Electro-osmotic flow through nanochannel with different surface charge configurations: A molecular dynamics simulation study

Abstract

Electro-osmotic flow behavior through rectangular graphene nanochannels with different charge (negative in nature) configurations is discussed in detail using non-equilibrium molecular dynamics (MD) simulations. Alternate patterning of charged and neutral stripes on the surface of the nanochannel lowers the water permeance and electro-osmotic flow velocity through the nanochannel. For all of the charge configurations, water permeance and electro-osmotic velocity through the nanochannel increase as surface charge density (σ) increases from 0.005 to 0.025 C m−2. This can be attributed to the increase in the number of counterions (Na+ ions) near the surface of the nanochannel. However, with further increase in σ, water permeance and electro-osmotic velocity through the nanochannel gradually decrease despite the increase in the number of counterions near the surface of the nanochannel. This is because of the significant increase in electrostatic interaction between the water molecules and the surface of the nanochannel. At a lower value of σ (σ≤0.025 C m−2), the overall interaction between the water molecules and the surface of the nanochannel is significantly dominated by van der Waals (vdW) interactions (electrostatic/vdW ≤0.40). The slip velocity of water molecules in the charged stripe portion of the wall (SlipCharge) is higher as compared to the slip velocity of water molecules in the neutral stripe portion (SlipNeutral) except at σ=0.2 cm−2. This difference between SlipCharge and SlipNeutral is highest at σ=0.025 C m−2 with SlipCharge > SlipNeutral, for all of the charge configurations.