Controlled flux technique

One other measurement technique that has been used to measure Kl over a shorter time period, and is thus more responsive to changes in wind velocity, is the controlled flux technique (Haupecker et al., 1995). This technique uses radiated energy that is turned into heat within a few microns under the water surface as a proxy tracer. The rate at which this heat diffuses into the water column is related to the liquid film coefficient for heat, and, through the Prandtl-Schmidt number analogy, for mass as well. One problem is that a theory for heat/mass transfer is required, and Danckwert s surface renewal theory may not apply to the low Prandtl numbers of heat transfer (Atmane et al., 2004). The controlled flux technique is close to being viable for short-period field measurements of the liquid film coefficient. 

Typical mass balance methods to measure the air-sea gas transfer have one major drawback the response time is of the order of hours to days, making a parameterisation with parameters such as wind forcing, wave field, or surface chemical enrichments nearly impossible. The controlled flux technique uses heat as a proxy tracer for gases to measure the air-sea gas transfer rate locally and with a temporal resolution of less than a minute. This method offers an entirely new approach to measure the air-sea gas fluxes in conjunction with investigation of the wave field, surface chemical enrichments and the surface micro turbulence at the water surface. The principle of this technique is very simple a heat flux is forced onto the water surface and the skin-bulk temperature difference across the thermal sublayer is measured. 

In Chapter 2, we used the control volume technique represented by equation (2.1) to transport mass into and out of our control volume. Inside of the control volume, there were source and sink rates that acted to increase or reduce the mass of the compound. Anything left after these flux and source/sink terms had to stay in the control volume, and was counted as accumulation of the compound. 

Bagger-Jorgensen et al. [17] found VMD to be a promising technique for retaining the aroma compounds in black currant juice. Their observations on the improved retention of aroma compound c/i-3-hexen-l-ol as a function of increased cross-flow was attributed to reduced temperature and concentration polarization, which in turn increases the aroma concentration at the vapor-liquid interface. The accumulation of aroma compounds at the boundary layer was eventually found to control flux in spite of increase in cross-flow rate. 

In a first realization, Jahne et al. (1989) forced a periodical heat flux density onto the water surface using a chopped heat source above the water surface. The temperature response at the water surface was detected with point measuring radiometer. In a further implementation of this technique, HauBcckcr (1996) developed the so-called passive controlled flux method that estimates the skin-bulk temperature difference under natural heat flux conditions assuming a surface renewal model. The naturally occurring heat fluxes at the ocean surface (latent, sensible and long wave radiative heat flux) cause the surface temperature to decrease or increase depending on the direction of these fluxes. The net heat flux forces a skin-bulk temperature difference AT across the thermal sublayer, commonly referred to as the cool skin of the ocean (compare Fig. 2). 

Like XPS, the application of AES has been very widespread, particularly in the earlier years of its existence more recently, the technique has been applied increasingly to those problem areas that need the high spatial resolution that AES can provide and XPS, currently, cannot. Because data acquisition in AES is faster than in XPS, it is also employed widely in routine quality control by surface analysis of random samples from production lines of for example, integrated circuits. In the semiconductor industry, in particular, SIMS is a competing method. Note that AES and XPS on the one hand and SIMS/SNMS on the other, both in depth-profiling mode, are complementary, the former gaining signal from the sputter-modified surface and the latter from the flux of sputtered particles. 

Metabolic control analysis (MCA) assigns a flux control coefficient (FCC) to each step in the pathway and considers the sum of the coefficients. Competing pathway components may have negative FCCs. To measure FCCs, a variety of experimental techniques including radio isotopomers and pulse chase experiments are necessary in a tissue culture system. Perturbation of the system, for example, with over-expression of various genes can be applied iteratively to understand and optimize product accumulation. 

After measuring the fluxes through the metabolic network, it is necessary to determine the extent to which each pathway or enzyme controls the net fluxes. Metabolic control analysis (MCA) is a technique used to elucidate how flux control is distributed in a metabolic network, thereby providing the information for identification of potential targets for metabolic engineering [8],... 

The primary methodologies for forming thin-film materials with atomic level control are molecular beam epitaxy (MBE) [4-9], vapor phase epitaxy (VPE) [10-12], and a number of derivative vacuum based techniques [13]. These methods depend on controlling the flux of reactants and the temperature of the substrate and reactants. 

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