Ultrafine particles are relevant to many practical applications, including froth flotation, emulsion and foam formation, printing, and drug delivery. Such particles have tremendous effect on the interfacial and hydrodynamic properties of the multiphase systems both in static and dynamic conditions. Combining experimental and theoretical approaches, this research scrutinizes a multidisciplinary subject focusing on hydrodynamic and physicochemical characteristics of the ultrafine particles at fluidic interfaces. In particular, the effect of ultrafine particles on the (i) surface activity of the surfactants of different charges, (ii) mobility of the interfaces, and (iii) kinetics of particles attachment are investigated. In the first part of this thesis the effect of negatively charged silica nanoparticles, as model particles for ultrafine particles, on the interfacial behavior of the cationic surfactant (CTAB) is investigated using Profile Analysis Tensiometry (PAT). The results indicate that neither surfactant concentration nor NPs alone can determine the surface tension of the system, and that the ratio of surfactants to nanoparticles is the decisive factor. Below a critical ratio, the surface tension values remain relatively constant as the free surfactant molecules are completely depleted from the solution due to the formation of nanoparticle-surfactant complexes. Above this critical ratio, the number of free surfactant molecules increases as the surfactant concentration increases, causing the surface tension to decrease. In this regime, free surfactant molecules and nanoparticle-surfactant complexes coexist and can co-adsorb at the interface. This is demonstrated by combining high amplitude compression and dynamic surface tension measurements. Furthermore, the effect of negatively charged nanoparticles on the surface activity of anionic surfactant (SDBS) in an aqueous phase is studied. Although recent studies indicate that nanoparticles increase the surface activity of surfactants through electrostatic repulsive forces, it is observed that the surface activity of surfactants is mainly affected by the change in ionic strength of the system due to the presence of nanoparticles. Several characteristic parameters including the equivalent concentration of the surfactant and the critical micelle concentration (CMC) are theoretically calculated and further employed to validate experimental observations. Based on the obtained results, a criterion to estimate the possible influence of the electrostatic repulsive forces for nanoparticles of different sizes and mass fractions is introduced.
In the next step, the effect of ultrafine particles on the mobility of interfaces is investigated. The shear stress of the axisymmetric flow field triggers a nonuniform distribution of the surfactants at the surface of a rising bubble, known as stagnant cap, which gives rise to Marangoni stress that can reduce the mobility of the interface. The conditions in technological processes; however, usually deviate from the linear rise of a single bubble in a quiescent unbounded liquid. Asymmetric shear can act on the bubble surface due to the vorticity in the surrounding flow, or due to the bubble-bubble interactions, which can significantly change the surfactant distribution at the interface. To better understand this effect, Particle Image Velocimetry (PIV) is applied in an experimental setup that is specifically designed to study the hydrodynamics of bubbles and droplets and their interfacial mobility under asymmetric shear flow. Series of PIV experiments are performed with a buoyant bubble/pendant drop at the tip of a capillary placed in a defined shear flow in the presence of surfactants and nanoparticles. A direct experimental observation of the circulating flow at the interface under asymmetric shear, which prevents the formation of the typical stagnant cap is observed. The results show that the interface remains mobile under these conditions regardless of the surfactant concentration. The response of the interface to the surrounding asymmetric flow under successive reduction of the surface area revealed that in the presence of nanoparticles, a contiguous network of particles forms at the interface through densification of surface structures that can drastically change the interfacial mobility of the bubbles and drops. The immobilization is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces, which provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field.
In the last part of this thesis, the kinetics of particles attachment to a buoyant bubble is
investigated. The results showed that the technique can be used to investigate the floatability of different particles as a function of various parameters such as hydrophobicity, particle size, and number density. Furthermore, a specific setup was developed to collect the attached particles and measure their mass and size distribution after collection. For a monomodal particle system, the results indicated almost identical size distribution before and after collection with a slight shift to smaller sizes after collection. For a bimodal particle system, on the other hand, results showed that the majority of the collected particles belong to smaller fractions. Next, the effects of ultrafine particles on the kinetics of particle attachment and the distribution and mobility of particles on the bubble surface is studied. It is shown that the ultrafine particles can increase the attachment rate of fine particles and at the same time decrease their packing density. The presence of ultrafine particles at the interface strongly influenced the distribution of the fine particles on bubble surface. The effect is more pronounced for pre-compressed bubbles, where the dense layer of ultrafine particles on the bubble surface completely prevents the attachment of the fine particles. It is also observed that the mobility of the fine particles at the interface changes significantly when ultrafine particles are adsorbed on the bubble surface.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:88953 |
| Date | 11 January 2024 |
| Creators | Eftekhari, Milad |
| Contributors | Eckert, Kerstin, Butt, Hans-Jürgen, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf (HZDR) |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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