Design and Engineering of Slippery Liquid-Infused Porous Surfaces by LbL Technique for Icephobic Surfaces and Hydrodynamic Cavitation

In this thesis a phenomenon that had been observed in nature and has been explained by fluid dynamics and surface engineering, was mimicked to study its properties and potential applications. The slippery liquid-infused porous surfaces (SLIPS) technology, which is inspired by pitcher plant, has been developed using Layer-by-Layer (LbL) assembly technique. The roughness of the surface was provided by deposition of a thin film of silica nanoparticles on a substrate and then the porosities of the surface was filled by a lubricant to have a non-stick, ultra-repellent, self-healing, icephobic and hydrophobic SLIPS. The charged silica nanoparticles with a diameter range of 40 to 80nm were synthesized using Stöber method and their size and surface charge were adjusted by controlling the TEOS/Ammonia ratio. The synthesized silica nanoparticles were deposited on the surface of the substrate using LbL assembly technique via dip coating and fluidic coating methods. The SEM, AFM, UV-Vis and ellipsometry results confirmed the deposition of a rough coating with root mean square roughness of 30 to 15nm, young modules of 5.3Gpa, 98% transparency in visible region and thickness of 100 to 200 nm. The icephobic porosities of the assembled thin films, which were filled by a lubricant were evaluated using a homemade ice adhesion strength measurement setup in an environmental chamber. The ice adhesion strength of the prepared SLIPS was measured as less than 5kPa. The cycling and aging tests, which were carried out on the SLIPS showed 35% reduction in the icephobicity of the SLIPS after 100 days and the ice adhesion strength of the coatings was about 5 times lower than untreated samples even after 50 icing deicing cycles. Surface topography and properties have an important influence on the generation of cavitating flow in microscale. For studying the effect of SLIPS and the surface roughness on the cavitating flow, the designed SLIPS structure was layer-by-layer assembled using fluidic method on the hydrodynamic cavitation microchips with various hydraulic diameters. The microfluidic devices were exposed to upstream pressures varying from 1 to 7.23 MPa and it has been observed that the inception of the cavitating flow and supercavitation condition have been occurred at much lower pressures in comparison with non-treated microfluidic devices. Introducing the cellulose nanofiber-stabilized perfluoropentane droplets to the SLIPS assembled micro channels, reduced the upstream pressure down to 1.7 MPa for generation of the supercavitation flow pattern within the device. The cellulose nanofibers were assessed by AFM after the cavitation process and it was observed that they were left undamaged during the cavitation process due to the lower upstream pressure, which in turn, increased the regeneration potential of the droplets for closed-loop applications

Tailoring the icephobic performance of slippery liquid-infused porous surfaces through the LbL method

There has been increasing interest in recent years in identifying an ice-removal procedure that is low cost and scalable and consumes a negligible amount of energy in order to prevent catastrophic failures in outdoor structures. One of the potential solutions to the structural problems caused by frigid and icy conditions is the use of slippery liquid-infused porous surfaces (SLIPS) to effect passive ice removal using easy, economical, and energy-free means. This work takes advantage of the highly flexible layer-by-layer (LbL) technology to customize and design surfaces that have a high degree of roughness using negatively and positively charged polyelectrolytes and negatively charged silica nanoparticles (NPs). SEM (scanning electron microscopy) images represent the silica nanoparticles deposition on the surface of the thin film. The roughness of these thin films has been demonstrated by AFM (atomic force microscopy) investigation. The main characteristics of these surfaces are their high contact angle and low water contact angle hysteresis, which is achieved by the fluorinated lubricant that is infused in the pores of the films. The ice adhesion strength of the thin films was measured using a homebuilt normal mode tensile test in an environmental chamber, which confirmed the icephobicity of the surface as having an adhesion strength of less than 5 kPa, implying that this surface is an excellent candidate for passive removal of ice. The thin films were aged for up to 100 days, and the results showed that the thin film could reduce the ice adhesion strength by 65%, even after this period. The ice adhesion strength of the thin film after icing/deicing cycles showed that 80% of the icephobicity of the thin film had been preserved even after 50 cycles

