Vol.:(0123456789) 1 3 Journal of Thermal Analysis and Calorimetry https://doi.org/10.1007/s10973-020-10056-8 Comparing the efect of single and mixture surfactants on the improvement in stability and thermal conductivity of CuO nanofuid: an experimental study Iman Fazeli 1  · Mohammad Reza Sarmasti Emami 1  · Mohammad Hossein Nazeri 1 Received: 23 January 2020 / Accepted: 6 July 2020 © Akadémiai Kiadó, Budapest, Hungary 2020 Abstract In the present study, the efect of single and mixed surfactants, including SDS and CTAB, on the stability and thermal con- ductivity of CuO nanofuid is studied. By using a standard two-step method, the nanofuids are prepared. The mass fraction of single surfactants and the CTAB-to-SDS mass fraction ratio of mixture surfactants are considered the parameters of this study. The visual sedimentation technique and zeta potential analysis are employed to evaluate the stability of nanofuid, and the DLS analysis is used to measure the mean diameter of particles. Also, the nanofuid’s thermal conductivity coefcient is measured using the KD2 apparatus and compared with the theoretical models. The results show that the nanofuid containing mixed surfactants has more stability than that of the single one so that the maximum stability is observed at a 95:5 mixture of CTAB–SDS. Also, it is observed that the thermal conductivity of the most stable mixture surfactant is lower than the single one at lower temperatures (20, 25, 30 °C), while at higher temperatures (35, 40, 45 °C), the mixture surfactant demonstrates higher thermal conductivity than the single one. The increases in thermal conductivity of CuO–water nanofuid containing mixture surfactant at 35 °C, 40 °C, and 45 °C are 13.476%, 16.381%, and 16.432%, respectively. Keywords Nanofuid · Stability · Surfactant · Thermal conductivity · Zeta potential Introduction The nanofuid is a suspension of two major parts includ- ing the base fuid such as water, oil, lubricants, ethylene glycol, propylene glycol, triethylene glycol, coolants, ionic liquids and the nanoparticle part like metal, metal alloys, metal oxides, nonmetal and carbon materials, ceramics, car- bides, the mixture of nanomaterials and nanoscale liquid droplets [1–13]. Nanofuids are applied in various felds of sciences and industries, for instance microelectronic, aero- space, transportation, nuclear reactors, military objectives, drilling, sublimation, electric coolants, lubrication, welding, energy storage, heating and cooling systems of buildings and manufactories [14–16]. The heat transfer approach is the most important application of the nanofuids, which has attracted much attention among the researchers during past decades [3, 17–20]. Changing the geometry of the system is one approach to enhance the heat transfer, and gener- ally, heat transfer in rough surfaces is higher than that of the smooth one [21, 22]. However, heat exchangers with complex geometry are faced with some restrictions such as installation, setting up, operating, and repair costs [23]. The conventional fuids, such as water, ethylene glycol, and oils, employed in diferent heating equipment are faced with some fundamental limitations like poor thermal conductiv- ity. Therefore, using nanofuids can overcome this defection by enhancing the thermal conductivity of the base fuid and increasing the heat transfer rate [24–26]. The thermal con- ductivity of Al 2 O 3 –EG/H 2 O and CuO–H 2 O nanofuids was investigated by Choi et al. [27]. The authors revealed that using nanopowders enhances the thermal conductivity of base fuid by about 20%. Thermophysical properties of the nanofuids such as density, specifc heat capacity, thermal conductivity, and viscosity are functions of the nanofuid stability [28–34]. Suitable stability of nanofuid for a long time and non- deposition are necessary for heat transfer applications. The * Mohammad Reza Sarmasti Emami m_r_emami@mazust.ac.ir 1 Department of Chemical Engineering, University of Science and Technology of Mazandaran, P.O. Box 48518-78195, Behshahr, Iran