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Title

INTERACTIONS BETWEEN NANOFLUIDS AND A SOLID SUBSTRATE: ROLE OF pH AND NANOFLUID PARTICLE CONCENTRATION ON THE THREE-PHASE CONTACT ANGLE

Creator

Horiuchi, Hiroki

Date

2013, 2013-12

Description

We investigate the effect of pH on the interactions between a silica slurry and a silica wafer substrate (TEOS) made by the chemical vapor...
Show moreWe investigate the effect of pH on the interactions between a silica slurry and a silica wafer substrate (TEOS) made by the chemical vapor deposition of tetraethylorthosilicate gas in order to understand the optimization of the Chemical Mechanical Polishing (CMP) process and develop a high-performance silica slurry. The nature of the interactions is probed by the solid-liquid interfacial energy and the electrostatic surface potential at the solid surface. An overview of the CMP process is discussed in Chapter 1. Conventional techniques used to measure the electrostatic potential at the solid/liquid interface, such as the streaming potential and potentiometric titration method, are reviewed. Although there are many techniques for measuring the surface potential of powders, such as potentiometric titration and zeta potential measurement, there is no well-established technique for measuring the surface potential of the silica wafer. Therefore, in this research, we developed a methodology to determine the surface potential and surface charge density of a silica wafer substrate in contact with a slurry. We developed a novel method for calculating the surface potential and surface charge density using the experimental data of the three-phase contact angle in conjunction with the Young- Lippmann and the Poisson-Boltzmann equations. The surface chemistry of silica is discussed in Chapter 2 to elucidate the origin of the surface charge due to the ionization of the silanol groups on a silica wafer. Since the silica wafer is always in contact with the aqueous solution during the CMP process, we specifically focus on the behavior of silica in aqueous solutions. xvi In Chapters 3 and 4, the three-phase contact angle (TPCA) on silica is measured as a function of the pH by the goniometric technique. The surface potential and surface charge density at the silica/water surface are calculated by a model based on the Young- Lippmann equation in conjunction with the Gouy-Chapman model for the electric double layer. In measurements of the TPCA on silica, two distinct regions are identified with a boundary at pH 9.5—showing a dominance of the surface ionization of silanol groups below pH 9.5 and a dominance of the dissolution of silica into the aqueous solution above pH 9.5. Since the surface chemistry changes above pH 9.5, the model is applied to solutions below pH 9.5 (ionization dominant) for the calculation of the surface potential and surface charge density at the silica/aqueous interface. In order to evaluate the model, a galvanic mica cell was made of a mica sheet and the surface potential was measured directly at the mica/water interface. The model results are validated by the experimental data from the literature, as well as the results obtained by the potentiometric titration method and the electro-kinetic measurements. The interactions between the nanofluid and solid surface are explored in Chapter 5. Measurements of three-phase contact angle of the nanofluid on a silica substrate show that the contact angle decreases as the volume fraction increases due to the formation of particle layers on the solid surface. We conclude that it is driven by the depletion attractions between the nanoparticles and a solid surface. In the calculations, the energy (based on Young’s equation) and the structural energy of silica particles (based on statistical mechanics---Henderson’s equation) are in good agreement, indicating that the formation of the nanoparticle layering occurs near the solid surface, as expected. We reference the measurement of the forces between two mica surfaces by Israelachvili and xvii Pashley (1983 and 1984) to calculate the interaction energy against the distance between the two surfaces; we found that the strong repulsive force (hydration force) at a short distance (less than 2 nm) gives rise to enough energy to change the three-phase contact angle. In addition, the calculated inter-particle energy due to the hydration force (by using a statistical mechanics approach, see Trokhymchuck et al. 2001) shows that the hydration force can dramatically increase the inter-particle energy in the case of a volume fraction of water that is from 40 to 45%. The inter-particle energy corresponds to the energy change of 3-5⁰ in the three-phase contact angle, which is same as that found in our experimental data obtained from the contact angle measurements. Finally, we conclude that the hydration force between the silica particles on the solid surface plays an important role in altering the interfacial energy between a solid and liquid.
PH.D in Chemical Engineering, December 2013
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