A new method for intense cavitation bubble generation on layer-by-layer assembled slips

The importance of surface topology for the generation of cavitating flows in micro scale has been emphasized during the last decade. In this regard, the utilization of surface roughness elements is not only beneficial in promoting mass transportation mechanisms, but also in improving the surface characteristics by offering new interacting surface areas. Therefore, it is possible to increase the performance of microfluidic systems involving multiphase flows via modifying the surface. In this study, we aim to enhance generation and intensification of cavitating flows inside microfluidic devices by developing artificial roughness elements and trapping hydrophobic fluorinated lubricants. For this, we employed different microfluidic devices with various hydraulic diameters, while roughness structures with different lengths were formed on the side walls of microchannel configurations. The surface roughness of these devices was developed by assembling various sizes of silica nanoparticles using the layer-by-layer technique (D2). In addition, to compare the cavitating flow intensity with regular devices having plain surfaces (D1), highly fluorinated oil was trapped within the pores of the existing thin films in the configuration D2 via providing the Slippery Liquid-Infused Porous Surface (D3). The microfluidic devices housing the short microchannel and the extended channel were exposed to upstream pressures varying from 1 to 7.23 MPa. Cavitation inception and supercavitation condition occured at much lower upstream pressures for the configurations of D2 and D3. Interestingly, hydraulic flip, which rarely appears in the conventional conical nozzles at high pressures, was observed at moderate upstream pressures for the configuration D2 proving the air passage existence along one side of the channel wall

Electrospinning of ternary composite of PMMA-PEG-SiO2 nanoparticles: comprehensive process optimization and electrospun properties

Electrospinning has been realized to be a promising method for creating nano-composite fibers due to its significant growth for producing innovative composites with advanced applications. In this method a polymeric solution subjected to an electrohydrodynamic process and slim charged liquid jet is formed inside a high potential electric field. The high voltage enables the production of continuously long fibers on a collector surface. Addition of different polymers and NPs to the one-component solution to modify the physicochemical characteristic and decorating the surface of electrospun fibers has proven to be challenging and imperative for many fields especially novel bioengineering and filtration applications. In this study, the effects of major parameters on the fabrication of electrospun fibers were extensively investigated. At the first step, formation of nanofibers on the surface of collector and optimization of process parameters were determined based on the mean diameter of resulting fibers, through SEM (Scanning Electron Microscopy) images. The optimum values for concentration, applied voltage, the distance between the tip of needle and collector, and flow rate determined to be 10 wt%, 12 kV, 20 cm, and 0.6 mL h−1, respectively. Afterwards, the hydrophilicity of fibers was modified by adding different poly (ethylene glycol) (PEG) concentrations (20, 30, and 40 wt%) to the polymeric solution. The contact angle analysis revealed that the poly (methyl methacrylate) (PMMA) and 30 wt% PEG fabricated fibrous mat exhibited a better wettability and 71.61% lower hydrophobicity compared to pure PMMA electrospun mats. In the next step, silica NPs (nanoparticles) were introduced to the polymeric solution of electrospinning in the form of an IPA (isopropanol)-based collide solution. The dispersed solution-based addition of silica NPs prevented the aggregation state of NPs in the nanofibers. The addition of silica nanoparticles also changed the thermal and mechanical properties of the ternary composite, which were analyzed in TGA (thermogravimetric analysis) and tensile tests. Noteworthy, the addition of 30% PEG and silica NPs increase 3 times the tensile strength and around 2 times elongation in comparison with pure PMMA electrospun mats. These results highlight that the hybrid composite leads to a promising new electrospun mat for filtration and bioengineering applications. © 2021 The Author

