We present a unique method for size-triggered microdroplet target transport, achieved on gradient-step spindle-knot fibers (GSFs). GSFs are controllably fabricated by developing a velocity-change coating method, the gradient features of which can have uni-directional,
middle or two-side symmetric spindle-knot modes to modulate directional droplet target transport. This finding offers an insight into how to effectively control the direction of liquid self-transport for water collection, and may also be extended to smart materials which
contribute to fluid control
