Dynamic Modes

The modification of the surface force apparatus (see Fig. VI-4) to measure viscosities between crossed mica cylinders has alleviated concerns about surface roughness. In dynamic mode, a slow, small-amplitude periodic oscillation was imposed on one of the cylinders such that the separation x varied by approximately 10% or less. In the limit of low shear rates, a simple equation defines the viscosity as a function of separation... 

Modification of an AFM to operate in a dynamic mode aids the study of soft biological materials [58]. Here a stiff cantilever is oscillated near its resonant frequency with an amplitude of about 0.5 nm forces are detected as a shift to a new frequency... 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

SIMS is, strictly speaking, a destructive teclmique, but not necessarily a damaging one. In the dynamic mode, used for making concentration depth profiles, several tens of monolayers are removed per minute. In static SIMS, however, the rate of removal corresponds to one monolayer per several hours, implying that the surface structure does not change during the measurement (between seconds and minutes). In this case one can be sure that the molecular ion fragments are truly indicative of the chemical structure on the surface. 

Anezykowski, B., Gotsmann, B., Fuchs, H., Cleveland, J.P. and Elings, V.B., How to measure energy dissipation in dynamic mode atomic force microscopy. AppL Surf. Sci., 140, 376-382 (1999). 

FIGURE 6.2 Diagrams of different AFM operating modes. (A) Contact mode and (B) dynamic mode for topographic imaging. (C) Force spectroscopy mode for interaction probing. Reprinted with permission from Liu and Wang (2010). 

Consequently, the lateral force between the tip and sample can be significantly reduced (Fig. 6.2B). Traditionally, contact mode typically could provide higher resolution, but recent advances in noncontact techniques have led to spatial resolution up to the atomic level in vacuums and liquids (Fukuma et al., 2005 Giessibl, 2003 Sugimoto et ah, 2007). Therefore, dynamic mode is preferred for soft and unstable samples. 

Supercritical fluid extraction can be performed in a static system with the attainment of a steady-state equilibrium or in a continuous leaching mode (dynamic mode) for which equilibrium is unlikely to be obtained (257,260). In most instances the dynamic approach has been preferred, although the selection of the method probably depends just as much on the properties of the matrix as those of the analyte. The potential for saturation of a component with limited solubility in a static solvent pool may hinder complete recovery of the analyte. In a dynamic system, the analyte is continuously exposed to a fresh stream of solvent, increasing the rate of extraction from the matrix. In a static systea... 

SFE can be carried out in three different ways. In a static extraction (no flow-rate), the extraction vessel is pressurised to the desired pressure with the extracting fluid and then simply left for a certain length of time. The main benefit of this method is that the fluid has time to penetrate the matrix. It is most applicable when the analyte has a high affinity for the solvent and a low affinity for the matrix and also when the solubility limit of the analyte in the fluid is much higher than the actual level reached during the extraction [89]. This method was popular in early SFE experiments but has declined in favour of dynamic SFE. Here, fresh SCF is continuously passed over the sample, extracting soluble compounds and depositing them in a suitable solvent or on a solid trap. The dynamic mode is particularly useful when the concentration of the solute... 

One general approach to enhancing sampling, which is the focus of this section, is based on the fact that both fast and slow dynamical modes contribute to the time evolution of biomolecular systems, but in most cases the motions of primary interest are the slow ones, which typically correspond to the largest structural changes [72, 73],... 

Figure 15. Mass misfit for two sequences of dynamic mode shifted by one mass. It is assumed that coincidence is obtained between Mo for the first sequence and Mo for the second sequence. The parameter SM (exaggerated) represents the misfit one mass down this peak.

An extremely important aspect in pharmaceutical research is the determination of drug optical purity. The most frequently applied technique for chiral separations in CZE remains the so-called dynamic mode where resolution of enantiomers is carried out by adding a chiral selector directly into the BGE for in situ formation of diastereomeric derivatives. Various additives, such as cyclodextrins (CD), chiral crown ethers, proteins, antibiotics, bile salts, chiral micelles, and ergot alkaloids, are reported as chiral selectors in the literature, but CDs are by far the selectors most widely used in chiral CE. 

