An Investigation On Regulation Of Electroosmotic Flow In Nanochannels Grafted With End-charged Polyelectrolyte Brushes

As a typical electrokinetic transport phenomenon,electroosmotic flows(EOFs)have been widely used in numerous applications ranging from electrokinetic energy conversion(EKEC)systems,sensing and separations of large molecules(DNA,for example)and gating of ions to biomedical care.It is one of the most important questions to regulate and control the EOFs in many applications of EOFs.Studies have proposed that nanochannels grafted with polymer brushes could adjust and control the EOFs.Recently,studies have proposed that nanochannels grafted with end-charged polyelectrolyte(PE)brushes demonstrate a massive augmentation in the strength of the EOFs.An enhanced drag is exerted by the PE brushes on the fluid and the ions are attracted towards the tail of PE brushes.Therefore,the enhancement of EOFs transport was observed in given grafting separation and degree of polymerization.We studied the effects of grafting separation and degree of polymerization on the EOFs velocity by explicitly modeling the EOFs through nanochannels grafted with end-charged PELs using molecular dynamics simulations.The main research contents include as follows:(1)EOFs in a polymer-coated nanoslit was modeled by using molecular dynamics(MD)simulations.An important quantity characterizing the effect of polymers on EOFs is the hydrodynamic radius of the polymer bead.We related the variation of the hydrodynamic radius to the structural properties.The results showed that the hydrodynamic radius of a polymer bead is smaller than its physical size and exhibited a dependence on hydrodynamic shielding by surrounding polymer beads.We confirmed the Navier-Stokes-Brinkman(NSB)model is reasonable for describing the effect of the polymers on EOFs.(2)We examined the variation of the velocity of EOFs with various grafting separation to cover mushroom and brush-like PE brush conformations.Navigated by the continuous NSB model,the variation of the velocity of EOFs with various grafting separations was rationalized.We related the variation of the velocity of EOFs to the structural properties of the PE brushes.In addition,the difference of EOFs velocity between the PE brushes and neutral polymers was studied and the enhancement in EOFs velocity by PE brushes was rationalized.(3)We examined the variation of the velocity of EOFs with various degrees of polymerization.The velocity function was applied to compare the competition between the driving effect from net ions and the resistant effect from PE brushes.The results showed that the velocity of EOFs increases as degree of polymerization decreases for smaller degrees of polymerization.Insights obtained in this work are important to guide the design of devices involving the enhancement of EOFs velocity using end-charged PE brushes.