2.1)Base fluid: Base fluid has the function of the carrier and naturally combining greasing (in combination with additives) and damping features. For the highest MRF effect the viscosity of the fluid should be small and not very temperature dependent. In this way the MRF effect will be the dominant effect comparing with the natural physical viscosity variation versus temperature and shear stress. Basically in off-state (without a magnetic) MRF behaves like the base fluid according to its chemical composition. There are some different types of liquids which could be used as a carrier fluid i.e. hydrocarbon oil, mineral oil or silicon oils. Due to metal particles content in the fluid it is just “thicker”, so the fluid will have with powder a defined offset in the viscosity in off-state. Usually the dynamic viscosity ? at ambient temperature is around 100mPa.
The primary function of the carrier liquid is to provide a low permeability and non magnetic base liquid in which the magnetically active phase particles remain suspended. The liquid has to posses low permeable in order to allow the particles to polarize with the utmost effectiveness. It, thus, enhances the Magneto Rheological effect. Examples of appropriate carrier liquid include silicone oils, mineral oils, paraffin oils etc. The silicone oil is most frequently used carrier liquid after hydrocarbon oil. In order to keep the off-state viscosity low, silicone oil with viscosity 100 cSt (which corresponds to 0.96 Pa-s dynamic viscosity), is used for synthesis of the M R Fluids samples
The base fluid is an inert or non-magnetic carrier fluid in which the metal particles are suspended. The base fluid should have natural lubrication and damping features. For better implementation of MRF technology the base fluid should have a low viscosity and it should not vary with temperature. This is necessary so that MRF effect i.e. variation of viscosity due to magnetic field becomes dominant as compared to the natural viscosity variation. Due to the presence of suspended particles base fluid becomes thicker. Commonly used base fluids are hydrocarbon oils, mineral oils and Silicon oils.
For the highest MRF effect the viscosity of the fluid should be small and almost independent of temperature. Carrier liquid is the major constituent of MR fluids (50-80 per cent by volume).The commonly used carrier liquids are: Mineral and Synthetic oil.The rate of change of viscosity with respect to the temperature is more in case of mineral oil. Hence, this limits the use of mineral oils as a carrier fluid in MR fluid for low temperature applications. Synthetic oil possess some important properties like higher flash point, does not thicken at high temperatures, lower friction, high shear strength and high viscosity index.
Silicone Oil Silicone oil has good temperature-stability and good heat transfer characteristics, oxidation résistance, very low vapor pressure, and high flash points. But, Silicone oil is very difficult to seal. There is little change in physical properties over a wide temperature span and a relative flat viscosity temperature slope and serviceability from -40 to 204°C. The different properties of these carrier oils are evaluated in Table(2).
Table(2) Properties of Carrier oils
Density at 25 ºC(kg/m3)
-25 ºC to -50 ºC
-30 ºC to -50 ºC
Carrier liquid is the major constituent of MR fluids (50-80 per cent by volume) which greatly influences the rheological properties of MR fluids. Its primary function is to provide a medium for magnetically active particulates to remain suspended during the absence of a magnetic field and to facilitate realignment once magnetic field is applied. But the type of carrier liquid used in a MR fluid may differ. In literature, different carrier liquids have been considered in the preparation of MR fluids and some are detailed here. These are polyvinyl- butyl and naphtha-thickened kerosene8,silicon oil910; white and light grade mineral oils 11a combination of synthetic oil, water and organic liquids12,etc.MR fluids are also available commercially which contain hydrocarbon-based fluid, silicon based fluid, etc. but these options are very costly.
While selecting carrier oils .The following points to be noted
(1)Low viscosity (2) Low density (3) Low freezing point (4) Easily available
(5)Low cost (6) High flash point (7) High fire point (8)Good thermal stability