Acoustic time constant

Eor IR-MALDI, the situation can be very different because of the larger penetration depth, resulting in a larger acoustic time constant of about 1 ns. Eor the desorption with an Er YAG laser, the pulse width of 100 ns is long compared to... 

Time scale for observation The response of the acoustic effect is limited by the acoustic propagation between the fringe distance and that time constant is rac = A/v. Therefore, in 0 < t rac, the density contribution is small and fast response of the population grating can be easily observed (although the Temp.G signal could contribute to the signal even in this time scale). In principle, even rather slow dynamics of the population, which is not disturbed by the thermal... 

Temperature can propagate with the speed of sound. When the critical point is approached more closely than a crossover value given by asymptote analysis, the characteristic time of the piston effect does not monotonicaUy decrease to zero, but tends to reach a constant value, which is the characteristic acoustic time. For CO2 contained in a 10 mm long container set at 1 K above its critical temperature, the crossover value is some mK. At these conditions, the... 

Percentiles are expressed as the percentage of time (for the stated period) during which the stated noise level was exceeded, i.e. 5 min Lgo of 80 dB(A) means that for the 5-min period of measurement for 90 per cent of the time the noise exceeded 80dB(A). Therefore Lo is the maximum noise level during any period and Lioo is the minimum. Leq (the equivalent continuous noise level) is the level which, if it were constant for the stated period, would have the same amount of acoustic energy as the actual varying noise level. 

In an elastic material medium a deformation (strain) caused by an external stress induces reactive forces that tend to recall the system to its initial state. When the medium is perturbed at a given time and place the perturbation propagates at a constant speed (or celerity) c that is characteristic of the medium. This propagating strain is called an elastic (or acoustic or mechanical) wave and corresponds to energy transport without matter transport. Under a periodic stress the particles of matter undergo a periodic motion around their equilibrium position and may be considered as harmonic oscillators. 

However, this can only be an estimate since it is unlikely that a bubble in a sound field vhll feel a constant pressure, Pq, exerted during its collapse (acoustic pressure is time dependent, i. e. P = P sin 27tft) nor will it be an empty void, being filled with either gas or vapour. As such, Eq. 2.25 must be modified (Eq. 2.27). 

It has been argued (Appendix 3, Eq. A.21) that the collapse time for a bubble, initially of radius R, is considerably shorter than the time period of the compression cyde. Thus the external pressure Pj (= P + Pjj), in the presence of an acoustic field, maybe assumed to remain effectively constant (Pj ) during the collapse period. Neglecting surface tension, assuming adiabatic compression (i. e. very short compression time), and replacing R, by R, the size of the bubble at the start of collapse, the motion of the bubble wall becomes... 

More recently, Yang and Thompson implemented this type of sensor in FI manifolds, which they consider ideal environments for relating the sensor s hydrodynamic response to the analyte s concentration-time profile produced by the dispersion behaviour of sample zones. Network analysis of the sensor generates multi-dimensional information on the bulk properties of the liquid sample and surface properties at the liquid/solid interface. The relationship between acoustic energy transmission and the interfacial structure, viscosity, density and dielectric constant of the analyte have been thoroughly studied by using this type of assembly [171]. 

A first study refers to liquid water [77]. The signals AS q,x) and A5[r,r] were measured using time-resolved X-ray diffraction techniques with 100 ps resolution. Laser pulses at 266 and 400 nm were employed. Only short times x were considered, where thermal expansion was assumed to be negligible and the density p to be independent of x. To prove this assumption, the authors compared their values of AS q, x) to the values of AS q) obtained from isochoric (i.e., p = const) temperature differential data [78-80]. Their argument is based on the fact that liquid H2O shows a density maximum at 4 °C. Pairs of temperatures Ti, T2 thus exist for which the density p is the same constant density conditions can thus be created in this unusual way. The experiment confirmed the existence of the acoustic horizon (Fig. 8). 

The infrared spectrum of partially deuterated polyethylene has been examined as a function of temperature 323). Bands associated with trans-trans (tt) bond pairs and trans-gauche (tg) bonds have been identified so it is possible to follow the changes in these isomers as a function of temperature. As the temperature increased, the intensity of the tg band increased, while the intensity of the tt band decreased. It is hypothesised that the extinction coefficients for the two bands are essentially equal and the nearly constant total intensity of the two bands seems to confirm this hypothesis. Thus it appears that each time a tt sequence disappears a tg sequence is created. Correlations of this infrared result with the intensity of the Raman longitudinal acoustic mode, suggest that the new thermally induced tg sequences occur mainly as point dislocations in the crystal 323). 

Recently Maret et al. (JL) have observed the longitudinal acoustic mode in oriented DNA fibres and films in a Brillouin scattering experiment. They observed the largest acoustic velocity for the driest samples, smallest for wet samples and at all times the observed velocity was larger than that for water itself. We have assumed that the velocities for propagation parallel to the helix axis are characteristic of acoustic modes in the DNA double helix and have used these values along with previously refined valence force field parameters (2,3) to fit the non-bonded force constants for the double helix. 

The frequency correlation time xm corresponds to the time it takes for a single vibrator to sample all different cavity sizes. The fluctuation-dissipation theorem (144) shows that this time can be found by calculating the time for a vertically excited v = 0 vibrator to reach the minimum in v = 1. This calculation is carried out by assuming that the solvent responds as a viscoelastic continuum to the outward push of the vibrator. At early times, the solvent behaves elastically with a modulus Goo. The push of the vibrator launches sound waves (acoustic phonons) into the solvent, allowing partial expansion of the cavity. This process corresponds to a rapid, inertial solvent motion. At later times, viscous flow of the solvent allows the remaining expansion to occur. The time for this diffusive motion is related to the viscosity rj by Geo and the net force constant at the cavity... 

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