Effect of grafting on microstructure, composition and surface and transport properties of ceramic membranes for osmotic evaporation

Concurrent coupling of atomistic simulation and mesoscopic hydrodynamics for flows over soft multi-functional surfaces

We develop an efficient parallel multiscale method that bridges the atomistic and mesoscale regimes, from nanometers to microns and beyond, via concurrent coupling of atomistic simulation and mesoscopic dynamics. In particular, we combine an all-atom molecular dynamics (MD) description for specific atomistic details in the vicinity of the functional surface with a dissipative particle dynamics (DPD) approach that captures mesoscopic hydrodynamics in the domain away from the functional surface. In order to achieve a seamless transition in dynamic properties we endow the MD simulation with a DPD thermostat, which is validated against experimental results by modeling water at different temperatures. We then validate the MD-DPD coupling method for transient Couette and Poiseuille flows, demonstrating that the concurrent MD-DPD coupling can resolve accurately the continuum-based analytical solutions. Subsequently, we simulate shear flows over grafted polydimethylsiloxane (PDMS) surfaces (polymer brushes) for various grafting densities, and investigate the slip flow as a function of the shear stress. We verify that a "universal" power law exists for the slip length, in agreement with published results. Having validated the MD-DPD coupling method, we simulate time-dependent flows past an endothelial glycocalyx layer (EGL) in a microchannel. Coupled simulation results elucidate the dynamics of the EGL changing from an equilibrium state to a compressed state under shear by aligning the molecular structures along the shear direction. MD-DPD simulation results agree well with results of a single MD simulation, but with the former more than two orders of magnitude faster than the latter for system sizes above one micron.

The long-term studies of osmotic membrane distillation

The results of osmotic membrane distillation carried out for 2.5 years were presented in this work. The influence of the process conditions, such as temperature and brine concentration on the permeate flux, was investigated. The saturated NaCl solutions and distilled water were used as a stripping solution and feed, respectively. A continuous regeneration of stripping solution was conducted using a method of natural evaporation from the surface of Białecki rings to the air surrounding the installation. The possibilities of application of Accurel PP S6/2 hydrophobic polypropylene membranes were tested. It was studied whether a saturation stripping solution does not cause scaling and wettability of membranes. It was found that most of the pores in the used membranes were non-wetted, and the salt retention over 99% was maintained during a study period. However, the obtained permeate flux was decreased by 10–20%. The SEM examinations revealed that it was caused by amorphous deposit, which was formed on the membrane surface on the brine side. The SEM–EDS analysis demonstrated that the deposit composition mainly included Si and O.

Surface modification of polyvinylidene fluoride (PVDF) membrane via radiation grafting: novel mechanisms underlying the interesting enhanced membrane performance

This study provided the first attempt of grafting hydrophobic polyvinylidene fluoride (PVDF) membrane with hydrophilic hydroxyethyl acrylate (HEA) monomer via a radiation grafting method. This grafted membrane showed an enhanced hydrophilicity (10° decrease of water contact angle), water content ratio, settling ability and wettability compared to the control membrane. Interestingly, filtration tests showed an improved dependence of water flux of the grafted membrane on the solution pH in the acidic stage. Atomic force microscopy (AFM) analysis provided in-situ evidence that the reduced surface pore size of the grafted membrane with the solution pH governed such a dependence. It was proposed that, the reduced surface pore size was caused by the swelling of the grafted chain matrix, with the pH increase due to the chemical potential change. It was found that the grafted membrane showed a lower relative flux decreasing rate than the control membrane. Moreover, flux of the bovine serum albumin (BSA) solution was noticeably larger than that of pure water for the grafted membrane. Higher BSA flux than water flux can be explained by the effects of electric double layer compression on the polymeric swelling. This study not only provided a pH-sensitive PVDF membrane potentially useful for various applications, but also proposed novel mechanisms underlying the enhanced performance of the grafted membrane.