Imparting superhydrophobic properties through janus wrinkled silica-gel particles

In nature, superhydrophobicity is associated with a number of possible evolutionary benefits that may be bestowed upon an organism, ranging from the prevention of encumbrance by water droplets, self-cleaning and removal of particulates and potential pathogens, and even to antimicrobial activity. Superhydrophobicity is a surface related property, where droplets of water easily roll-off without stickicking or wetting the surface and can be achieved through hierarchical textured and low surface tension materials. The interest in this kind of behaviour comes from the possibility of exploiting superhydrophobic surfaces for many applications in different field (medical devices, infrastructures, fabrics, transports, etc.). In this PhD project (granted by Procter & Gamble), superhydrophobic surfaces were chosen as possible solution of a company’s problem related to the difficulty in emptying of plastic dispensers containing viscous liquid products. Therefore, a possible solution was to render the plastic surface superhydrophobic to reduce the drag and help the easy-emptying of the dispensers. We developed a new type of particles that can impart a superhydrophobic behaviour to the applied substrate. Janus wrinkled silica-gel particles combine all the aspects that are necessary to achieve superhydrophobic surfaces: enabling formation of hierarchical roughness and low surface tension (without using any fluorinated molecule, not allowed in the dispensers manufacturing). Such particles were applied to substrates by drop-casting and dip-coating methods, and through a camera we monitored the behaviour of different water solutions on particles coatings. Superhydrophobic surfaces have been obtained possessing high values of water contact angle and very low values of contact angle hysteresis (2°) and roll-off angle values (1.8°), which are close to zero. These results have been interpreted based on the multiple scales of hierarchical roughness that has been constructed by deposition of the particles. This thesis is divided in five chapters. In chapter 1, superhydrophobicity and how nature and man exploit this surface property are described so as methods to achieve such behaviour. Then, 10 the easy-emptying issue is discussed and the use of Janus wrinkled silica-gel particles as possible solution is motivated. Chapter 2 collects the theoretical backgrounds of this multidisciplinary project. Indeed, production of Janus wrinkled silica-gel particles involve different techniques and methods from different branches of science. Superhydrophobicity is described from a theoretical point of view: models and parameters that characterize the wettability of surfaces with regards to liquids are reported. Sol-gel method is briefly described, paying attention to the mesoporous particles formation mechanism. In the end, Pickering emulsion formation, necessary for Janus particles production, is described. In chapter 3, all the synthetic procedures are reported. The chapter is divided in paragraphs that reflects the multi-step synthetic procedure. First the synthesis of wrinkled silica-gel particles, then the formation of colloidosomes by Pickering emulsification of wrinkled particles, wax and water. Subsequently, the surface functionalization for obtaining Janus wrinkled particles. The characterization techniques are described in chapter 4, together with the apparatus used in this work and the sample preparations related to such characterization techniques. We used microscopic techniques as SEM, TEM and AFM for monitoring the structures of particles (SEM, TEM), the hierarchical roughness of the coatings (SEM, AFM) and the Janus nature of the particles (TEM). DLS and Zeta potential measurements were used for monitoring particles size and stability, especially in surfactant dispersions, necessary for Pickering emulsion preparation. ATR and XPS allowed evaluating the presence of functional groups (ATR, XPS) after each functionalization. Chapter 5 collects all the results of Janus wrinkled silica-gel particles: from the particles synthesis, to colloidosomes formation through Pickering emulsion, their functionalization and the particles application as coating on different substrates. In the end, the wettability properties results are reported to verify the excellence of this approach achieving superhydrophobic behaviour by Janus wrinkled silica-gel particles coatings

Light Electrospun Polyvinylpyrrolidone Blanket for Low Frequencies Sound Absorption

Light polymeric soundproofing materials (density = 63 kg/m3) of interest for the transportation industry were fabricated through electrospinning. Blankets of electrospun polyvinylpyrrolidone (average fiber diameter = (1.6 ± 0.5) or (2.8 ± 0.5) μm) were obtained by stacking disks of electrospun mats. The sound absorption coefficients were measured using the impedance tube instrument based on ASTM E1050 and ISO 10534–2. For a given set of disks (from a minimum of 6) the sound absorption coefficient changed with the frequency (in the range 200–1600 Hz) following a bell shape curve with a maximum (where the coefficient is greater than 0.9) that shifts to lower frequencies at higher piled disks number and greater fiber diameter. This work showed that electrospinning produced sound absorbers with reduced thickness (2–3 cm) and excellent sound-absorption properties in the low and medium frequency range

Thermally Driven Selective Nanocomposite PS-PHB/MGC Nanofibrous Conductive Sensor for Air Pollutant Detection

