4.3 SEDIMENTATION TEST: Sedimentation is the tendency of the particles in suspension to settle down in the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them. These forces may be due to gravity, centrifugal acceleration or electromagnetism. The sedimentation of particles, in the M R Fluid, is determined by neglecting the magnetic effect on the M R Fluid particles. The inherent density difference of the carrier fluid and the iron particles results in the sedimentation of the particles, leaving an upper volume as supernatant fluid (the clarified fluid above the mud line). The upper mud line is the boundary between the supernatant fluid and turbid part of carrier oil. The sedimentation of particles is measured by visually observing the position change of the mud line. For analyzing the sedimentation of the Magneto Rheological Fluid samples, a setup is used and is shown in Fig 4. This set up contains a tube holding device and a ruler which is placed adjacent to the test tube to measure the height of supernatant fluid. With passage of time, the height of supernatant fluid increases and the height of turbid part decreases.
Fig. 4 (a) Sedimentation measurement in actual experiment and (b) Schematic diagram of the setup used to determine sedimentation ratio.
The sedimentation is observed visually at day time to get a clear view of the observation being made for the height of supernatant and the turbid part of the M R Fluid sample. Prepared samples were filled up to 10 cm in a cylindrical glass test tube for few hours at vertical stationary position. The height of the supernatant fluid (Hs) is measured after every 5 minutes with the help of ruler and stop watch. Thus the sedimentation ratio is calculated.
The sedimentation ratio is defined as the ratio of the height of the supernatant oil to the total height of the M R Fluid filled and is given as
SR= Hs /HT X 100 % (2)
Where, SR is the sedimentation ratio, HS is the height of supernatant fluid and HT is the total height of fluid filled in the tube.
This leads to new research challenges and further questions
1) Influencing factors such as particle shape, size working mode
2) Most critical issues is stability against sedimentation
3) Achieving high yield stress
4) High Manufacturing cost
5) Operation in wide range of temperature