# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 3 Semi-Active Fluid Viscous Dampers
### Rheological Fluid Viscous Dampers  
Rheological fluids are those fluids that change their viscosity in the presence of a magnetic field (MR fluids) or an electric field (ER fluids). This process is completely reversible, and the fluids return to their original viscosity as soon as the magnetic or electric field is removed. The specific behavior of MR and ER fluids is detailed in Sections 3.1.1 and 3.1.2, respectively. The increment in the viscosity is caused by the structure of the fluids' particles, which are polarized under the electrostatic or magnetic field and form columns following the direction of the applied field, as shown in Figure 10. The formation of the column structure allows sustaining shear forces that are applied perpendicularly to the electric field, thereby increasing the viscosity of the fluid [109, 110]. Whenever the column structure is activated, the fluid particles require a minimum amount of force to be moved. The force over which the particles start moving is called yield strength. The higher the applied electric/magnetic field, the higher the force required to move the fluids' particles. A common feature of both types of devices is the presence of two surfaces, which are electrodes in ER dampers and poles in MR dampers. They are used to generate either an electric (ER dampers) or a magnetic (MR dampers) field. ER dampers are divided into three typologies depending on how the rheological fluid operates. MR dampers, on the other hand, are divided into four types, with three of them being the same as those for ER dampers, while the fourth one is unique to MR devices [46]. The first typology operates in flow mode (Figure 11a), which is characterized by two stationary surfaces, and the flow rate of the fluid is modified by varying the intensity of the electric or magnetic field. The second typology is based on the shear mode of operation (Figure 11b), which is based on the application of a relative motion between the surfaces perpendicular to the direction of the electric or magnetic field; this motion produces a shear force on the ER or MR fluid that triggers the flow. The intensity of the applied shear force can be modified through the intensity of the electric or magnetic field. The third mode of operation is the squeeze-flow mode (Figure 11c), which consists of the relative movement of the surfaces in the direction parallel to the electric or magnetic field. The movement of the electrodes induces an alternate tension/compression state of stress in the fluid [46]. Figure 11d shows the magnetic gradient pinch mode, which is a mechanism distinctive to MR devices. This mode works with magnetic poles placed along the flow path of the fluid and separated by a non-magnetic material. This arrangement creates an elliptical magnetic field near the wall of the orifice, which allows the diameter of the orifice to be controlled by changing the  
Figure 10: Behavior of ER and MR fluids (adapted from [46]). intensity of the magnetic field; the stronger the magnetic field, the higher the reduction in the orifice's diameter [111]. Table 1 provides typical ranges of the properties associated with ER and MR fluids in terms of yield strength, voltage power supply, maximum magnetic/electric field, viscosity, density, and size of the particles. Both MRs and ERs provide response times in the order of milliseconds. ER fluids can be easily produced in a laboratory and require a simpler system to be excited by an electric field. On the other hand, MR fluids provide a higher yield strength than ER fluids and require a lower voltage to be activated. Moreover, MR fluids have great stability over a wider range of temperatures [112; 113]. However, MR fluids are heavier due to the presence of iron particles, and they are affected by sedimentation and aggregation [114]. For this reason, sometimes additives are included in the fluids in order to improve the strength between particles.