One of the most questionable issues in wetting is the force balance that includes the vertical component of liquid surface tension. On soft solids, the vertical component leads to a microscopic protrusion of the contact line, that is, a ‘wetting ridge’. The wetting principle determining the tip geometry of the ridge is at the heart of the issues over the past half century. Here we reveal a universal wetting principle from the ridge tips directly visualized with high spatio-temporal resolution of X-ray microscopy. We find that the cusp of the ridge is bent with an asymmetric tip, whose geometry is invariant during ridge growth or by surface softness. This singular asymmetry is deduced by linking the macroscopic and microscopic contact angles to Young and Neuman laws, respectively. Our finding shows that this dual-scale approach would be contributable to a general framework in elastowetting, and give hints to issues in cell-substrate interaction and elasto-capillary problems.

Organic electronics increasingly impacts our everyday life with a variety of devices such as displays for TV or mobile appliances, smart cards and radio-frequency identifi cation (RFID) tags. [ 1 , 2 ] This blossoming domain could greatly profi t from effective ways to fabricate conducting or semiconducting organic nanowires. [ 3 ] Specifi cally, the three-dimensional (3D) and individual integration of each nanowire is essential [ 4 ] for many new device concepts, but so far this was not possible. Here we show the demonstration of accurate and versatile 3D direct writing of conducting polymer nanowires based on guiding a monomer meniscus by pulling a micropipette during oxidative polymerization. This is an important step for organic electronic integration with high density and enhanced freedom in circuit design.Conducting polymers such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) are very interesting materials because they combine tunable electrical transport characteristics and excellent mechanical properties. [ 5 ] In particular, conducting polymer nanowires are quite important for a broad range of nanodevices such as fi eld effect transistors, [ 3 ] bio-and chemical sensors, [ 6 , 7 ] and non-volatile memories. [ 8 ] Such nanowires are fabricated by soft lithography, [ 9 , 10 ] dip-pen lithography [ 11 ] and electrospinning. [ 12 ] However, these methods are still limited to in-plane patterning of low-aspect-ratio nanowires, whereas for advanced applications 3D patterning is essential.Direct ink writing and probe-based drawing are used for 3D wire patterning. The fi rst method, based on the extrusion of concentrated ink through a nozzle, was applied for 3D microfabrication with metals, oxides, and polymers. [13][14][15][16] However, bringing the wire diameter below micrometer-level is not easy due to the size and concentration of the ink particles. Probe-based drawing can fabricate polymer nanowires. [ 17 ] However, high-density integration is limited by the large pre-deposited polymer droplet (a few tens to hundreds of micrometers).An alternate technique for 3D electrodeposition was recently developed: writing nanowires with a nanoscale electrolyte meniscus. [ 18 , 19 ] This method was so far demonstrated for 3D metallic nanowires but not for conducting polymers.Here we show that this type of technique can in fact be used for conducting polymers offering high accuracy, excellent versatility and marked advantages with respect to alternate solutions. In essence, we obtained a stretched monomer meniscus by pulling a micropipette fi lled with a Py solution, exploiting oxidative polymerization in air. The wire radius so produced was accurately controlled down to ∼ 50 nm by tuning the pulling speed.The technique was successfully tested with specifi c focus on essential features for advanced organic nanodevice integration. Specifi cally, we produced dense arrays of different types of freestanding nanocomponents: straight wires, complex-shape wires, branche...

When a liquid drop impacts a solid surface, air is generally entrapped underneath. Using ultrafast x-ray phase-contrast imaging, we directly visualized the profile of an entrapped air film and its evolution into a bubble during drop impact. We identified a complicated evolution process that consists of three stages: inertial retraction of the air film, contraction of the top air surface into a bubble, and pinch-off of a daughter droplet inside the bubble. Energy transfer during retraction drives the contraction and pinch-off of a daughter droplet. The wettability of the solid surface affects the detachment of the bubble, suggesting a method for bubble elimination in many drop-impact applications.

A bubble reaching an air–liquid interface usually bursts and forms a liquid jet. Jetting is relevant to climate and health as it is a source of aerosol droplets from breaking waves. Jetting has been observed for large bubbles with radii of R≫100 μm. However, few studies have been devoted to small bubbles (R<100 μm) despite the entrainment of a large number of such bubbles in sea water. Here we show that jet formation is inhibited by bubble size; a jet is not formed during bursting for bubbles smaller than a critical size. Using ultrafast X-ray and optical imaging methods, we build a phase diagram for jetting and the absence of jetting. Our results demonstrate that jetting in bubble bursting is analogous to pinching-off in liquid coalescence. The coalescence mechanism for bubble bursting may be useful in preventing jet formation in industry and improving climate models concerning aerosol production.

A vortex is a flow phenomenon that is very commonly observed in nature. More than a century, a vortex ring that forms during drop splashing has caught the attention of many scientists due to its importance in understanding fluid mixing and mass transport processes. However, the origin of the vortices and their dynamics remain unclear, mostly due to the lack of appropriate visualization methods. Here, with ultrafast X-ray phase-contrast imaging, we show that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact. Interestingly, we find a row of vortex rings along the drop wall, as demonstrated by a phase diagram established here, with different power-law dependencies of the angular velocities on the Reynolds number. These results provide important insight that allows understanding and modelling any type of vortex rings in nature, beyond just vortex rings during drop splashing.