STUDY ON SPREADING CHARACTERISTICS OF NANOFLUIDS DROPLET IMPINGING ON SOLID SURFACE1)

The spreading characteristics of a nanofluids droplet impinging on the solid surface are the key factors in efficient heat and mass transfer technology which based on droplet deposition. However, the dynamic behaviors and characteristics of the nanofluid droplet haven't been fully understood since the presence of the non-Newtonian fluid behavior and the interaction between the microstructure of nanoparticle and micro-flow field in the droplet which complicates the spreading process. In this study, we prepared homogeneous and stable nanofluids by dispersing different nanoparticles (multiwall carbon nanotube (MWCNT), graphene and nano-graphite powder) to epoxy resin with two-step method. The rheological behaviors of these nanofluids have been measured and analyzed. The evolution of droplet morphology during the spreading process has been captured by means of high speed camera visualization technique. Based on the image processing technique, the transient dimensionless height, transient spreading factor and dynamic contact angle (DCA) of the droplet have been studied. The results show that the nanoparticles bring the base fluid non-Newtonian shear thinning property. The shear viscosity of test fluid plays important role during the spreading phase and the nanofluid with a lower shear viscosity over the entire range of the shear rate results inlarger variations of the spreading factor and dimensionless height. Nanofluids droplet impacting on the hydrophobic surface could be faster to reach equilibrium condition. The inertial force of impacting droplet dominants the initial spreading process, the spreading variation and velocity are proportional to the impact velocity. This study can provide theoretical basis and specific guidance for the development of gain cooling technology and the manufacture of micro high thermal and electrical conductivity materials.