Magnetic Nanofluids –Physics and Applications John Philip SMARTS, NDED, Metallurgy and Materials Group Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamilnadu, India E-mail: firstname.lastname@example.org Magnetic nanofluids and nanoemulsions are wonderful model system for fundamental studies and practical applications[1-5]. Recently, we have used magnetic nanofluids to obtain insight into the mechanism of heat transport in nanofluids. Our investigation on the role of microconvection induced by Brownian motion of nanoparticles on thermal conductivity enhancement in stable nanofluids containing nanoparticles of average diameters 2.8–9.5 nm confirm that microconvection is not the key mechanism responsible for thermal conductivity enhancements in nanofluids whereas aggregation has a more prominent role. Further, our studies confirm that the large enhancement in k is due to the efficient transport of heat through percolating nanoparticle paths. We demonstrate that magnetically polarizable nanofluids offer promising applications in „smart‟ cooling devices. Our investigation of magnetic-field-induced changes on transmitted light intensity in a magnetic nanoparticle suspension show a dramatic decrease in the transmitted light intensity at a critical magnetic field. The critical magnetic field follows power-law dependence with the volume fraction of the nanoparticles suggesting a disorder-order structural transition. The circular pattern observed on a screen placed perpendicular to the incident beam confirms the formation of rodlike structures along the direction of propagation of the light. By taking advantage of the ordering property of the ferrofluid emulsions, we have developed new applications for this material to detect defects in ferromagnetic materials and optical filtering. We have developed a force apparatus to measure weak forces between emulsion drops. Our force measurement apparatus facilitates “insitu” measurement of weak forces (10-13 N to 10-11 N) between individual colloidal droplets of the size of about 200nm. The sensitivity for inter-space distance measurement is 0.1 nm . This apparatus has been used to probe polymer-surfactant complexation in presence of surfactants. During my talk, I will give an overview on our activities in the area of nanofluids. P.D.Shima, J. Philip and B. Raj Appl. Phys. Lett. 94, 223101 (2009). J.Philip, P.D.Shima and B. Raj, Appl. Phys. Lett. 91, 203108 (2007). J.Philip, P.D.Shima and B. Raj, Appl. Phys. Lett. 92, 043108 (2008) J. Philip, J.M.Laskar and Baldev Raj Appl. Phys. Lett. 92, 221911 (2008) J.Philip, G.Prakash, T.J.Kumar, P.K.Sundaram , Baldev Raj Phys. Rev. Letts 89 268301(2002). 6. J.Philip, G.Prakash, T.J.Kumar & B. Raj, Macromolecule 36 9230 (2003) 1. 2. 3. 4. 5.