Abstract
Electrical conductivity is an important property for technological applications of nanofluids that has not been widely studied. Conventional descriptions such as the Maxwell model do not account for surface charge effects that play an important role in electrical conductivity, particularly at higher nanoparticle volume fractions. Here, we perform electrical characterizations of propylene glycol-based ZnO nanofluids with volume fractions as high as 7%, measuring up to a 100-fold increase in electrical conductivity over the base fluid. We observe a large increase in electrical conductivity with increasing volume fraction and decreasing particle size as well as a leveling off of the increase at high volume fractions. These experimental trends are shown to be consistent with an electrical conductivity model previously developed for colloidal suspensions in salt-free media. In particular, the leveling off of electrical conductivity at high volume fractions, which we attribute to counter-ion condensation, represents a significant departure from the "linear fit" models previously used to describe the electrical conductivity of nanofluids.
Highlights
Nanofluids are created by suspending nanometer size particles in a base fluid [1] and allow for the engineering of fluid properties by changing the type, size, and amount of particles
When dispersed in a fluid, these particles gain surface charge due to the protonation or deprotonation of a surface group such as a hydroxyl ligand (-OH) [4]. This surface charge, which can be adjusted in electrolyte solutions by altering the pH of the suspension [5,6] or chemically treating the particle surface [6], causes an electrical double layer (EDL) of counter-ions to form near the particle surface
Zeta potential The zeta potentials measured for the propylene glycol (PG)-based ZnO nanofluids are given in Table 1 and show only a slight dependence on particle size
Summary
Nanofluids are created by suspending nanometer size particles in a base fluid [1] and allow for the engineering of fluid properties by changing the type, size, and amount of particles. We apply electrokinetic models developed for colloidal suspensions to nanofluids for the first time to explain the large measured increases in electrical conductivity. Using propylene glycol (PG) without any dispersants as a salt-free medium, we measure the electrical conductivities of 20, 40, and 60 nm diameter ZnO nanoparticle dispersions up to 7% volume fraction, applying Ohshima’s model to determine the limiting ionic conductance of the system. This model does not account for overlapping EDLs and is commonly used to investigate the electrophoresis and sedimentation of colloidal suspensions of spherical particles Based on this cell model, Ohshima [16] derived separate analytical expressions for the electrical conductivity K of a salt-free suspension that apply when the particle surface charge Q = 4πεrε0aζ is either less than or greater than a critical surface charge given by: Qcrit. Increasing the amount of counter-ions in this case adds to the condensation region and leaves the charge and potential outside that region unchanged, causing the electrical conductivity to plateau
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