. The electrochemical reaction between cathode and anode is provided under the influence of a constant electric potential of 250 mV. The obtained signal in voltage drop is then conditioned through an amplifier and low-pass filter. The relationship between mass transfer at the probe and the limiting diffusion current is used to convert the voltage values into shear forces [77, 78]. 

Surface conductivity of glass or of thin films on glass is often measured in terms of sheet resistance (ohms per square, or Q/D) using a four-point probe technique. Electrically contacting point probes are placed at the four comers of a square on the surface or the fihn. A current I is allowed to pass through two adjacent probes, and the potential difference V developed across the other two probes is measured. Sheet resistance in this arrangement is calculated as... 

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