## 2 Active Systems
### Field-Active Fluids
#### 2.1.1 Electrorheological Fluids  
Back in 1896, [PERSON] [10] discovered that the viscosity of certain fluids, like glycerine or castor oil, exhibited a slight reversible change when an electric field was applied perpendicular to the direction of the flow. Active research on electrorheological fluids (ERFs) started in the 1940s, being worldwide recognized through the pioneering work of [PERSON]. His patent in 1947 [11] comprised what he termed an electro-fluid clutch, an invention that exploits a novel phenomenon of electricity: \"if two plates are separated by certain substantially dielectric fluids containing certain substances the fluid mixture will tend to cause the two plates to act as a unit as long as an electrical potential difference exists between the plates\". Later on, in 1949, he reported the electrorheological phenomena, working with certain suspensions of silica particles in low-viscosity oils. These suspensions exhibited induced shear resistances when bounded by electrodes providing an electric field of the order of 3000 V/mm. He also reported that at low shear stresses, the fluid behaved like a solid, but like a viscous fluid with higher viscosity at larger stresses. The origin of this change in the rheology of these suspensions is in the electrically induced fibration of small particles, that is the tendency to form elongated condensed structures of particles parallel to the field [12]. His research on the electrorheological effect is considered as the origin of the science of electrorheology [13]. Additionally, [PERSON] also patented a field controlled hydraulic device [14] and improved compositions of the type disclosed in his patent granted in 1947 [15]. For all his contributions, the electrorheological phenomena is also known as the Winslow effect.  
Basically, the dielectric or conductivity mismatch between the particles and continuous phase gives place to the electrorheological phenomena. Thus, ERFs consist of particles dispersed in a carrier fluid, where the polarizable components can be either the fluid or the particles. Nevertheless, it is common to find ERFs consisting of colloidal suspensions in which polarizable particles are dispersed in nonpolarizable solvents [16]. ERFs can increase their apparent viscosity by orders of magnitude almost instantaneously upon the application of an electric field, from fluid-like to solid-like, with a yield stress [17; 18; 19; 20; 21; 22; 23; 24] characterizing its strength [25]. That microstructural transition, in which the particles form chains connecting the electrodes once the electric field is active, is reversible once the voltage is removed, as depicted in Figure 1. This fast, strong and reversible response allows the development of simple and efficient electromechanical systems able to control vibrations and dissipate energy in shocks [26; 27]. However, the development of applications has been hindered due to the weakness of the Electrorheological effect (<10 kPa, much below 30 kPa required by many mechanical devices [28]) until the discovery of the giant electrorheological (GER) effect [29], which can reach a yield strength (>10 kPa [28]) above the theoretical upper bound on conventional ERFs. Additionally, while the static yield stress of ERFs shows a quadratic dependence on the electric field, GER effect exhibits a quasi-linear dependence [30].