Patterning nanoscale flow vortices in nanochannels with patterned substrates

We report the generation of complex toroidal nanoscale vortices in nanochannel flows, through the employment of patterned surface patches with contrasting wettability characteristics. Through extensive molecular-dynamics simulations, we further delineate that complicated interactions between the wettability gradient-driven vortex structures and surface roughness-influenced flow features may give rise to interesting patterns in the flow field. We also establish the orientation specificities of these nanoscale vortices, and demonstrate that the surface roughness effects may potentially act to alter the flow rotationalities realized out of modulated surface patterning effects to a considerable extent, which may have interesting consequences toward designing nanoscale mixing elements.

Figure 1

(Color online) (a) Three-dimensional view of the system simulated. Triangles and asterisks indicate wall atoms, whereas circles represent fluid atoms. (b) Flow structures over a patterned arrangement of wall atoms. The “plus $(+)$” indicates hydrophilic wall atoms whereas the “asterisk $\left(\ast \right)$” indicates hydrophobic wall atoms. The continuous lines indicate the streamlines of the flow.

Figure 2

(Color online) Flow pattern over the channel wall in “horizontally patched” and “diagonally patched" topologies. The “plus $(+)$” indicates hydrophilic wall atoms whereas the “asterisk $\left(\ast \right)$” indicates hydrophobic wall atoms. The continuous lines indicate the streamlines of the flow.

Figure 3

Density field corresponding to the cases presented in Fig. 2

Figure 4

(Color online) Flow patterns over rough channel surfaces with (a) and (c) “horizontally patched” and (b) and (d) “diagonally patched” configurations. Cases (a) and (b) correspond to $N=2$, whereas cases (c) and (d) correspond to $N=1$. The “plus $(+)$” indicates hydrophilic wall atoms whereas the “asterisk $\left(\ast \right)$” indicates hydrophobic wall atoms. The continuous lines indicate the streamlines of the flow.

Figure 5

Density field corresponding to the cases presented in Fig. 4

Figure 6

Dimensionless vorticity $\left(C_z\right)$ fields corresponding to the cases presented in Fig. 2

Figure 7

Dimensionless vorticity $\left(C_z\right)$ fields corresponding to the cases presented in Fig. 3

Figure 8

Slip length map corresponding to the cases presented in Fig. 2

Figure 9

Slip length map corresponding to the cases presented in Fig. 4