Water droplet transport on fibers is of great importance for achieving high water collection efficiency from fog. Here, we exploit a new droplet sliding mechanism to accelerate the droplet coalescence and collection for highly efficient fog harvesting by coating hydrophilic microfibers with superhydrophobic layers of assembled carbon nanoparticles. We find that during the initial water collection, unlike the pinned droplets having axisymmetric barrel shapes wrapped around uncoated microfibers, the hanging droplets on coated microfibers with non-wrapping clamshell shapes are highly mobile due to their lower contact hysteresis adhesion; these are observed to oscillate, coalesce, and sweep the growing droplets along the horizontally placed microfibers. The driving force for droplet transport is mainly ascribed to the coalescence energy release and fog flow. After introducing small gravity force by tilting coated microfibers with a small angle of 5°, we find that it can effectively drive the oscillating mobile droplets for directional transport by rapidly sweeping the droplets with a much higher frequency. Finally, the water collection rate from fog on uncoated microfibers over a prolonged duration is found to be improved over 2 times after superhydrophobic coating, and it is further enhanced over 5 times after a small tilting angle of 5°.