Shape stability and bundling of ultrathin nanowires

Ultrathin nanowires are promising nanoscale materials. They can reach length-to-diameter aspect ratios exceeding 1000, making them suitable building blocks for optoelectronic devices such as transparent conducting films. An organic ligand shell surrounds their inorganic core, provides colloidal stability, and guides their one-dimensional growth. Two unresolved issues limit their application. Nanowires can agglomerate into elongated bundles, but efficient use of this superstructure is difficult since we do not yet understand the bundling mechanisms. Furthermore, nanowires are prone to the Plateau-Rayleigh instability: thin wires tend to fragment into discrete spheroidal particles to reduce their surface energy, limiting their lifetime and reliability. This thesis investigates superstructure formation and nanowire stability and the link between both topics. Bundles are shown to emerge in non-polar solvents for entropic reasons. Solvent or unbound ligand molecules align in proximity to the ligand shell, thus losing entropy. Bundling decreases this loss in entropy by reducing contact with the bulk solvent. The structural stability of nanowires is enhanced or degraded by the ligand shell, depending on the relationship between free energy and local surface curvature. Kinetic barriers in ad- and desorbing ligands and rearrangement of surface atoms slow down the break-up. Bundling further stabilizes the wires by confining the space available to them.Ultradünne Nanodrähte, bestehend aus einem anorganischen Kern und einer organischen Ligandenhülle, können Aspektverhältnisse von über 1:1000 erreichen und sind potenzielle Materialien für optoelektronische Technologien wie transparente Elektroden. Einer breiteren Anwendung stehen zwei Herausforderungen entgegen. Nanodrähte können zu Bündeln agglomerieren, aber eine effiziente Nutzung dieser Superstruktur ist schwierig, da unser Verständnis der zugrundeliegenden Bündelungsmechanismen unvollständig ist. Zudem sind Nanodrähte instabil: gemäß der Plateau-Rayleigh-Instabilität zerbrechen sie zur Reduktion ihrer Oberflächenenergie in kleinere Nanopartikel, was ihre Langzeit-Anwendung verhindert. Sowohl Formstabilität und Superstruktur als auch der Zusammenhang zwischen beiden Themen wurden in dieser Dissertation untersucht. Bündel entstehen in unpolaren Lösemitteln, weil sich Lösemittelmoleküle oder freie Liganden parallel zur Ligandenhülle ausrichten und dabei Entropie verlieren. Durch die Anordnung in Bündeln wird der Kontakt zum Lösemittel reduziert, sodass der Entropieverlust geringer ausfällt. Die Formstabilität von Nanodrähten wird von der Ligandenhülle verbessert oder verschlechtert, je nach Zusammenhang zwischen freier Energie und Oberflächenkrümmung. Kinetische Barrieren in der Ad- und Desorption von Liganden und der Reorganisation der Oberfläche verlangsamen den Zerfall. Bündel verbessern die Formstabilität, indem sie den für die Nanodrähte verfügbaren Raum begrenzen

Low Friction and High Solid-Solid Contact Ratio—A Contradiction for Laser-Patterned Surfaces?

Recording of Stribeck-like curves is a common way to study the effect of laser-patterned surfaces on the frictional efficiency. However, solely relying on the coefficient of friction when identifying the lubrication regime and the underlying working principles can be misleading. Consequently, a ball-on-disc tribometer was combined with an electrical resistivity circuit to simultaneously measure Stribeck-like curves and solid-solid contact ratios for polished and laser-patterned samples. Line-like surface patterns with different periodicities were produced by direct laser interference patterning on steel substrates (AISI304). The reference shows a Stribeck-like behavior well correlating with the contact ratios. The behavior deviates for high sliding velocities (high contact ratios) due to a loss of lubricant induced by centrifugal forces pulling the lubricant out of the contact zone. In contrast, the solid–solid contact ratio of the laser-patterned samples is around 80% for all sliding velocities. Those values can be explained by higher contact pressures and the structural depth induced by the surface topography which make a full separation of the surfaces unlikely. Despite those high values for the contact ratio, laser-patterning significantly reduces friction, which can be traced back to a reduced real contact area and the ability to store oil in the contact zone

Low friction and high solid-solid contact ratio—A contradiction for laser-patterned surfaces?

Recording of Stribeck-like curves is a common way to study the effect of laser-patterned surfaces on the frictional efficiency. However, solely relying on the coefficient of friction when identifying the lubrication regime and the underlying working principles can be misleading. Consequently, a ball-on-disc tribometer was combined with an electrical resistivity circuit to simultaneously measure Stribeck-like curves and solid-solid contact ratios for polished and laser-patterned samples. Line-like surface patterns with different periodicities were produced by direct laser interference patterning on steel substrates (AISI304). The reference shows a Stribeck-like behavior well correlating with the contact ratios. The behavior deviates for high sliding velocities (high contact ratios) due to a loss of lubricant induced by centrifugal forces pulling the lubricant out of the contact zone. In contrast, the solid–solid contact ratio of the laser-patterned samples is around 80% for all sliding velocities. Those values can be explained by higher contact pressures and the structural depth induced by the surface topography which make a full separation of the surfaces unlikely. Despite those high values for the contact ratio, laser-patterning significantly reduces friction, which can be traced back to a reduced real contact area and the ability to store oil in the contact zone

Breakdown of continuum models for spherical probe adhesion tests on micropatterned surfaces

Funding Information: SB, DY, EA, and RH acknowledge funding from the European Research Council (ERC) under the European Union's Seventh Framework Program (FP/2007–2013)/ERC Advanced Grant No. 340929 . RMM acknowledges the Alexander von Humboldt Foundation for awarding the “Virtual Humboldt Cluster on the Mechanics and Physics of Adhesion and Grip”.Peer reviewedPublisher PD

Low Friction and High Solid-Solid Contact Ratio—A Contradiction for Laser-Patterned Surfaces?

