CHAPTER 2 RHEOLOGY THEORIES AND MATERIAL PROPERTIES
2.2 Materials and nanofluids
0 [1 ]
1
mCa
KCa
2-10
where KCa and m are coefficients used in Carreau model and similar to the Ca K and m in Cross model. The advantage of Carreau model is that the coefficient m is outside of bracket, and gives convenience to mathematical deducing.
2.2 Materials and nanofluids
The nanofluids researched in this study are suspensions fabricated by mixing polypropylene glycol (PPG) liquids, as solvents, and fumed silica particles, as solute.
With various concentrations and combinations of these two material, nanofluids could exhibit different behavior, which have been experimentally and numerically studied in this study. In this part, the properties of PPG and fumed silica will be introduced respectively, and the behaviors and principles of the nanofluid will be discussed.
2.2.1 Polypropylene Glycol, PPG
Propylene glycol is an organic compound with the chemical formula C3H8O2, as shown in Fig. 2-3 (a), and appears as a clear and colorless liquid. As noted by American Agency for Toxic Substances and Disease Registry (ATSDR) [30], the propylene glycol is a “generally recognized as safe” (GRAS) additive for food and medications, and rarely causes toxic effects.
As the polymer type of propylene glycol, the polypropylene glycol series also show as clear and colorless liquid. The comparison of chemical structure between propylene glycol and polypropylene glycol is shown in Fig. 2-7, the subscript n in Fig 2-7 (b) represents the chain length of this polymer; the larger value of n, the longer of PPG chain length will be. To distinguish various types of PPG in the polymer family, most common
way is to name the molecular weight after PPG, such as PPG400, PPG1000, or PPG3000.
Note that molecular weight means the weight of 6 1023molecular, measured in gram;
for example, the weight of 6 1023 PPG1000 molecular will be 1000 g. With larger molecular weight, the chain length of PPG is longer and viscosity is higher. The boiling point is in the range of 230oC to 300oC, which is sufficient for engineering applications, depending on the chain length. Pure PPG fluid is Newtonian fluid possessing constant viscosity, but when undergoes very high shear rate, viscosity will slightly descent. PPG with longer chain length possesses higher boiling point and viscosity.
On the other hand, according to the MSDS (Material Safety Data Sheet) of PPG, pure or high concentration of PPG fluid will cause slightly hazards to skin or eye contact.
However, PPG is a safe and stable liquid if it is used properly. PPG is used as defoamer or surfactant in cosmetic, drink, candy, and yeast industry; also, due to the stable viscosity behavior, it is used as a rheology modifier to adjust viscosity of fluids.
2.2.2 Fumed Silica
Silicon is one of the most abundant resources in the earth, and always combined with oxygen in different crystals, such sand, gemstone, and quartz; among them, silicon dioxide, SiO2, also called silica, is the most common and stable element presenting in plenty of natural objects, including plants, waters and foods. “Fumed silica”, literally, is fumed-like silica type, which means that the crystal particles of silica is so tiny and slight that it could behave as fume under wind blowing. The size of fumed silica crystal particles, briefly ignore the term “crystal” hereafter, are in the range of 5 nm to 50 nm.
There are several ways to produce silica, and each way produces product that possesses different properties and size. The most common producing process, also be the process to produce fumed silica used in this study, is the flame hydrolysis method [31] and
the material is silicon tetrachloride, SiCl4, hydrogen and oxygen. As shown in Fig. 2-8, the first step of this process is to vaporize silicon tetrachloride, and then mix the silicon tetrachloride gas with oxygen, and burn with hydrogen. After passing through flame, a mixture of hot gases containing hydrochloric acid gas and fumed-like silica solid (nanoscale particles) are formed.
For the reason that fumed silica is stable and non-toxic, it is used in a lot of liquid products to modify the rheology properties. For example, original paints are composed of different pigments, which are colorful dilute fluids, and difficult to utilize on vertical stuffs; hence, fumed silica will be used as additive to thickening paints; through aggregation effect of nanoparticles, paints would instantly thicken when painting on vertical objects. Another example with same purpose is white glue.
The viscosity of fluid that contains fumed silica does not directly depend on the particle size. Due to different surface treatment, fumed silica will present totally different properties, such as hydrophilic or hydrophobic.
2.2.3 Nanofluids: The Combinations of Fumed Silica and Polypropylene Glycol Nanofluids are made from PPG and fumed silica; because fumed silica will not dissolve but disperse in PPG, nanofluids are classified as suspensions. The rheology classification of nanofluids, similar to typical suspension, is shear thickening fluid (STF).
The general behavior of a suspension fluid was introduced by Prof. Barnes [32], who noticed it should be avoid that some studies called shear thickening effect as “dilatancy”, because this term implies “an increase of volume on deformation”. Different from the idealized shear thickening behavior that discussed in the last part, the general shear thickening behavior, as sketched in Fig. 2-9, presented in logarithmic coordinates, could be divided into three parts, starting with Newtonian part (or slightly shear thinning part),
shear thickening part, and shear thinning part respectively. These three parts are respectively divided by minimum and maximum viscosities and the corresponding shear rate are and c . Prof. Barnes also mentioned that there are two conditions for m suspensions exhibiting shear thickening effect; the first one is the concentration of suspension must be sufficient, and the second one is the particles in fluid must be either neutral or repel one another by virtue of electrostatic, steric, or entropic interactions.
To further explain the phenomenon of shear thickening effect, Hoffman [33]
postulated a model, with suspension confined in two parallel plates, as shown in Fig. 2-10 (a). Under slowly moving of upper plate, particles in the suspension arrange in layers along y-direction with distances 2h in between. With the increase of moving velocity, the distance between each layer will reduce; according to Fig. 2-10 (b), when 2h is small enough, the van der Walls-London attraction will start to pull layers together. Under this circumstance, the original 2D particle structure will start to form a 3D particle structure, and then causes fluid viscosity steeply increasing. This phenomenon is defined as “aggregation” or “hydro cluster”. However, if the upper plate is keeping accelerating, the distance 2h will continually decrease, and the van der Walls-London attraction will becomes repulsion; this force will push layers far from each other, and causes fluid viscosity decrease again. On the other hand, Boersma et al. [34] used computer to simulate shear thickening effect under different shear rate, and defined few stages of forming aggregation; among them, three typical stages are chosen and shown in Fig. 2-11, such as layered structure, imperfect layered structure, and clusters. Later, in 1996, Srinvasa and Saad [35], prepared the same nanofluids adopted in this study (combined with PPG and fumed silica) and verified the aggregation phenomenon through scanning electron microscope (SEM), as shown in Fig 2-12.
It is noteworthy that even although most of suspensions possess van der Walls-London attraction between particles, the shear thickening effect will only occurs in the suspensions with small particles, especially for nanoscale particles; in other words, van der Walls-London attraction is not capable enough to attract two large particle together. Therefore, the particle size plays a very important role in the nanofluid to determine rheological properties.