  We propose a water pump that actively transports water molecules through
nanochannels. Spatially asymmetric thermal fluctuations imposed on the channel
radius cause unidirectional water flow without osmotic pressure, which can be
attributed to hysteresis in the cyclic transition between the wetting/drying
states. We show that the water transport depends on fluctuations, such as
white, Brownian, and pink noises. Because of the high-frequency components in
white noise, fast switching of open and close states inhibits channel wetting.
Conversely, pink and Brownian noises generate high-pass filtered net flow.
Brownian fluctuation leads to a faster water transport rate, whereas pink noise
has a higher capability to overcome osmotic pressure in the opposite direction.
A trade-off relationship exists between the resonant frequency of the
fluctuation and the flow amplification. The proposed pump can be considered as
an analogy for the reversed Carnot cycle, which is the upper limit on the
energy conversion efficiency.
