Electroosmotic Flow in Mixed Polymer Brush-Grafted Nanochannels

Abstract

Mixed polymer brush-grafted nanochannels-where two distinct species of polymers are alternately grafted on the inner surface of nanochannels-are an interesting class of nanostructured hybrid materials. By using a coarse-grained molecular dynamics simulation method, we are able to simulate the electrokinetic transport dynamics of the fluid in such nanochannels as well as the conformational behaviors of the mixed polymer brush. We find that (1) the brush adopts vertically-layered and longitudinally-separated structures due to the coupling of electroosmotic flow (EOF) and applied electric field; (2) the solvent quality affects the brush conformations and the transport properties of the EOF; (3) the EOF flux non-monotonically depends on the grafting density, although the EOF velocity in the central region of the channel monotonically depends on the grafting density.

Keywords: dissipative particle dynamics; electroosmotic flow; molecular dynamics; polymer brush.

Figure 1

(a,b) Typical simulation snapshots, and (c, d) density and shear rate profiles at ${\rho }_{\mathrm{g}}=0.1{\mathsf{\sigma }}^{-2}$. (a,c) correspond to the B–solvent attraction case; (b,d) correspond to the A–B attraction case. Cations, anions, and solvent molecules are not shown. Color scheme: negatively-charged A chains (green), neutral B chains (mauve), grafted beads (blue), charged surface particles (cyan), and neutral surface particles (orange). EOF: electroosmotic flow.

Figure 2

Fluid velocity profiles at grafting densities (a) ${\rho }_{\mathrm{g}}=0.02{\mathsf{\sigma }}^{-2}$; (b) $0.1{\mathsf{\sigma }}^{-2}$; and (c) $0.3{\mathsf{\sigma }}^{-2}$ for different interaction cases. Only the velocity profile to the centerline along the z axis is shown. The abbreviations are defined as follows: (1) P corresponds to the case of pure repulsion between any two components; (2) AB represents A–B attraction; (3) AS and BS represent A–solvent and B–solvent attraction, respectively; (4) BAS corresponds the case of A–B and A–solvent attraction; (5) likewise, ABS indicates B–A and B–solvent attraction.

Figure 3

(a) Electroosmotic flow velocity $u_{\mathrm{e}}$; (b) flow flux Q; (c) inclination angle $\mathsf{\theta }$; and (d) shape factor $R_{\mathrm{ee}}^2/R_{\mathrm{g}}^2$ as a function of the grafting density. In (c,d), the solid and dashed lines represent the A and B layers, respectively. The abbreviations (P, AB, AS, BS, BAS, and ABS) have the same meanings as in Figure 2.