Layer-by-layer assembly of nanofilms from colloidally stable amine-functionalized silica nanoparticles

The assembly of uniform thin films with functional silica nanoparticles (FSNPs) by the layer-by-layer (LbL) technique has been studied to provide rough, robust, and functional surfaces. Initially, monodisperse silica nanoparticles (SNPs) have been synthesized by the reverse microemulsion method. Then SNPs have been functionalized using amino silane coupling agents with various chain lengths and amine positions such as (3-aminopropyl)trimethoxy silane (APS), N-[3-(trimethoxysilyl)propyl]-N,N,N-trimethylammonium chloride (NPC), N-(6-aminohexyl)aminopropyltrimethoxy silane (AHAPS), and 3-aminopropyl(dimethyl)ethoxy silane (APDMES). The amine-functionalized silica nanoparticles have been characterized using zeta potential, Fourier Transform-Infrared Spectroscopy (FT-IR), and 1H Nuclear Magnetic Resonance (NMR) spectroscopy to prove the existence of amine groups on the silica surface. Scanning electron microscopy (SEM) images have revealed that the LbL assembled thin film coatings of FSNPs are uniform and monodisperse. The thickness measurements of the coatings have been conducted by spectroscopic ellipsometry. Water contact angle measurements were performed to analyze the alteration of surface features with functional groups. The roughness, topography, and elastic modulus of the assembled thin-film coatings were probed by atomic force microscopy (AFM). The dispersion of AHAPS-FSNPs has indicated superior colloidal stability than other FSNPs because of their secondary amines creating steric hindrance. Moreover, AHAPS-FSNPs film exhibited the lowest coating thickness and the highest elastic modulus comparing with the other FSNPS because of their crosslinking and hydrogen-bonding ability. Briefly, in this work, LbL assembly of the amine FSNPs was established by an extensive investigation to form rough and functional surfaces for many demanding applications

Cavitation inception and evolution in cavitation on a chip devices at low upstream pressures

The concept of "hydrodynamic cavitation on a chip"offers facile generation of cavitating flows in microdomains, which can be easily scaled up by arranging short microchannels (micro-orifices) in cascade formations. In this regard, microscale cavitation in an energy-efficient test rig has the potential of increasing utilization possibilities of cavitation in a wide range of applications such as liquid-phase exfoliation. In this study, a new experimental test rig was constructed to generate microscale hydrodynamic cavitation. This setup enables cavitation bubble generation at low upstream pressures through the control of the downstream pressure of the device. Particular attention was directed to the classification of flow patterns, scale effects, and cavitating flow evolutions with an in-depth categorization of underlying mechanisms such as Kelvin-Helmholtz instability. Cavitation inception appeared in the form of a single bubble. The appearance of different attached cavitating flow patterns within the microfluidic device was accompanied by new physics, which revealed that cavitation generation and development are affected by the existence of various fluid flow phenomena, particularly the jet flow. The outcome of this study makes hydrodynamic cavitation on a chip attractive for applications, where the cavitation effects are sought in the presence of multiphase fluid flows

Chemical effects in "hydrodynamic cavitation on a chip": the role of cavitating flow patterns

Amongst the advanced oxidation processes (AOPs), hydrodynamic cavitation (HC) has emerged as one of the most cost-efficient, simple and ecologically friendly approaches in the recent decade. This type of the cavitation, in contrast to its counterpart (acoustic cavitation), has a huge potential to upscale to the industrial levels. In the recent years, the micro-scale HC (HC on a chip concept) has exhibited favorable efficacy in terms of nucleation type, surface effects and flow pattern dominancy. In this study, the chemical effects of the HC on a chip concept are shown for the first time by considering the effects of the cavitating flow patterns. So, this is the first attempt to understand the effects of the inception and developed cavitating flow patterns on the chemical reactions during the bubble collapse in the micro-scale. In addition, a particular attention is paid to the chemical reaction effects before the cavitation bubble observation in this investigation. Our results indicated that the triiodide releasing amount was interestingly maximum before the inception occurred, especially at the first cycle. The released amount decreased at the inception and increased for the case of the developed twin cavities. We also showed that, comparing to our previous studies, the cavitation arrived at a relatively lower upstream pressure in the open loop cavitation test rig. Therefore, the outcome of this approach reveals the significance of the in-depth investigations of the complex and very transient nature of the cavitation at different flow patterns. Furthermore, this study implied that reactors benefitting HC on a chip concept will be environmentally friendly tools for producing products from the wastes and worthless materials in the near future