In the nonisothermal or dynamic mode, a sample is set into a calorimeter and the temperature is raised at a certain constant heating rate up to the operational temperature. The total heat of reaction is independent of the heating rate (recommended range is 2-20°C/min [1,140]). Three methods are possible ... 

Further analysis shows that, for this particular example, the fastest modes (group I) correspond directly to the gas temperatures at the interior collocation points and those of groups II and III correspond to the concentrations. One should not infer from this conclusion that a general correspondence between the dynamical modes and the physical variables always exists. In this example, the correspondence results from the major differences in the magnitudes of the various groups. 

Indentation hardness determinations were performed in dynamic mode ( 1500 mm/sec impact speed) using a pendulum impact device and in quasistatic mode ( 0.008 mm/sec impact speed) with a custom-built indentation tester. The spherical indenters were of 2.54 cm diameter and 65.6 g mass, and the pendulum length was 92.3 cm with a release angle of 30°. Quasistatic indentation forces were selected to produce indentations of a similar size to the dynamic indentation test (1.5 to 2.0 mm radius). The compact indentations were measured using a white light interferometer (Zygo Corporation, Middlefield, Connecticut, U.S.A.) and the dent depth, dent diameter, apparent radius of curvature, and pendulum initial and rebound heights were used to calculate the indentation hardness of the compacts. 

Extractions can be carried out in dynamic, static, or combination modes. In the dynamic mode, the supercritical fluid continuously flows through the sample in the extraction vessel and out the restrictor to the trapping vessel. In the static mode, the supercritical fluid circulates in a loop containing the extraction vessel for some period of time before being released through the restrictor to the trapping vessel. In the combination mode, a static extraction is performed for some period of time, followed by a dynamic extraction. 

For the analyses of very volatile samples, there is a technique known as head space chromatography which can be used either in a static or dynamic mode (cf. Chapter 20). 

Dynamic mode instead of working in a closed environment, a carrier gas such as helium is either passed over the surface of the sample or bubbled through it in order to carry the volatile components into a trap where they are adsorbed and concentrated (Fig. 20.5). The sample is then introduced into the chromatograph by thermal desorption. This purge-and-trap technique is semi-quantitative and delivers a sample without residue. 

Figure 20.5—Dynamic mode of headspace sample analysis. The sample is recovered by thermal desorption ( stripping ) from a cartridge appropriate for the compound being measured.

Fig. 2.49 Dynamic structure factor at two temperatures for a nearly symmetric PEP-PDMS diblock (V = 1110) determined using dynamic light scattering in the VV geometry at a fixed wavevector q = 2.5 X 10s cm-1 (Anastasiadis et al. 1993a). The inverse Laplace transform of the correlation function for the 90 °C data is shown in the inset. Three dynamic modes (cluster, heterogeneity and internal) are evident with increasing relaxation times.

In the critical phenomena, the contribution from the different hydro-dynamic modes to the transport coefficients are calculated. On the other hand, in the extended hydrodynamic theory, only the Enskog values of the transport coefficients are used. Thus, while the critical phenomena considers only the long-time part of the memory function, the extended hydrodynamic theory uses only the short-time part of the memory function. None of the theories involve any self-consistent calculation. 

Once the extraction is complete, the static/dynamic selection valve is repositioned to the dynamic mode to allow flow. Subsequently, pressure and density are rapidly reduced to prevent significant losses of the supercritical fluid from the syringe pump tank and the extraction effluent, which is being transferred for collection. With a non-re-stricted transfer, the flow of supercritical fluid effluent is rapid. This desire for rapid depressurization led to the development of a delivery nozzle which would ensure collection of the extracted solutes without losses. Details of this delivery system can be found in the next section. 

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