The potentials to use the working temperature to tune both the sensitivity and the selectivity of a chemical sensor based on a nanostructured and nanocomposite polymer layer have been investigated and described. Thus, in a single step, a peculiar chemical layer was grown up onto IDE (Interdigitated Electrode) microtransducers by electrospinning deposition and using a single-needle strategy. The 3-component nanofibers, obtained from a mixture of polystyrene and polyhydroxibutyrate (insulating thermoplastics) and a known concentration of mesoporous graphitized carbon nanopowder, appeared highly rough on the surface and decorated with jagged islands but homogeneous in shape and diameter, with the nanofillers aggregated into clusters more or less densely packed through the fibers. The resulting sensor was conductive at room temperature and could work between 40 and 80°C without any apparent degradation. As the fibrous sensing layer was heated, the current increased and the sensitivity to some classes of VOCs such as an oxidizing gas drastically changed depending on the working temperature. More in detail, the sensor resulted highly sensitive and selective to acetic acid at 40°C but the sensitivity fell down, decreasing by 96%, when the sensor operated at 80°C. On the other hand, although an increase in temperature caused a general decrease in sensitivity to the tested VOCs (with a maximum of 14, 81, and 78% for amine, acetone and toluene, respectively) and water vapors (with a maximum of 55%), higher temperature affected only slightly the amine permeation, thus modifying the partial selectivity of the sensor to these chemicals. Conversely, when the operating temperature increased, the sensitivity to the detected gas, NO2, increased too, reporting a ~2 ppb limit of detection (LOD), thus confirming that the temperature was able to drive the selectivity of nanocomposite polymeric sensors

New polyvinylpyrrolidone (PVP) based soundproofing materials through electrospinning

In this work, new light (density=63 Kg/m3) soundproofing materials were obtained through electrospinning of high molecular weight (1300 KDa) polyvinylpyrrolidone (PVP). The sound absorbers were obtained in the form of blankets of thin disks of 10 cm diameter. For a given set of disks (from a minimum of 6) the sound absorption coefficient αa changes, in the frequency range 200-1600Hz, following a bell-like curve with a maximum (close to 1.0) that shifts to lower frequencies the higher the piled disks number (and mass). The acoustic properties of electrospun woven non woven mats are compared to the traditional soundproofing materials ones. The effect, on sound absorption, of assembling together PVP and traditional soundproofing glass wool is also shown. The experimental results suggest that sound energy dissipation occurs primarily because of resonance of the sound wave with the natural frequency of vibration of the disks pile (acting as a membrane)

Low-Cost Benzene Toluene Xylene Measurement Gas System Based on the Mini Chromatographic Cartridge

Benzene, toluene and xylene (BTX) are an important part of the volatile organic compounds (VOCs) to be detected and monitored in the air, due to their toxicity towards human health. One of the most reliable technique used in BTX detection is gas chromatography (GC), which presents a high sensitivity. On the other hand, it has important drawbacks, such as high costs, the need for qualified personnel and frequent maintenance. To overcome these drawbacks, this work reports the development of a low cost and portable BTX gas detection system based on a mini chromatographic cartridge, a photo ionization detector (PID), a simple control unit (based on Arduino architecture) and a mini pump. In order to separate the BTX components, we propose the use of a cartridge 80 mm in length, composed of several commercial chromatographic column sections. To test the system performances, we have injected different amounts (from about 0.3 to 5.3 µg) of benzene, toluene and xylene and two of the most frequent possible interferents (ethanol, acetone). Experimental results have shown different retention time values (i.e., 25 ± 0.5 s, 51 ± 1.2 s and 117 ± 4 s, respectively) for benzene, toluene and xylene

Forming nanostructured surfaces through Janus colloidal silica particles with nanowrinkles: A new strategy to superhydrophobicity

Proper nano-structuring coupled with low surface energy properties is exploited to obtain super-hydrophobic surfaces. A biomimicking hierarchically structured coating upon a material surface is obtained by depositing Janus nano-structured wrinkled colloidal particles. The morphology of the surface is thus ruled by different scales: the size of the wrinkles (few tens of nanometer), particle radius (several hundreds of nanometer) and interparticle distance. Janus wrinkled silica particles have one side exposing hydroxyl groups while the other side is partially silanized with dichlorodimethylsilane. To obtain these Janus particles, wax colloidosomes with surface covered by starting rough particles were successfully prepared. The exposed surface of rough wrinkled silica particles was then silanized, thus obtaining a Janus structure. The Janus character was prooved in different manners: the simple so called “visual test”, through Energy Filtered Transmission Electron Microscopy (was proven in different EFTEM) and by further functionalization with polypropylene grafted maleic anhydride (PPgMA). A glass microscope slide was covered by drop casting the particles obtaining super-hydrophobic features of the surface characterized by a high water contact angle (149°) and, more interestingly, by a very low water contact angle hysteresis (CAH=2°) and roll-off angle (ROA=1.8°). Superhydrophobicity is confirmed by the very low values of apparent surface free energy estimated with the OWRK (Owens-Wendt-Rabel e Kaelble) method. SEM and AFM analyses prove the multiple scale hierarchical roughness, from nano to micro size, that resembles the one present at the surface of the lotus leaves