Flow-induced currents in nanotubes: a Brownian dynam- ics approach

– Motivated by recent experiments [1] reporting that carbon nanotubes immersed in a flowing fluid displayed an electric current and voltage, we numerically study the behaviour of a collection of Brownian particles in a channel, in the presence of a flow field applied on similar but slower particles in a wide chamber in contact with the channel. For a suitable range of shear rates, we find that the flow field induces a unidirectional drift in the confined particles, and is stronger for narrower channels. The average drift velocity initially rises with increasing shear rate, then shows saturation for a while, thereafter starts decreasing, in qualitative agreement with recent theoretical studies [2] based on Brownian drag and “loss of grip”. Interestingly, if the sign of the interspecies interaction is reversed, the direction of the induced drift remains the same, but the flow-rate at which loss of grip occurs is lower, and the level of fluctuations is higher. Recent experiments show that liquid flow past single walled carbon nanotube (SWNT) bundles generates voltage [1] and electric current [2] in the nanotube, along the direction of flow. The current and voltage are found [1,2] to be highly sublinear functions of the fluid flow rate. The direction of flow-induced current relative to fluid flow is determined [1, 2] by the nature of the ions in the vicinity of the nanotubes. The experiments in [1,2] also showed that the one-dimensional nature of the SWNTs was essential for the generation of a net electrical signal in the sample: experiments on graphite did not generate a measurable signal and those on multiwalled carbon nanotubes produced a signal approximately 10 times weaker than that for the single walled nanotubes, the dimensions of the sensing element and flow speeds of the ionic liquid remaining same. Electrokinetic [3] and phonon-wind [4] based explanations give a linear current versus flow rate dependence. There is also a recent proposal [5] involving stick-slip and barrier hopping of ions giving a sublinear dependence. Ghosh et al. [2] explain the phenomenon as follows: Thermal fluctuations in the ionic charge density in the fluid near the nanotube produce a stochastic Coulomb field on the carriers in the nanotube. At thermal equilibrium, i.e., if there is no mean fluid flow, the fluctuation-dissipation theorem tells us