New nanofiber composition for multiscale bubble capture and separation

Bubble dynamics inside a liquid medium and its interactions with hydrophobic and hydrophilic surfaces are crucial for many industrial processes. Electrospinning of polymers has emerged as a promising fabrication technique capable of producing a wide variety of hydrophobic and hydrophilic polymer nanofibers and membranes at a low cost. Thus, knowledge about the bubble interactions on electrospun hydrophobic and hydrophilic nanofibers can be utilized for capturing; separating; and transporting macro-, micro-, and nanobubbles. In this study, poly(methyl methacrylate) (PMMA) and PMMA-poly(ethylene glycol) (PEG) electrospun nanofibers were fabricated to investigate gas bubble interactions with submerged nanofiber mats. To improve their durability, the nanofibers were reinforced with a plastic mesh. The ultimate tensile strengths of PMMA and PMMA-30%PEG nanofibers were measured as 0.35 and 0.30 MPa, respectively. With the use of reinforcement mesh, the mechanical properties of final membranes could be improved by a factor of 70. The gas permeability of the electrospun and reinforced nanofibers was also studied using the high-speed visualization technique and a homemade setup to investigate the effect of electrospun nanofibers on the bubble coalescence and size in addition to the frequency of released bubbles from the nanofiber mat. The diffusion rate of air bubbles in hydrophobic PMMA electrospun nanofibers was measured as 10 L/s for each square meter of the nanofiber. However, the PMMA-30%PEG mat was able to restrict the diffusion of gas bubbles through its pores owing to the van der Waals force between the water molecules and nanofiber surface as well as the high stability of the thin water layer. It has been shown that the hydrophobic electrospun nanofibers can capture and coalesce the rising gas bubbles and release them with predictable size and frequency. Consequently, the diameter of bubbles introduced to the hydrophobic PMMA membrane ranged between 2 and 25 mm, whereas the diameter of bubbles released from the hydrophobic electrospun nanofibers was measured as 8 ± 1 mm. The proposed mechanism and fabricated electrospun nanofibers can enhance the efficiency of various systems such as heat exchangers, liquid-gas separation filters, and direct air capture (DAC) systems

Synthesis of novel Schiff base cobalt (II) and iron (iii) complexes as cathode catalysts for microbial fuel cell applications

In this study, the synthesis and characterization of a new Schiff base and its cobalt(II) and iron(III) complexes were performed fully characterized by common spectroscopic techniques such as 1H-NMR, 13C-NMR, FT-IR, UV–Vis and MS and elemental analysis. The cathodes prepared with only activated carbon, Co-Schiff base complex, and Fe-Schiff base complex mixed with activated carbon as the carrier were examined in single chamber air cathode microbial fuel cells (MFCs). The spectroscopic results confirm the structure of novel Schiff base and its complexes with cobalt (II) and Fe(III). MFC results showed that Fe-Schiff base complex generated higher voltage generation using glucose as the carbon source. Cyclic voltammetry results showed the conductivity and catalytic features of the cathodes developed in this study. Scanning electron microscopic results showed the distribution the complexes on the cathode surface. In conclusion, a novel Schiff base and its complexes with cobalt (II) and iron (III) can be employed into MFC technology to be used in green electricity production, and might help decreasing the operating costs of wastewater treatment plants