The evaporation of water from open u-shaped microchannel grooves was investigated with particular emphasis on the roles of channel width and air flow conditions. Given the small dimensions of the microchannels, all measurements were conducted in a range where convection and diffusion are of equal importance and known correlations for the calculation of mass transfer coefficients cannot be applied. The evaporation rates were measured using a new optical method and a gravimetric method. Both measurement methods yielded mass transfer coefficients that are in agreement with each other. The observed relation between mass transfer coefficient, air velocity and channel width vastly differs from the predictions obtained from macroscopic structures. With respect to diagnostic devices we conclude that analyte concentration in an open microchannel groove strongly increases even within short times due to the evaporation process and we show that wider channels are more favourable in terms of minimizing the relative evaporation rate.
Water content profiles within thin aqueous films of lactose and bovine serum albumin (BSA) were tracked during drying using inverse microscope Raman spectroscopy (IMRS). These film drying experiments provide useful insights into component diffusion within droplets during spray drying in the food industry. Virtually no segregation of lactose and BSA occurred in the films during drying at 30 C. Thus, in film drying simulations at this temperature, lactose and BSA can be assumed to be a single homogenous solute, and binary diffusion of water and this single solute can be modeled. A solute-fixed coordinate system drying model properly predicted the water content profiles and shrinkage of the films during the constant rate period. This represents a partial validation of the model. The drying model predicted significant gradients in water content in films dried at a higher temperature of 80 C, which have the potential to drive segregation of lactose and BSA. However, IMRS analysis of films dried at 80 C revealed no segregation of lactose and BSA. We propose that lactose and BSA are relatively immobile in the bulk of the film during drying, forming a homogenous gel that allows the significantly smaller water molecules to diffuse through. The observed accumulation of BSA as a monolayer at the surface of the dried films appears to be a local phenomenon most likely driven by the surface activity of BSA.
The knowledge of mass transport parameters in polymer/protein blend films is of substantial interest for the preparation of glucose biosensor test strips where drying and rehydration need to be controlled. In this study, the diffusion of water in bovine serum albumin (BSA) and poly(vinylpyrrolidone) (PVP) films and blends of BSA with PVP and polyvinylalcohol is investigated and the applicability of predictive models for the diffusion coefficient in mixtures is explored. Water concentration profiles are measured with Raman spectroscopy during drying experiments under controlled conditions. Concentration-dependent diffusion coefficients of water in the films are then determined by fitting simulated drying curves to measured data. Comparison of the resulting diffusion coefficients in the blends with a predictive logarithmic model shows good agreement. The application of such predictive models could greatly facilitate the development of polymer/protein blends in the future.
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