Materials with superhydrophobic properties are usually generated by covering the surfaces with hydrophobic nanoscale rough features. A major problem, however, for any practical application of such strongly water-repellent surfaces is the mechanical fragility of the nanostructures. Even moderate forces caused by touching or rubbing the surfaces are frequently strong enough to destroy the nanostructures and lead to the loss of the superhydrophobic properties. In this article, we study the mechanical stability of superhydrophobic surfaces with three different topographies: nano- and microscale features and surfaces carrying a combination of both. The surfaces are generated by silicon etching and subsequent coating with a monolayer of a fluoropolymer (PFA). We perform controlled wear tests on the different surfaces and discuss the impact of wear on the wetting properties of the different surfaces.
We report on a method to generate surfaces whose wettability can be reversibly switched between a superhydrophobic and Wenzel state or a Wenzel and superwetting state just by a short UV or VIS irradiation. To achieve this, we generate a silicon surface with a nanoscale roughness ("black silicon") and attach a polymer monolayer to it. The polymer contains a fluorinated azobenzene moiety which can be switched between the cis and trans state depending on the wavelength of the light used during illumination. The surface energy of the polymer coating is carefully adjusted to the energy value which separates distinct wetting regimes of the nanorough surface. This coupling of light induced switching to a transition of the wetting regimes can cause changes in the water contact angle as high as Δθ = 140° in the advancing CA or more than 175° in the receding CA even when the surface energy is changed only in a rather small range. Short irradiation times with UV or VIS light are enough to change the roll-off angle from <5° to no roll off at all and back. We discuss the requirements necessary so that large changes in the contact angle occur during photoswitching processes on rough surfaces.
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Recently developed flexible active matrix sensors employ organic and large area electronics to enable artificial sensing skin applications. In the proof‐of‐concept presented here, an active matrix proximity sensor‐array is formed by monolithic integration of screen‐printed sensors with organic transistors on a single sheet of a polymer film, using only seven industrially scalable patterning steps. A ferroelectric co‐polymer of polyvinylidendifluoride (PVDF) serves as the pyro‐sensor material, and bottom‐gate top‐contact organic thin film transistors switch the sensors during operation. An on/off ratio well above 103 and a low off‐current below 8 × 10−11 A of the organic transistors result in a dynamic range of 44 dB and allow to detect a human hand approaching to a distance of 20 cm with a signal‐to‐noise ratio of 28 dB. A basic matrix operation is demonstrated by the correct tracking of a moving human hand.
Stretchable
conductive films were obtained by screen printing and
thermal treatment of a homogenous ink comprising a thermally reducible
silver formate complex, an acrylate monomer, and a radical initiator.
In the curing process, both the filler nanoparticles and the polymer
matrix are generated in situ, at temperatures as low as 100 °C.
The obtained conductors, consisting of percolated silver nanoparticles
embedded in a polymeric matrix, typically show a resistivity of (2–4)
× 10–5 Ω·m. When applied on an elastomeric
substrate, the composite is stretchable up to 200% with very low R/R
0 values, which is unprecedented
for stretchable silver composite inks. Quasi-in situ confocal laser
scanning microscopy of the strained samples revealed an initial fracture
strain above 40%, which is unusually high for metal–nanoparticle
films. The described system was compared to some commercial stretchable
screen-printing inks and proved superior with regard to both R/R
0 and resistance to cyclic
tensile loading.
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