Colloid Polym Sci (2006) 284: 562–567 DOI 10.1007/s00396-005-1414-2 SHORT COMMUNICATION Katsufumi Tanaka Yutaka Fujioka Atsushi Kubono Ryuichi Akiyama Received: 31 May 2005 Accepted: 24 September 2005 Published online: 13 December 2005 # Springer-Verlag 2005 Electrically developed morphology of carbon nanoparticles in suspensions monitored by in situ optical observations under sinusoidal electric field Abstract In situ optical observations were performed for suspensions com- posed of carbon nanoparticles under the sinusoidal electric field with an amplitude around 20 kV/mm (volt per micrometer) and various frequencies. For extremely diluted suspensions of mixed fullerenes or multiwalled car- bon nanotubes (MWNTs) in a silicone oil, the dark-field optical microscopy was effective for the in situ observa- tion of the particle behavior under the electric field. The nanoparticles in a fullerene suspension under the sinu- soidal electric field with a frequency of 100 Hz (in short, 100 Hz electric field) were aggregated to form a rigid spherical microstructure around the halfway between the electrodes. On the other hand, the nanoparticles in an MWNT suspension under 100 Hz electric field were also aggregated but aligned to form a chain-like micro- structure which spans the electrodes. Both of the aggregated particles were stable even after the removal of the electric field, and they were redis- persed by application of 10 Hz electric field. Keywords In situ optical observations . Morphology . Carbon nanoparticles . Suspensions . Sinusoidal electric field Introduction Carbon nanoparticles, such as fullerenes or carbon nano- tubes, have attracted intense interest because of their characteristic structures and physical properties of single particles [1–3]. At the same time, composite materials or suspensions of these nanoparticles, which are incorporated into appropriate media, such as polymers, are also of great interest for applications [4]. In view of material processing, however, most nanoparticles are supplied as powders, which are aggregates of the primary or secondary particles, and the processing of the powders as supplied is usually a hard problem. Therefore, studies on nanoparticles in di- luted and concentrated suspensions are important to have desirable nanocomposite materials. Especially, it is attrac- tive to manipulate and monitor dynamic behavior of nano- particles under external fields such as shear field [5–9], magnetic field [10], and electric field [11–13]. For instance, a chain-like macrostructure of nanoparticles in suspension or polymer composite would be developed by external electric field as well as magnetic field. Up to now, the particle behavior is mostly investigated after processing using external fields. In situ monitoring of the particle behavior during manipulation is highly demanded for pre- cise processing of nanoparticle materials. From the results of in situ monitoring, macroscopically developed mor- phology of nanoparticles in polymer composite would be controlled precisely by appropriate conditions of external fields. In the present paper, in situ optical observations of dynamic behavior of carbon nanoparticles in suspensions are reported under the sinusoidal electric field with an amplitude around 20 kV/mm (volt per micrometer) and various frequencies using the dark-field as well as the bright-field optical microscopy. Preliminary results of the nanoparticle behavior under the electric field are also reported, which was effectively monitored using the dark- field microscopy. Further, electrically developed morphol- K. Tanaka (*) . Y. Fujioka . A. Kubono . R. Akiyama Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan e-mail: ktanaka@kit.ac.jp Tel.: +81-75-7247840 Fax: +81-75-7247800 Present address: A. Kubono Department of Materials Science and Chemical Engineering, Shizuoka University, Hamamatsu 432-8561, Japan