RESUMO
In electrodialysis desalination processes, the operating current density is limited by concentration polarization. In contrast to other membrane processes such as ultrafiltration, in electrodialysis, current transport above the limiting current is possible. In this work, the origin of the overlimiting current at cation-exchange polymers is investigated. We show that, under certain experimental conditions, electroconvection is the origin of the overlimiting conductance. The theory concerning electroconvection predicts a shortening of the plateau length of membranes with increased conductive or geometrical heterogeneity. We investigate the influence of these two parameters and show that the creation of line undulations on the membrane surface normal to the flow direction, having distances in the range of approximately 50-200% of the boundary-layer thickness, lead to an earlier onset of the overlimiting current. The plateau length of the undulated membranes is reduced by up to 60% compared to that of a flat membrane. These results verify the existence of electroconvection as a mechanism destabilizing the laminar boundary layer at the liquid-membrane interface and causing ionic transport above the limiting current density.
RESUMO
BACKGROUND & AIMS: Liver biopsy is the gold standard for the diagnosis of non-alcoholic steatohepatitis (NASH), but is an invasive method. There is a need for non-invasive methods that can reflect the histopathological severity of NASH. The aim of this study was to compare the ultrasonography and computerized tomography findings with the histopathological severity in patients with NASH. MATERIAL AND METHODS: Twenty-two consecutive patients with biopsy proven NASH and 20 age- and sex-matched healthy individuals were enrolled. Clinical and demographic data were collected at the time of liver biopsy. Histopathological grading and staging were made by an expert pathologist. Each patient underwent ultrasonography and computerized tomography. RESULTS: Liver ultrasonographic findings were not correlated with histopathological grade and stage (r: 0.134, P > 0.05; r: 0.130, P > 0.05). Mean liver densities obtained by computed tomography of NASH patients were lower than that of controls (P < 0.05) and liver/spleen density ratios were lower than that of controls (P < 0.05). These results were significantly correlated with histopathological grade (r: -0.716, P < 0.001; r: -0.663, P: 0.001), but not with the histopathologic stage (r: -0.416, P: 0.05; r: -0.356, P: 0.1). CONCLUSIONS: Ultrasonography findings do not reflect histopathological severity in patients with NASH. Computed tomography attenuation of the liver is significantly correlated with histopathologic grade but not with histopathological stage.
