Surface contact sampling

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Over the past 10 years it has been demonstrated by a variety of in situ and ex situ techniques187,188 485 487 488 534 that flame-annealed Au faces are reconstructed in the same way as the surfaces of samples prepared in UHV,526-534 and that the reconstructed surfaces are stable even in contact with an aqueous solution if certain precautions are taken with respect to the potential applied and the electrolyte composition 485,487,488 A comprehensive review of reconstruction phenomena at single-crystal faces of various metals has been given by Kolb534 and Gao etal.511,513... 

In the oxamyl tomato study, the DFR samples were obtained using a 5-cm Birkestrand sampler (10-cm disk size using the upper and lower surfaces). The sample consisted of 40 leaf disks or 400 cm from each subplot. The samples were collected impartially or in a nondirected approach from the middle two rows. The plot was four rows wide and the tractor came into contact with the first and fourth rows as the application was made. The middle two rows should be undisturbed by this movement and should therefore provide a more representative sample. 

WBDs) and the second group wore swimming suits to give maximum skin exposure. The cotton WBDs were chosen to normalize regional differences in surface contact and to increase the sample size relative to cotton-patch dosimeters (Durham and Wolfe, 1962). 

Another source of acrylonitrile in water is leachate from chemical waste sites. Preliminary data from the Contact Laboratory Program (CLP) Statistical Database indicates that acrylonitrile has been detected in surface water samples collected at two of 862 hazardous-waste sites (including NPL and other sites) being investigated under Superfund. The median concentration of the positive samples was 100 pg/L (CLPSD 1988). Acrylonitrile was detected in 12 groundwater samples collected at 5 sites, also at a median concentration of 100 pg/L. 

Cavelier C, Foussereau J, Gille P, et al. 1988. Nickel allergy Tolerance to metallic surface-plated samples in nickel-sensitive humans and guinea pigs. Contact Dermatitis 19 358-361. 

The current transfer problem that had been identified with low temperature superconducting composites deserves additional mention for the high temperature superconductors, that in the bulk material are frequently not fully dense. Making the electrical connection in such a manner as to obtain uniform current distribution throughout the cross section of the material is difficult. The method described by Jin, et al. (24) with embedded wires or particles may provide for a significant improvement but the present techniques used to determine the critical current by a surface contact on the ceramic sample are subject to this problem. A discussion for the multifilamentary wire of NbsSn is provided by Goodrich and Fickett (30) and this discussion is likely to be similar to the high temperature materials that are not fully dense. 

The use of contact plates is the recommended technique. The RODAC plate method is the simplest, but it is useful only for flat surfaces. Swab samples afford examination of corners, crevices, and other area inaccessible to RODAC plates. 

The cap may be set on the top of a clean surface topside against the surface. The sample side of the cap should not come in contact with any surface, including the fingers or hand of the individual obtaining the sample. 

The modulation frequency is typically in the range from 100 Hz to 3 kHz, and thus much lower than the resonance frequencies of the cantilever and the scanner. This enables better control of the forces exerted on the sample. The z-mod-ulation amplitude can be varied between 10 nm and 1 pm to ensure that that the tip is retracted from the surface. Shear forces are reduced permitting investigation of soft samples because of the small duration of the tip-surface contact, between 10 3 and 10 4 s. Pulse force mode SFM has been used to map adhesion of heterogeneous polymers in dependence of temperature and molecular weight as well as map electrostatic double-layer interactions [158-160]. 

Appropriate SPE sorbent selection is critical to obtaining efficient SPE recovery of semivolatile organics from liquids. Henry [58] notes that an SPE sorbent must be able to sorb rapidly and reproducibly, defined quantities of sample components of interest. Fritz [73] states that successful SPE has two major requirements (1) a high, reproducible percentage of the analytical solutes must be taken up by the solid extractant and (2) the solutes must then be easily and completely eluted from the solid particles. The sorption process must be reversible. In addition to reversible sorption, SPE sorbents should be porous with large surface areas, be free of leachable impurities, exhibit stability toward the sample matrix and the elution solvents, and have good surface contact with the sample solution [68,73],... 

For post-type DTA s in which thermocouple junctions measure the temperature of the container of the sample (e.g. platinum or poly crystalline alumina crucibles), good mechanical contact between the sample and the bottom of the crucible will improve instrument sensitivity to transformations. Surface contact may be optimized by using samples shaped to match the crucible, or finely crushed granules, as opposed to more spherical or odd-shaped chunks. Optimum mechanical contact minimizes the lag time between when a reaction occurs and when heat propagates to/from the point of temperature measurement, and the reaction is recorded. 

The aim of the present study was a closer investigation of the properties of LDPE surfaces subjected to photosulfonation processes. Until today no zeta potential studies had been carried out on photosulfonated LDPE. In the present investigation such modified LDPE surfaces were characterized by FTIR spectra, contact angle testing and zeta potential measurements were carried out. To improve the stability of modified LDPE surfaces, polyethylene samples were crosslinked by e-beam irradiation and... 

The samples treated with KOH and washed with water were protonated by treating with 0.2 M HC1 aqueous solution at 22 °C for 5 min to yield a polyamic acid surface. The samples were washed with water (2x3 min) and isopropanol (2x3 min) and dried under vacuum. This modified surface was extensively analyzed since the polyamic acid surface is of great interest in electronic applications (14). Treatment with HC1 does not modify the polyimide, but acidifies all of the potassium polyamate surface to the polyamic acid surface. The water contact angles increased from 23°(0a)/5°(0r) to 5878°, indicating that the surface became less polar. The polyamic acid surface is less polar than the potassium polyamate surface, thus these results are consistent with the proposed reaction shown in Scheme I. 

Fig. 2.2. Mycelium of Hebeloma crustuliniforme after colonization of a potassium feldspar surface for seven months. The sample was prepared by fixation and critical point drying followed by gold coating and analysis by scanning electron microscopy. Hyphae (H) and bacteria (B) are visible. Scale bar = 10 pm. The hyphal surface contact is mediated by a fdm of extracellular mucilage (arrow) and bacteria are seen in the mucilage.

---