Back feed behavior

Frisch B, Hardin PE, Hamblen-Coyle MJ, Rosbash M, Hall JC 1994 A promoterless DNA fragment from the period locus rescues behavioral rhythmicity and mediates cyclical gene expression in a restricted subset of the Drosophila nervous system. Neuron 12 555—570 Glossop NR, Houl JH, Zheng H, Ng FS, Dudek SM, Hardin PE 2003 VRILLE feeds back to control circadian transcription of clock in the Drosophila circadian oscillator. Neuron 37 249-261... 

In membrane filtration, some components (dissolved or particulate) of the feed solution are rejected by the membrane and these components are transported back into the bulk by means of diffusion. The rate of diffusion will depend on the hydrodynamics (laminar or turbulent) and on the concentration of solutes. If the concentration of solute at the surface is above saturation (i.e., the solubility limit) a gel is formed. This increases the flow resistance with consequential flux decrease. This type of behavior, for example, is typical of UF with protein solutions. 

Nevertheless, machines with concave die rings and internal press rollers do have advantages. For example, if the feed material exhibits a certain elastic behavior, because the forces in the relatively long and slender nip increase slowly, a more complete conversion of temporary elastic into permanent plastic deformation takes place. Fig. 8.41b is another presentation of the forces at work. Feed, ideally deposited in a uniform layer on the die, is pulled into the space (nip) between roller and die and compressed. Friction between roller, die, and material as well as interparticle friction in the mass are responsible for the pull of the feed into the nip and for densification. Smooth surfaces of roller and/or die may result in slip. Axial grooves in the roller, which may also favor build-up of a thin layer of material, and the above mentioned residual layer of densified feed on the die effectively reduce slip. Low interparticle resistance to flow or a distinct plasticity result in a more or less pronounced tendency of the mass to avoid the squeeze" (back-flow), thus reducing densification and potentially choking the machine (see above). 

Dissolved oxygen (DO) concentration in steady state was observed to be dependent on the experimental conditions (Lopez et al. 2(X)4). DO decreases when there is an unbalance between H2O2 addition rate and organic loading rate (OLR) (Fig. 6.6.9). This suggests DO as a suitable parameter for monitoring the reaction behavior and a control variable to apply a feed-back control system (L6pez et al. 2007). 

This behavior has also been discussed in terms of alkaline state (to the left of the critical point of MR) and acid state (to the right of the critical point) of the cation-exchange membrane [232], [239]. This difference in internal states of the membrane is connected to the presence in the conductive channels of the membrane of a population of either hydroxyl ions (back migration from the catholyte when the electric current is substantially transported by sodium ions low MR values) or protons (significant portion of the current transported by protons high MR values). The model has remarkable practical consequences, particularly in terms of the sensitivity of the membrane with respect to impurities contained in the feed solutions. 

Peterson, L.H. (1963) Systems behavior, feed-back loops and high blood pressure research. Circ. Res., 12, 585-596. 

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