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Non-expert summaryPhotoresists are an example of long chain molecules which dissolve in the presence of alkali. Dissolution involves the transport of hydoxyl ions (OH-) into the layer: they react with the photoresist and, depending on the temperature (above or below the upper critical solution temperature, UCST), it will either be transported by diffusion through the fluid boundary layer (above UCST) or form a gel phase (below the UCST), introducing an additional diffusion step in the reaction and mass transfer processes. A physico-chemical model of the process is presented here, applied to novolak resins, which can be applied to other polymeric systems including printing inks and dairy protein foulants. Experiments are performed with a spinning disk apparatus, which gives well defined mass transfer conditions.
Three-dimensional advective–diffusive boundary layers in open channels with parallel and inclined walls
Non-expert summaryThe authors consider the steady laminar advective transport of a diffusive component released at the base of a narrow three-dimensional longitudinal open channel with non-absorbing side walls and rectangular or truncated-wedge-shaped cross-sections: the findings are relevant to heat and mass transfer applications in confined U-shaped or V-shaped channels (or trenches) such as might arise in the decontamination and cleaning of narrow gaps, crevices and boundary features on walls or other surfaces, and well as transport processes in chemical or biological microfluidic devices. The fluid flows along the channel in the laminar regime and there is no flux or slip on the side walls. Numerical simulations are conducted for various duct shapes and the rate of mass transfer from the base is calculated: this is used to evaluate the dimensionless mass transfer coefficient, the Sherwood number. The results for 3-D (constant cross section, long in the direction of flow) are compared favourably with results for a simplified, 2-D, calculation.
Non-expert summaryThe authors conduct a comprehensive numerical study of convective mass transport from 2-D rectangular cavities in low‐Reynolds‐number flows, i.e. the flow set up in a long rectangular trench by the motion of a steady shear flow across the top. They calculate the velocity field in the trench and the associated mass transport (enhancement of diffusion). The flow field is calculated by a high‐order implementation of the boundary‐integral method, while the convective diffusion equation is solved using the spectral‐element method. Results are presented in the form of concentraton contours and local mass fluxes, for cavity aspect ratios from 1:1 to 4:1 and for Péclet numbers from 0 to 100,000. They investigate the effects of inlet flow profile and system boundaries on the system.
Non-expert summaryWhen a steady shear flow passes over a long (2-D) cavity it sets up a circulation cell in the cavity, where the flow in the cavity is contained within the cavity (with a boundary called the separatrix) and mass transport into and out of the cavity is predominantly by diffusion. This is particularly true at small scales. The authors investigate numerically and experimentally two methods for enhancing mass transport from these 'cells' - by geometrically modifying the boundary driving the flow, and making the driving flow time-dependent. Both modifications destabilize one of the wall attachment points of the separatrix, allowing fluid exchange between the cavity and channel. The range of Reynolds and Reynolds-Strouhal numbers studied is 7.7 <= Re <= 46.5 and 0.52 <= ReSr <= 12,55 in the spatially dependent mode and 12 <= Re <= 93 and 0.26 <= ReSr <= 5.02 in the time-dependent mode. The transport is described theoretically via lobe dynamics, which characterizes the instability of the separatrix. They find that the resulting mass transfer between the cavity and the outer shear flow, through the distabilized separatrix is enhanced by several orders of magnitude compared to a diffusive mass transfer.
Non-expert summaryThis is a relatively early experimental study of mass transfer of a sparingly soluble material from a flat, solid surface when it is exposed to a normally impinging turbulent jet of water. The nozzle Reynolds numbers ranged from 25,000 to 125,000. The surface was coated with trans-cinnamic acid, and thickness profiles were measure over time to determine the local rate of mass transfer. The mass transfer flux is used to calculate the local Sherwood number (dimensionless mass transfer coefficient). In the wall-jet region these were found to be independent of the nozzle to plate distance, and were correlated as Sh = 1.3*Re^0.84*(x/d)^- 1.27. The authors found reasonable agreement with published heat transfer data . The average Sherwood numbers in the impingement region were found to decrease rapidly beyond a transition zone of 6.5 diameters from the nozzle: mass transfer rates are thus weak beyond this zone.
Non-expert summaryThis is an experimental and modelling study of the removal of a passive tracer contained in small, thin, viscous drops attached to a flat inclined substrate using the flow of a thin gravity-driven film. The drop cannot be detached either partially or completely from the surface by the mechanical forces exerted by the cleaning fluid on the drop. Convective mass transfer is established across the interface between the drop and the flowing liquid film and the (dilute) tracer diffuses into the film flow, which takes it away. The Peclet number, comparing the rate of mass transfer in the drop to the rate in the liquid film, is small (< 1) . Two models are presented: a simple empirical model based on film mass transfer coefficients; and a fuller theoretical model solving the quasi-steady two-dimensional advection–diffusion equation in the film, coupled with a time-dependent one-dimensional diffusion equation in the drop. A range of values of the Peclet number (0.01 to 1) is considered in the fuller model. Good agreement is observed between the experimental data and the models.