Hans-Jürgen Butt

The precise control of liquid motion with light is fundamental to advancing next‐generation microfluidic devices and microreactors. Fluoroalkylated photoswitches that combine low surface tension with photoinduced reversible solid‑to‑liquid transitions would be ideal for this purpose, yet they have remained unrealized because fluoroalkyl chains typically suppress solid‑state photoisomerization. Here, we overcome this limitation by designing fluoroalkylated azobenzenes that exhibit low surface tension and photoinduced reversible solid‑to‑liquid transitions. This dual functionality stems from tailored molecular and crystalline structures that enable efficient trans ‐ cis photoisomerization in the solid state. The fluoroalkylated azobenzenes function as non‑lubricating solids in their trans form and become liquid lubricants upon photoconversion to their cis form. By infusing them into microstructured substrates, we create surfaces that can be optically switched between a liquid‐pinning state and a slippery state. This photoswitching capability enables the on‑demand creation and reconfiguration of guiding pathways to control the sliding of various liquids. This work establishes spatiotemporally controlled lubrication via photoinduced reversible solid‑to‐liquid transitions as a new mechanism for designing microfluidics, microreactors, and adaptive surfaces.

The precise control of liquid motion with light is fundamental to advancing next‐generation microfluidic devices and microreactors. Fluoroalkylated photoswitches that combine low surface tension with photoinduced reversible solid‑to‑liquid transitions would be ideal for this purpose, yet they have remained unrealized because fluoroalkyl chains typically suppress solid‑state photoisomerization. Here, we overcome this limitation by designing fluoroalkylated azobenzenes that exhibit low surface tension and photoinduced reversible solid‑to‑liquid transitions. This dual functionality stems from tailored molecular and crystalline structures that enable efficient trans ‐ cis photoisomerization in the solid state. The fluoroalkylated azobenzenes function as non‑lubricating solids in their trans form and become liquid lubricants upon photoconversion to their cis form. By infusing them into microstructured substrates, we create surfaces that can be optically switched between a liquid‐pinning state and a slippery state. This photoswitching capability enables the on‑demand creation and reconfiguration of guiding pathways to control the sliding of various liquids. This work establishes spatiotemporally controlled lubrication via photoinduced reversible solid‑to‐liquid transitions as a new mechanism for designing microfluidics, microreactors, and adaptive surfaces.

The microscopic and fundamental origin of slide electrification, which happens when water drops move across insulating surfaces while accumulating and depositing electrical charges, is still debated. Charge transfer is often attributed to ion transfer at the receding contact line. However, it remains an open question whether ion transfer alone can fully account for the observed charge separation. Here we examined slide electrification of polar, self-ionizing liquids and non-polar liquids. By cooling them below the melting temperature, we were able to compare this process to tribocharging of the respective frozen components. Despite reduced ion mobility at sub-freezing temperatures, the frozen polar compounds continued to accumulate substantial charge. Non-polar liquids exhibited lower charging and nearly identical charging behaviour in both their liquid and frozen phases on different substrates. As non-polar liquids contain few free ions, these observations indicate an alternative charging mechanism, which could be electron transfer. Our findings suggest that slide electrification operates through at least two mechanisms, with the dominant charge transfer pathway shifting between ions and electron transfer depending on electronegativity, phase and temperature.

ZUSAMMENFASSUNG: Dynamische dreidimensionale (3D) Muster weisen gegenüber ihren statischen Gegenstücken überlegene Fähigkeiten auf und spielen eine entscheidende Rolle in Anwendungen wie wiederbeschreibbarer Datenspeicherung und aktualisierbarer Informationsverschlüsselung. Die effiziente Herstellung und Rekonfiguration von 3D‐Mustern erfordert jedoch geeignete Methoden und Materialien. In dieser Arbeit untersuchen wir die Herstellung und Rekonfiguration dynamischer 3D‐Muster durch Laserdirektschreiben mittels Hochkonversion in Kombination NIR‐lichtempfindlichen Nanokompositen. Wir haben ein System entwickelt, das einen kostengünstigen kontinuierlich betriebenen NIR‐Laser nutzt, um reversible Photoreaktionen im Laserfokus auszulösen, und so durch Bewegung des Laserfokus dynamische 3D‐Muster zu erzeugen. Durch die Integration von Hochkonversions‐Nanopartikeln (UCNPs) mit photoschaltbaren Materialien in eine Polymermatrix, einschließlich Spiropyran, Diarylethen und Azobenzol, ermöglichen wir eine lokalisierte Photoisomerisierung durch fokussierte Laserbestrahlung. Die Reversibilität des Photoschaltprozesses macht die resultierenden 3D‐Muster löschbar, wiederbeschreibbar und dynamisch. Diese Technik stellt eine universelle und wirtschaftliche Methode für die dynamische 3D‐Photostrukturierung dar und ist besonders vielversprechend für den Fortschritt in den Bereichen Informationsverschlüsselung, optische Speicherung und Datensicherheit.

