www.MaterialsViews.com C O M M Direction Controlled Driving of Tiny Water Drops on Bioinspired Artifi cial Spider Silks U N IC A By Hao Bai , Xuelin Tian , Yongmei Zheng ,* Jie Ju , Yong Zhao , and Lei Jiang * IO N Directional driving of liquid drops is of signifi cant interest in many applications, such as microfl uidic devices, [ 1–6 ] fog harvesting, [ 7 ] fi ltration, [ 8 ] and condensers. [ 9 ] For this purpose, great progress has been made in driving drops larger than hundreds of micrometers [ 9–23 ] by introducing chemical, [ 1 , 10 , 12–15 , 23 ] thermal, [ 16–19 ] or shape [ 20–22 ] gradients on surfaces. However, driving micrometer-sized drops is much more diffi cult because they encounter a larger contact angle (CA) hysteresis effect. [ 7 , 24 ] In nature, the wetted silk of cribellate spider offers new insights into solving this problem by combining different gradients together. [ 7 ] Here, inspired by the spider silk, we fabricated a series of artifi cial spider silks with spindle-knots in which the chemical compositions and surface nanostructures were subtly designed. Our investigations demonstrated that tiny water drops (tens of picoliters) could be driven with controllable direction (“toward” or “away from” the knot) by optimizing the cooperation of curvature, chemical, and roughness gradients on the fi ber surfaces. The study will pave the way for designing smart materials and devices to drive tiny water drops in a controllable manner. When a nylon fi ber was immersed into polymer solution and drawn out horizontally, a string of polymer drops, which became spindle-knots after being dried, formed on the fi ber due to the Rayleigh instability [ 25 ] of the polymer solution. The surface energy of the spindle-knots was tailored by choosing different polymers including poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(vinylidene fl uoride) (PVDF), which have intrinsic water contact angles of 56.7 ° , 68.4 ° , 93.3 ° , and 92.7 ° , respectively (see Supporting Information, Figure S1). On the other hand, the surface roughness (porous nanostructures) of the spindle-knots was also designed through phase separation