Measurements of the surface tension of nanofluids and development of a new correlation Jabez Chinnam, Debendra K. Das * , Ravikanth S. Vajjha, Jagannadha R. Satti Department of Mechanical Engineering, University of Alaska Fairbanks, P.O. Box 755905, Fairbanks, AK 99775-5905, USA article info Article history: Received 13 June 2014 Received in revised form 26 June 2015 Accepted 1 July 2015 Available online xxx Keywords: Concentration Nanofluids Particle size Propylene glycol Surface tension Temperature dependence abstract Surface tension measurements were performed on four nanofluids containing aluminum oxide (Al 2 O 3 ), zinc oxide (ZnO), titanium dioxide (TiO 2 ) and silicon dioxide (SiO 2 ) nanoparticles suspended in a base fluid of 60% propylene glycol and 40% water by mass (60:40 PG/W). First, benchmark tests for the surface tension of water were performed, for which accurate data are available in the published literature. Measured data agreed well with the published data confirming the accuracy of the apparatus, as well as the experimental procedure. Following the benchmark tests, measurements were performed on nano- fluids over a temperature range of 30  C to 70  C for particle volumetric concentrations ranging from 0 to 6 % and particle sizes in the range of 15e50 nm. From the experimental data, it was observed that the surface tension of nanofluids decreased with an increase in temperature. At a constant temperature, an increase in the particle volumetric concentration of a nanofluid caused a decrease in the surface tension. For nanofluids at fixed volumetric concentration and temperature, the surface tension was found to be lower for smaller particle sizes except the ZnO nanofluid. A statistical analysis performed on the experimental data yielded a single correlation valid for all the nanofluids tested. This surface tension correlation is a function of temperature, volumetric concentration and the size of the nanoparticles, which predicts results successfully with an average deviation of 2.6% from the measured values. © 2015 Elsevier Masson SAS. All rights reserved. 1. Introduction Surface tension is a force between molecules that develops at the interface between two immiscible fluids and lies on the plane of the interface. As discussed by White [1], at a gaseliquid interface, molecules deep within the liquid repel each other due to their close packing. But, molecules at the surface are less dense and attract each other. At the gaseliquid interface, the upper half of the neighbors of liquid molecules are missing, with the net effect that the interface experiences tension and behaves as a stretched membrane. This intermolecular force of attraction between adja- cent molecules is also regarded as the surface energy per unit area of interface expressed in the unit N.m/m 2 . Traditionally, surface tension is expressed in force per unit length, as milliNewtons/meter or dynes/centimeter. Detailed studies of the phenomena at inter- facial surfaces are presented by Adamson [2]. Generally, the surface tension results available in the literature are for pure liquids. However, when particles are introduced into the liquid the surface tension gets affected, because particles introduce changes at the molecular level. Research has shown that the addition of particles, chemical or surfactant will change the surface tension. The most common measurements of surface tension widely listed in the literature are for the liquideair interface. In this paper we present surface tension measurements for different types of nanofluideair interfaces. The surface tension plays an important role in many heat transfer applications, particularly in boiling and condensation, where bubbles are formed creating interfaces between the liquid and the vapor. 1.1. Boiling heat transfer coefficient enhancement The nucleate boiling is a desirable boiling regime for designing heat transfer equipment. The heat transfer coefficient h under this regime depends on several thermophysical properties listed in Eq. (1) described in Bergman et al. [3]. h ¼ f h DT ; gðr l  r v Þ; h fg ; g; L; r; c p ; k; m i (1) * Corresponding author. E-mail address: dkdas@alaska.edu (D.K. Das). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts http://dx.doi.org/10.1016/j.ijthermalsci.2015.07.008 1290-0729/© 2015 Elsevier Masson SAS. All rights reserved. International Journal of Thermal Sciences 98 (2015) 68e80