Colloids and Surfaces A: Physicochem. Eng. Aspects 339 (2009) 211–216 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa Behavior of carbon cone particle dispersions in electric and magnetic fields Eldrid Svåsand a,b,∗ , Kai de Lange Kristiansen a,c , Ørjan G. Martinsen b,d , Geir Helgesen a,b , Sverre Grimnes b,d , Arne T. Skjeltorp a,b a Institute for Energy Technology, POB 40, NO-2027, Norway b Department of Physics, University of Oslo, NO-0316, Norway c Department of Chemical Engineering, University of California, Santa Barbara, CA 93106, USA d Department of Clinical and Biomedical Engineering, Rikshospitalet, Oslo, Norway article info Article history: Received 4 November 2008 Received in revised form 12 February 2009 Accepted 13 February 2009 Available online 25 February 2009 Keywords: Carbon cones Electrorheology Magnetorheology abstract The behavior of carbon cone (CC) particle dispersions in electric and magnetic fields is presented. The behavior of CC dispersions in an ac electric field was studied by the use of electrorheological (ER) mea- surements. Low ac electric fields were sufficient for fibrilar structures to form. However the structures formed were relatively weak as determined by ER measurements and this was attributed to the high con- ductivity of the particles. This is also consistent with the low relaxation frequency found by impedance spectroscopy. The behavior of a dispersion of CC added ferrofluid was studied using magnetorheological measurements. The relative increase in viscosity was found to be around 1.2–1.5 for low magnetic fields and low shear rates. This was attributed to purely hydrodynamic effects caused by CC particle–particle interactions. © 2009 Elsevier B.V. All rights reserved. 1. Introduction In some particle suspensions subjected to an electric or mag- netic field, the particles form fibrilar structures which may change the properties of the suspension from liquid-like to solid-like. These kinds of suspensions are called electrorheological (ER) and mag- netorheological (MR) fluids and they are used in a wide range of applications [1,2]. By measuring the rheological properties of these fluids the strength or rigidity of the fibrilar structures may be studied. The solid phase of such suspensions may be composed of various materials such as silicate ceramics, conductive organics and polymers. Numerous articles also report the use of carbona- ceous particles of various kinds [3–8]. Some of these suspensions function as ER fluids. In other cases it is the resulting fibrilar struc- tures that are interesting as they may be used for conductive paths in composites [4,7]. In this work a unique material called carbon cone material is used as the solid phase of various suspensions. Lit- tle experimental work has been done regarding this material, hence the basic physical properties are mostly unknown. The material is produced in an industrial scale process named Kværner Carbon Black & Hydrogen process [9]. Particles with conical and disk shape result from the process. The large scale process greatly reduces the material cost making the investigation of this materials inter- ∗ Corresponding author at: Institute for Energy Technology, Department of Physics, Instituttveien 17, POB 40, NO-2027 Kjeller, Norway. E-mail address: eldrid.svasand@gmail.com (E. Svåsand). esting for possible future applications. Compared to carbon black, also produced in industrial scale processes, the carbon cone mate- rial contain particles with semicrystalline structure. In addition the conical particles represent perfect structures with a closed tip, cre- ating unique electronic properties in the tip area [10]. At the present, pure samples representing only one geometry does not exist and finding an efficient purification process has proven difficult. Hence this study investigates the behavior of the mixed carbon particles in a dispersion in an electric and magnetic field. The influence of the particle concentration and the electric/magnetic field on the apparent viscosity is investigated. The ER effect is discussed with regard to the particle conductivity and the relaxation frequency of the dispersion. Possible applications are also discussed. 2. Experimental 2.1. Materials The carbon cone (CC) particles provided by n-Tec AS [11], were used as the dispersant phase in both ER and MR fluids. The particles were used as produced. The particles have various geometries con- sisting of approximately 20% cones, 70% disks and 10% amorphous carbon. The size distribution typically varies from a few hundred nanometers for the amorphous carbon to about 5 m for the largest disks, with most of the particles having dimensions of 1–3 m, as determined by SEM analysis, Fig. 1. For electrorheological studies the dispersive phase used was polydimethylsiloxane (silicon oil) (Dow Corning, density  = 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.02.025