  Single-crystal diamond nanomechanical resonators are being developed for
countless applications. A number of these applications require that the
resonator be operated in a fluid, i.e., a gas or a liquid. Here, we investigate
the fluid dynamics of single-crystal diamond nanomechanical resonators in the
form of nanocantilevers. First, we measure the pressure-dependent dissipation
of diamond nanocantilevers with different linear dimensions and frequencies in
three gases, He, N$_2$, and Ar. We observe that a subtle interplay between the
length scale and the frequency governs the scaling of the fluidic dissipation.
Second, we obtain a comparison of the surface accommodation of different gases
on the diamond surface by analyzing the dissipation in the molecular flow
regime. Finally, we measure the thermal fluctuations of the nanocantilevers in
water, and compare the observed dissipation and frequency shifts with
theoretical predictions. These findings set the stage for developing diamond
nanomechanical resonators operable in fluids.
