1,700 research outputs found
Stability of the submerged superhydrophobic state via rare event molecular dynamics simulations
No full textIn the present thesis the stability of the superhydrophobic state on submerged nanotextured surfaces with complex chemistry and geometry is investigated via rare event molecular dynamics simulations. Superhydrophobicity stems from the presence of vapor or gas pockets trapped within the surface texture sustaining the liquid atop of the textures. This superhydrophobic --or Cassie-- state gives rise to remarkable macroscopic properties, such as enhancing liquid slip at the surface or resulting into anti-fouling capabilities. These and many other properties make superhydrophobic surfaces promising for a wide range of technological applications. Superhydrophobicity can be lost via two mechanisms: i) liquid intrusion, i.e. the filling of the liquid inside the textures, leading to the fully wet Wenzel state, or ii) vapor cavitation, i.e. the formation, growth, detachment of a (supercritical) vapor or gas bubble, and the ensuing replacement of the vapor pocket by the liquid. Understanding the mechanism of the Cassie-Wenzel and cavitation transitions is crucial in order to define quantitatively the stability of the superhydrophobic state. These transitions are typically characterized by large free-energy barriers. The presence of these barriers make the study of the transitions particularly challenging due to the diverse timescales present in the problem. Indeed, these transitions are rare events, i.e., they happen on timescale which cannot be simulated by standard molecular dynamics methods. Thus, in order to tackle this issue, rare event methods are used which allow one to compute the free-energy barriers and the mechanism of the transition. This information is essential for developing new design criteria which can pave the way to a new generation of surfaces with more stable superhydrophobic properties in submerged conditions. In the first part of the thesis, re-entrant surface textures are investigated. The focus on this geometry is due to the increasing interest on textures which show omniphobic properties, i.e., which allows the formation of the Cassie state also for low surface tension liquids. Our atomistic results are compared with a macroscopic sharp-interface continuum model. While the two models are generally in fair agreement, quantitative differences were found in the cavitation free-energy barriers, with the macroscopic model overestimating them. The major qualitative difference concerns the behaviour of the system in the Wenzel state, where only the atomistic model can capture the presence of the confined liquid spinodal. These results also allowed us to develop design criteria for textured submerged superhydrophobic surface. The role of the chemistry of the surface was also studied. Pure hydrophobic, pure hydrophilic, and mixed, i.e. internally hydrophobic and externally hydrophilic surfaces, were considered. It was found that the free energy of the mixed chemistry surface closely resemble the superimposition of the one of pure hydrophilic and hydrophobic chemistries. The mixed chemistry shows improved stability of gas pockets against both liquid intrusion and vapor cavitation. Finally, the combined effect of complex chemistry and geometry, such as re-entrant pore morphologies, was also investigated. This latter part of the work was primarily inspired by the natural case of Salvinia molesta, which is a floating fern capable of remaining dry even after a long underwater immersion. Salvinia leaves show similar features with respect to the proposed textures; it is characterized by hairs with a peculiar re-entrant structure with a heterogeneous chemistry: a hydrophobic interior and an hydrophilic patch on the hairs top. In the second part of the thesis, the Cassie-Wenzel transition was investigated on a submerged 3D nano pillared surface. Here, a state-of-the-art technique, the string method, was employed in order to compute the most probable transition mechanism and the corresponding free-energy barrier. The coarse-grained fluid density field was used as a collective variable to characterize the transition. Results are both of applicative and of methodological interst. For the former, the string method revealed the actual transition mechanism, which proceeds by breaking the 2D translational symmetry of the surface textures. These results are interpreted in terms of a sharp-interface continuum model suggesting that nanoscale effects, e.g., line tension, play a minor role in the considered conditions. Concerning to the former, the effect of the choice of the collective variables, i.e. different level of coarse-graining of the fluid density field, was studied. Results show the correct level of coarse-graining suited to correctly capture the transition mechanism and the free-energy barrier
