Coulombically Driven Rolling of Nanorods on Water

We use molecular dynamics simulations to examine the possibility of rolling nanorods on the surfaces of polar liquids. Asymmetric charging of nanorod surfaces, generated by light excitation of its photoactive hydrophobic surfactants, can induce asymmetric Coulombic coupling to the polar liquid surfaces. We demonstrate that under this driving nanorods with diameters of 3–10 nm can roll on water with translational velocities of $1–5\mathrm{nm}/\mathrm{ns}$. The efficiency of this motion is controlled by the chemistry and dynamical phenomena at the nanorod-water interface.

Figure 1

Model nanorod rolling on the water surface by Coulombically induced torque (see movie in Ref. 31).

Figure 2

(top) The translational $v_{\mathrm{tran}}$ and (bottom) rotational $v_{\mathrm{rot}}$ velocities of the driven CG roller as a function of the vdW parameter of the surfactant beads, ${\epsilon }_s$. The simulation results obtained in the STO and PER recharging regimes are presented at $T_1=300\mathrm{K}$ and $T_2=340\mathrm{K}$. Two points also show the velocities obtained at $T_1$ for a STO-driven AT roller with alkane surfactants (empty circle) and a CG roller with ${\epsilon }_s=0.57\mathrm{kcal}/\mathrm{mol}$ (empty diamond), both approximately submerged in the same amount. In the inset (top), the rolling efficiency $S=v_{\mathrm{tran}}/R{v}_{\mathrm{rot}}$ is also shown.

Figure 3

The angular velocity $v_{\mathrm{rot}}$ of the CG roller as a function of the recharging time $t_C$.

Figure 4

The translational velocity $v_{\mathrm{tran}}$ of the CG roller with ${\epsilon }_s=0.12$ as a function of the force density $f$ applied to unit length of the roller.