Dynamic three‐dimensional (3D) patterns exhibit superior capabilities compared to their static counterparts, playing crucial roles in applications such as rewritable data storage and updatable information encryption. However, the efficient fabrication and reconfiguration of 3D patterns require suitable methods and materials. In this work, we explore the fabrication and reconfiguration of dynamic 3D patterns using upconversion laser direct writing in combination with near‐infrared (NIR) light‐responsive nanocomposites. We have developed a system that utilizes a cost‐effective continuous‐wave (CW) NIR laser to induce reversible photoreactions at the laser focus, thereby fabricating dynamic 3D patterns by moving the laser focus. By integrating upconversion nanoparticles (UCNPs) with photoswitchable materials, including spiropyran (SP), diarylethene (DTE), and azobenzene (AZO), within a polymer matrix, we enable localized photoisomerization through focused laser irradiation. The reversibility of the photoswitching process renders the resulting 3D patterns erasable, rewritable, and dynamic. This technique represents a universal and economical method for dynamic 3D photopatterning, particularly promising for advancing information encryption, optical storage, and data security technologies.

Water drops spontaneously become electrically charged when moving on different surfaces, such as plant leaves, insect wings, building walls, window glass and plastic ^1–9 . This process, known as contact or sliding electrification, is analogous to tribocharging between solids ^10–13 . The electric potential of water drops charged in this way can exceed 1 kV (refs. ^14–16 ). A vital but as yet unanswered question is whether the charge in water drops causes corrosion. Here we analyse the effect of series of water drops hitting metal surfaces, which are protected by a non-conductive coating. Before hitting the coated metal, the drops acquire a charge spontaneously by sliding over an insulating surface. We demonstrated that these charged drops can cause the coating to break down electrically and lead to corrosion of the metal. As spontaneously charged water drops form naturally, this previously overlooked corrosion mechanism may contribute to the degradation of cultural heritage sites, buildings, ships, cars and other metal components. Our findings can improve anticorrosion strategies and emphasize the need for protective materials capable of resisting charge-induced damage from water drops.

We report the segmental dynamics of poly(n-ethyl methacrylate) melts forming nanotubes inside cylindrical alumina nanopores (AAOs). Long nanotubes were fabricated at the inner pore walls by the polymer precursor film during the complete wetting transition at high temperatures. Using differential scanning calorimetry and dielectric spectroscopy, we show that the segmental dynamics in the nanotubes are bimodal and slower than in bulk with a broad distribution of relaxation times. These features reflect the dynamic properties of the “dead layer” formed by the adsorbed polymer chains at the pore walls.

Controlling the motion of non-Newtonian drops on surfaces is crucial for applications ranging from inkjet printing to biomedical devices and food processing. While the macroscopic behavior of viscoelastic drops sliding on tilted hydrophobic surfaces has been characterized—showing reduced velocities and elongation compared to Newtonian fluids—the underlying microscopic mechanisms remain poorly understood. To address this gap, we developed a high-speed, high-resolution reflection microscope that enables direct visualization of the contact line of sliding drops. We used water-soluble polyelectrolyte solutions based on polyacrylamide and allowed drops to slide on hydrophobic substrates composed of Teflon amorphous fluoropolymer and polydimethylsiloxane-coated glass slides. The substrate tilting angle was varied between 20° and 45°. We reveal how viscoelasticity influences the dynamics of the receding contact line and drop motion. Our experiments demonstrate that viscoelasticity can destabilize the receding contact line, triggering filament formation. This instability previously observed in the coating of thin viscoelastic films is reported here for sliding drops, for the first time on smooth surfaces. We further highlight the critical role of polymer charge in this process: while cationic and nonionic polymers promote filament formation, anionic polymers do not, a difference we attribute to the distinct wetting properties of the solutions. In conclusion, we clarify the interplay between rheology, surface interactions, and drop dynamics.

Water drops sliding on hydrophobic surfaces spontaneously separate charges at their rear. It is unclear how this charge separation affects the contact angles of a sliding drop. We slide grounded and insulated drops on hydrophobic surfaces at low capillary numbers ( ≤ 10 − 4 ). We find that the drop charge leads to spontaneous electrowetting that decreases the contact angles. The deposited charges lead to a surface charge effect and decrease the contact angle. Both phenomena compensate each other at the receding contact line, resulting in an insignificant change in the receding contact angle of a sliding drop.