Collapse of superhydrophobicity on nanopillared surfaces
No full textThe mechanism of the collapse of the superhydrophobic state is elucidated for submerged nanoscale textures forming a three-dimensional interconnected vapor domain. This key issue for the design of nanotextures poses significant simulation challenges as it is characterized by diverse time and length scales. State-of-the-art atomistic rare events simulations are applied for overcoming the long time scales connected with the large free energy barriers. In such interconnected surface cavities wetting starts with the formation of a liquid finger between two pillars. This break of symmetry induces a more gentle bend in the rest of the liquid-vapor interface, which triggers the wetting of the neighboring pillars. This collective mechanism, involving the wetting of several pillars at the same time, could not be captured by previous atomistic simulations using surface models comprising a small number of pillars (often just one). Atomistic results are interpreted in terms of a sharp-interface continuum model which suggests that line tension, condensation, and other nanoscale phenomena play a minor role in the simulated conditions
Intrusion and extrusion of a liquid on nanostructured surfaces
No full textSuperhydrophobicity is connected to the presence of gas pockets within surface asperities. Upon increasing the pressure this 'suspended' state may collapse, causing the complete wetting of the rough surface. In order to quantitatively characterize this process on nanostructured surfaces, we perform rare-event atomistic simulations at different pressures and for several texture geometries. Such an approach allows us to identify for each pressure the stable and metastable states and the free energy barriers separating them. Results show that, by starting from the superhydrophobic state and increasing the pressure, the suspended state abruptly collapses at a critical intrusion pressure. If the pressure is subsequently decreased, the system remains trapped in the metastable state corresponding to the wet surface. The liquid can be extruded from the nanostructures only at very negative pressures, by reaching the critical extrusion pressure (spinodal for the confined liquid). The intrusion and extrusion curves form a hysteresis cycle determined by the large free energy barriers separating the suspended and wet states. These barriers, which grow very quickly for pressures departing from the intrusion/extrusion pressure, are shown to strongly depend on the texture geometry
Unraveling the Salvinia Paradox: Design Principles for Submerged Superhydrophobicity
No full textThe complex structure of the Salvinia molesta is investigated via rare event molecular dynamics simulations. Results show that a hydrophilic/hydrophobic patterning together with a re-entrant geometry control the free energy barriers for bubble nucleation and for the Cassie-Wenzel transition. This natural paradigm is translated into simple macroscopic design criteria for engineering robust superhydrophobicity in submerged applications
TUTELA DEL LAVORO E LIBERTA' D'IMPRESA NEI PROCESSI DI ESTERNALIZZAZIONE
No full textL’elaborato analizza le conseguenze lavoristiche della successione fra imprenditori, muovendo da una ricognizione delle varie tipologie di esternalizzazione con le relative esigenze e principali criticità.
L’indagine si concentra in primo luogo sul trasferimento d’azienda, esaminando la normativa e la giurisprudenza europee per passare poi alla disciplina di diritto interno, alle procedure sindacali e a uno specifico focus sul trasferimento delle aziende in crisi.
Successivamente l’autore si sofferma sull’appalto, prendendone in particolare considerazione gli indici di genuinità, i criteri di distinzione dalla somministrazione illecita di manodopera e la tutela delle maestranze in caso di avvicendamento fra imprese.
Da ultimo, la ricerca approfondisce le c.d. “clausole sociali”, sia di prima che di seconda generazione, valutandone la compatibilità con il diritto eurounitario e con la costituzione nonché riflettendo sui possibili rimedi in caso di loro violazione.The author analyzes the labour consequences of the succession between entrepreneurs, starting from a recognition of the various types of outsourcing with the related needs and main critical issues.
The survey focuses primarily on the transfer of businesses, examining European legislation and case-law and then moving on to internal legislation, trade union procedures and a specific focus on the transfer of companies in crisis.
The author then dwells on the contract, taking into account in particular the indications of authenticity, the criteria of distinction from the illicit administration of labour and the protection of workers in the event of turnover between companies.
