Contact conductance

The above measurements all rely on force and displacement data to evaluate adhesion and mechanical properties. As mentioned in the introduction, a very useful piece of information to have about a nanoscale contact would be its area (or radius). Since the scale of the contacts is below the optical limit, the techniques available are somewhat limited. Electrical resistance has been used in early contact studies on clean metal surfaces [62], but is limited to conducting interfaces. Recently, Enachescu et al. [63] used conductance measurements to examine adhesion in an ideally hard contact (diamond vs. tungsten carbide). In the limit of contact size below the electronic mean free path, but above that of quantized conductance, the contact area scales linearly with contact conductance. They used these measurements to demonstrate that friction was proportional to contact area, and the area vs. load data were best-fit to a DMT model. 

It is very difficult to estimate the magnitude of the contact conductance G. Normally the total conductance of the heat exchanger is determined, and G - is calculated from Eq. (9.48). Only in the case that rhe plate fins are welded to the pipes with a metallurgical contact is the contact conductance infinite, leading to zero contact resistance, that is 1 /G,. = 0. 

In the time constant (relaxation) method, the waveform of P is a negative step which produces a relaxation of the sample temperature from TB + ST to TB. The measure of P(T) may be critical when the power P is comparable with the spurious power or when the thermal conductance G is steeply variable with the temperature (i.e. G oc T3 in the case of contact conductances). 

Stojkovic S, Joachim C, Grill L, Moresco F (2005) The contact conductance on a molecular wire. Chem Phys Lett 408 134... 

For example, the small tolerance and low surface roughness of the plate in manufacture are critical for assuring the high electrical contact conductivity, low fluid flow resistance, and low water holdup to meet performance require-menfs of fhe plates. Moreover, to play the role of removing generated water in the cathode side—particularly to avoid flooding when fhe current density is high, the surface of the cathode plate may need hydrophobicity [11] so as to better adjust hydrophobic and hydrophilic properties of plate materials in cathode and anode plates. This area needs further study. 

Two types of conductivity detectors exist the contact conductivity detector, where the electrodes are in direct contact with the electrolyte, and the contactless coupled conductivity detector (C D also called oscillometric detector). With this detector, two stainless-steel tubes that act as electrodes are mounted on a capillary at a certain distance from each other. By applying an oscillation frequency, a capacitive transition occurs between the actuator electrode and the liquid inside the capillary. After having passed the detection gap between the electrodes, a second capacitive transition between the electrolyte and the pickup electrode occurs (see Figures 7 and 8 which is an example of separation of cations). In different reviews, Zemann and Kuban and Hauser discuss the advantages of this technique which include rather simple mechanical parts and electronics, and Kuban et al. compared several C D detectors. This technique has also been used as a detector for analysis by microchip CE. C" D detectors are available to be mounted on existing CE instruments. 

Conductivity detection is a universal detection mode in which the conductivity between two inert electrodes comprising the detector cell is measured. The different arrangements employed for the construction of these detectors include apparatus with a galvanic contact of the solution with the sensing electrodes (contact conductivity detection) [51] and detection systems without galvanic contact of the solution with the sensing electrodes (contactless conductivity detection) [1]. 

There are two main varieties of bulk conductivity detectors contact and contactless. In a contact conductivity detector, the electrodes contact the column effluent directly. The electrodes are usually made of stainless steel, platinum, or gold in order to minimize electrochemical reactions, but they are still subject to fouling over time. In the absence of electrochemical reactions, there is no charge transfer between the solution and the electrodes, so the conductivity measurement is made with an oscillating or alternating voltage. 

FIGURE 7.7 Simplified circuit diagram for a contact conductivity detector. 

For kilohertz measnrement frequencies and above, the contribution of Qi can generally be neglected. McWhorter and Soper employed a simple two-electrode contact conductivity detector with reverse-phase ion pair chromatography, and achieved a detection limit of 3.46pg in a lOOnL volume (464nM) for KCl [54]. Four-electrode setups, in which two leads are nsed to measure current and the other pair to measure voltage, can also be nsed in order to eliminate the effect of the impedance of the electrical leads on the measnrement. 