Recording of Stribeck-like curves is a common way to study the effect of laser-patterned surfaces on the frictional efficiency. However, solely relying on the coefficient of friction when identifying the lubrication regime and the underlying working principles can be misleading. Consequently, a ball-on-disc tribometer was combined with an electrical resistivity circuit to simultaneously measure Stribeck-like curves and solid-solid contact ratios for polished and laser-patterned samples. Line-like surface patterns with different periodicities were produced by direct laser interference patterning on steel substrates (AISI304). The reference shows a Stribeck-like behavior well correlating with the contact ratios. The behavior deviates for high sliding velocities (high contact ratios) due to a loss of lubricant induced by centrifugal forces pulling the lubricant out of the contact zone. In contrast, the solid–solid contact ratio of the laser-patterned samples is around 80% for all sliding velocities. Those values can be explained by higher contact pressures and the structural depth induced by the surface topography which make a full separation of the surfaces unlikely. Despite those high values for the contact ratio, laser-patterning significantly reduces friction, which can be traced back to a reduced real contact area and the ability to store oil in the contact zone

Breakdown of continuum models for spherical probe adhesion tests on micropatterned surfaces

The adhesion of fibrillar dry adhesives, mimicking nature's principles of contact splitting, is commonly characterized by using axisymmetric probes having either a flat punch or spherical geometry. When using spherical probes, the adhesive pull-off force measured depends strongly on the compressive preload applied when making contact and on the geometry of the probe. Together, these effects complicate comparisons of the adhesive performance of micropatterned surfaces measured in different experiments. In this work we explore these issues, extending previous theoretical treatments of this problem by considering a fully compliant backing layer with an array of discrete elastic fibrils on its surface. We compare the results of the semi-analytical model presented to existing continuum theories, particularly with respect to determining a measurement system- and procedure-independent metric for the local adhesive strength of the fibrils from the global pull-off force. It is found that the discrete nature of the interface plays a dominant role across a broad range of relevant system parameters. Accordingly, a convenient tool for simulation of a discrete array is provided. An experimental procedure is recommended for use in conjunction with this tool in order to extract a value for the local adhesive strength of the fibrils, which is independent of the other system properties (probe radius, backing layer thickness, and preload) and thus is suitable for comparison across experimental studies

Effect of celecoxib vs placebo as adjuvant therapy on disease-free survival among patients with breast cancer

Importance: Patients with breast cancer remain at risk of relapse after adjuvant therapy. Celecoxib has shown antitumor effects in preclinical models of human breast cancer, but clinical evidence is lacking. Objective: To evaluate the role of celecoxib as an addition to conventional therapy for women with ERBB2 (formerly HER2)-negative primary breast cancer. Design, Setting, and Participants: The Randomized European Celecoxib Trial (REACT) was a phase 3, randomized, double-blind study conducted in 160 centers across the UK and Germany testing 2 years of adjuvant celecoxib vs placebo among 2639 patients recruited between January 19, 2007, and November 1, 2012, with follow-up 10 years after treatment completion. Eligible patients had completely resected breast cancer with local and systemic therapy according to local practice. Patients with ERBB2-positive or node-negative and T1, grade 1 tumors were not eligible. Randomization was in a 2:1 ratio between celecoxib or placebo. Statistical analysis was performed from May 5, 2019, to March 5, 2020. Interventions: Patients received celecoxib, 400 mg, or placebo once daily for 2 years. Main Outcomes and Measures: The primary end point was disease-free survival (DFS), analyzed in the intention-to-treat population using Cox proportional hazards regression and log-rank analysis. Follow-up is complete. Results: A total of 2639 patients (median age, 55.2 years [range, 26.8-86.0 years]) were recruited; 1763 received celecoxib, and 876 received placebo. Most patients' tumors (1930 [73%]) were estrogen receptor positive or progesterone receptor positive and ERBB2 negative. A total of 1265 patients (48%) had node-positive disease, and 1111 (42%) had grade 3 tumors. At a median follow-up of 74.3 months (interquartile range, 61.4-93.6 years), DFS events had been reported for 487 patients (19%): 18% for those who received celecoxib (n = 323; 5-year DFS rate = 84%) vs 19% for those who received placebo (n = 164; 5-year DFS rate = 83%); the unadjusted hazard ratio was 0.97 (95% CI, 0.80-1.17; log-rank P =.75). Rates of toxic effects were low across both treatment groups, with no evidence of a difference. Conclusions and Relevance: In this randomized clinical trial, patients showed no evidence of a DFS benefit for 2 years' treatment with celecoxib compared with placebo as adjuvant treatment of ERBB2-negative breast cancer. Longer-term treatment or use of a higher dose of celecoxib may lead to a DFS benefit, but further studies would be required to test this possibility. Trial Registration: ClinicalTrials.gov Identifier: NCT02429427 and isrctn.org Identifier: ISRCTN48254013