Finally, the research deepens the "social clauses", both first and second generation, assessing their compatibility with European law and with the constitution and reflecting on possible remedies in case of their violation
Ultra Low Carbon Vehicles: New Parameters for Automotive Design
No full textAs the influence of vehicle emissions on our environment has become better understood, the UK government has recently placed urgent emphasis on the implementation of low carbon
technologies in the automotive industry through: the UK Low Carbon Industrial Strategy. The overall objective is to offer big incentives to consumers and support for the development of infrastructure and engineering solutions. This scheme however does not consider how the development of functional and experiential user value might drive consumer demand, contributing to the adoption of low carbon vehicles (LCVs) in the mass market.
With the emergence of the North East of England as the UK’s first specialised region for the development of ultra-low carbon vehicles (ULCVs), ONE North East, as a development agency for the region's economic and business development, and Northumbria University Ideas-lab have supported a project to facilitate innovation through the collaboration of technology, research and development (R&D) and business. The High Value Low Carbon (HVLC) project aims to envisage
new user value made possible by the integration of low carbon vehicle platforms with new process and network technologies. The HVLC consortium represents vehicle manufacturers and their suppliers as well as technology based companies and through an ongoing process of design concept generation the project offers a hub for innovation led enterprise.
Whilst new technological developments in areas such as power generation, nano materials, hydrogen fuel cells, printed electronics and networked communications will all impact on future automotive design, the mass adoption of low carbon technologies represents a paradigm shift for the motorist. This paper aims to describe how the mapping of new parameters will lead to new transport scenarios that will create the space for new collaborative research on user experiences supported by innovative technologies and related services
Self-recovery superhydrophobic surfaces: modular design
No full textSuperhydrophobicity, the enhanced hydrophobicity of surfaces decorated with textures of suitable size, is associated with a layer of gas trapped within surface roughness. The reduced liquid/solid contact makes superhydrophobicity attractive for many technological applications. This gas layer, however, can break down with the liquid completely wetting the surface. Experiments have shown that the recovery of the "suspended" superhydrophobic state from the wet one is difficult. Self-recovery - the spontaneous restoring of the gas layer at ambient conditions - is one of the dreams of research in superhydrophobicity as it would allow to overcome the fragility of superhydrophobicity. In this work we have performed a theoretical investigation of the wetting and recovery processes on a set of surfaces characterized by textures of different dimensions and morphology in order to elucidate the optimal parameters for avoiding wetting and achieving self-recovery. Results show that texture size in the nanometer range is a necessary but not sufficient condition for self-recovery: the geometry plays a crucial role, nanopillars prevent self-recovery, while surfaces with square pores exhibit self-recovery even at large positive pressures. However, the optimal morphology for self-recovery, the square pore, is suboptimal for the functional properties of the surface, for example, high slippage. Our calculations show that these two properties are related to regions of the texture separated in space: self-recovery is controlled by the characteristics of the bottom surface, while wetting and slip are controlled by the cavity mouth. We thus propose a modular design strategy which combines self-recovery and good functional properties: Square pores surmounted by ridges achieve self-recovery even at 2 MPa and have a very small liquid/solid contact area. The macroscopic calculations, which allowed us to efficiently devise design criteria, have been validated by atomistic simulations, with the optimal texture showing self-recovery on atomic time scales, τ ∼ 2 ns
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Matteo Maria Boiardo (1441-1494)
No full textMatteo Maria Boiardo – contemporary of Sir Thomas Malory and forerunner of Shakespeare, Spenser, Milton, Cervantes, Tolkien, and C.S. Lewis – is best known as the author of the Orlando Innamorato [Orlando in Love], a poem in ottava rima organically merging Carolingian epic and Arthurian romance. He was also a lyric and pastoral poet, playwright, and translator of classical texts into the Italian vernacular. Matteo was active in Italy during the years of 1463 throughout 1494, the year of his death
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