Galloway M, Stryjewski W, Henry A et al. Contact conductivity detection in polyfmethyl methacrylate)-based microfluidic devices for analysis of mono- and polyanionic molecules. Analytical Chemistry 74, 2407-2415, 2002. 

This type of detection has achieved much development in the last few years due to its simplicity. A specific revision on conductimetric (and potentiometric) detection in conventional and microchip capillary electrophoresis can be found in Ref. [57]. It is considered a universal detection method, because the conductivity of the sample plug is compared with that of the solution and no electroactivity of the analytes is required. Two electrodes are either kept in galvanic contact with the electrolyte (contact conductivity) or are external and coupled capaci-tively to the electrolyte (contactless mode). An alternating current potential is applied across the electrodes and the current due to the conductivity of the bulk solution is measured. As the signal depends on the difference in conductivity between solution and analyte zones, the choice of the electrolyte is crucial. It is necessary that it presents different conductivity without affecting sensitivity. 

Proteins, peptides PMMA Conductimetry Contact conductivity Platinum [152,154,155]... 

CEC was used to separate double-stranded DNA (dsDNA) on microchips [58,152]. In these cases, Ci8-modified PMMA microchips were employed with ion-pair reverse phase chromatography and DNA fragments with different sizes were analysed by contact conductivity detection. 

M. Galloway and S.A. Soper, Contact conductivity detection of polymerase chain reaction products analyzed by reverse-phase ion pair microcapillary electrochromatography, Electrophoresis, 23 (2002) 3760-3768. M. Masar, M. Dankova, E. Olvecka, A. Stachurova, D. Kaniansky and B. Stanislawski, Determination of free sulfite in wine by zone electrophoresis with isotachophoresis sample pretreatment on a column-coupling chip, J. Chromatogr. A, 1026 (2004) 31-39. 

Contact conductivity 835-836, 848 Contactless 832, 835, 848 Contamination of food, food residues, and food safety 467 CoPC el70... 

In freeze-drying of pharmaceuticals in vials at 0.1 mbar, the contribution of radiation can be 20-30%, of gas conductivity 50-60% and of contact conductivity 10-30% of total energy transfer. 

Abbreviations AOD, Acousto-optical deflection BCB, bisbenzyocyclobutadiene CCD, indirect contact conductivity detection CL, chemiluminescence ECD, electron capture detector FCS, fluorescence correlation spectroscopy FRET, fluorescence resonance energy transfer ICCD, integrated contact conductivity detection GMR, giant magnetoresistive LED-CFD, light emitting diode confocal fluorescence detector LIF, laser-induced fluorescence LOD, limit of detection MALDI, matrix-assisted laser desorption ionization PDMS, poly(dimethylsiloxane) PMMA, poly(methylmetha-crylate) SPR, surface plasmon resonance SVD, sinusoidal voltammetric detection TLS, thermal lens spectroscopy. 

An on-column contact conductivity detector was fabricated on a PMMA chip to analyze amino acids, peptides, proteins, and oligonucleotides. To minimize faradaic reaction and to only measure ohmic resistance (or conductivity), a bipo-... 

Describe two advantages and one disadvantage of the contactless and contact conductivity detectors. [1159] (3 marks)... 

For design purposes the contact conductance values given in Table 2-2 may be used in the absence of more specific information. Thermal contact resistance can be reduced markedly, perhaps as much as 75 percent, by the use of a thermal grease like Dow 340. 

Fins are frequently installed on tubes by a press-fit process. Consider a circumferential aluminum fin having a thickness of l.O mm to be installed on a 2.5-cm-diameter aluminum tube. The fin length is 1.25 cm, and the contact conductance may be taken from Table 2-2 for a l00-/xin ground surface. The convection environment is at 20°C, and h = 125 W/m2 °C. Calculate the heat transfer for each fin for a tube wall temperature of 200°C. What percentage reduction in heat transfer is caused by the contact conductance ... 

Figure 4 Separation of 100 pM amino acid mixture consisting of (1) alanine, (2) valine, (3) glutamine, and (4) tryptophan in an unmodified PMMA microchip using indirect, contact conductivity detection. Electrophoretic conditions 3-s electroki-netic injection time =150 V/cm for the electrophoresis. (Reprinted with permission from Ref. 20.)...

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