The present subject matter relates to electrorheological fluids and the role of particle-fluid wetting surfactants in inducing the electrorheological effect formed by particles in fluid suspension. Of particular interest is the role of particle-fluid wetting surfactants in lowering sedimentation rates.
Electrorheological (ER) fluids are a type of colloidal suspensions, comprising micro-particles or nanoparticles dispersed in non-conducting oil. The rheological properties (apparent viscosity) of ER fluids can be continuously and reversibly adjusted from fluid to solid and back again in response to an electric field. Specifically, under application of a 1-5 kV/mm field, ER fluids will exhibit solid-like behavior, such as the ability to transmit shear stress. The transition time from liquid-like behavior to solid-like behavior can occur on the order of 1 to 10 ms. This phenomenon is known as the ER effect, and this change in apparent viscosity is dependent on the applied electrical field. The change is not a simple viscosity change. Instead, the ER effect is more correctly defined as an electric field dependent shear yield stress, wherein the yield point of the ER fluid is determined by the electric field strength. After the yield point is reached, the fluid shears as a fluid, and consequently the resistance to motion of the ER fluid can be controlled by adjusting the applied electric field.
One problem encountered with ER fluids is that the yield strength is too low for many practical applications. The yield stress of known ER fluids is typically not more than 5 kPa at 3 kV/mm which is inadequate for most of the potential uses of ER fluids. A further problem is the tendency for ER fluids to undergo sedimentation.
The discovery of the giant ER (GER) effect was realized by using urea-coated nanoparticles and has broken the theoretical upper limits of the traditional ER effect. With a controllable solid-liquid phase transition having a reversible response time of <10 ms, GER fluids can sustain higher yield stress over many other ER fluids. Despite improved performance, GER fluids still display stability issues with respect to particle sedimentation, and thus have not improved upon this aspect of ER fluids, generally.
Various attempts at improvements to these drawbacks to ER and GER fluids have been made. Adding surfactant to the solvent phases or making the particle less dense can either decrease the density mismatch or modify the surface or particle morphology, or both. While surfactants have been shown to improve the sedimentation property of the GER fluids, they tend to generally lower the GER effect and required a current density increase as a result. Previous experiments conducted with GER fluids indicate the activity of a surfactant depends strongly on its polarity. Lipophilic surfactants stabilized the suspension but at the expense of about 30% decrease in the yield stress, simultaneous with a reduction in the current density. Hydrophilic surfactants hardly stabilized the suspension but an increase of yield stress was observed that was not accompanied by an increase in current density.
Shen et al. (Wetting-induced Electrorheological Effect, J. Appl. Phys. 99, 106104 (2006)) demonstrated that by adding a small amount of oleic acid to nanoparticles of barium titanyl oxalate coated with urea suspended in hydrocarbon oil produced a high yield stress. However, this dramatic increase of the dynamic yield stress also coincided with a sharp increased current density. Likewise, Li et al. (Giant Electrorheological Fluid Comprising Nanoparticles: Carbon Nanotube Composite, J. Appl. Phys. 107, 093507 (2010)) added oxide-carbon nanotube composites to the synthesis of urea-coated particles. The particles, when dispersed in different types of silicone oil, were shown to have enhanced anti-sedimentation property. The yield stress, however, has shown a 10% reduction.
Carlson, in U.S. Pat. No. 5,032,307, attempts to bypass sedimentation problems by using a surfactant as the particle component of an ER fluid; Carlson teaches water-miscible electrorheological materials containing a carrier fluid, a combined non-abrasive, anionic surfactant-particle component, and an activator.
Okada et al., in U.S. Pat. No. 5,558,803, discloses an ER fluid capable of generating a large shear stress while exhibiting excellent current property and durability. Okada et al. rely on dielectric particles and a dielectric particle absorbing structure.
Pialet et al., in U.S. Pat. No. 5,558,811, discloses good dispersive stability by use of an aromatic hydroxyl compound substituted with a hydrocarbyl group containing at least 6 carbon atoms in a carbon-based hydrophobic base fluid.
In an effort to overcome the drawbacks of known ER and GER fluids, the present subject matter is directed to compositions and methods for introducing surfactant additives to GER fluids that enhance stability without the usual drawbacks. Specifically, the instant subject matter seeks to circumvent the known restriction that increased yield stress is accompanied by increased current density. Accordingly, by adding a polar molecule additive, the inventors have found that dynamic yield stress can be enhanced over 50%, while the current density is reduced dramatically. The reversible response time remains the same and the sedimentation stability is greatly enhanced. Long-term reliability problems are reduced as a result of the low sedimentation rates and improved redispersion rated in the fluids. The improved GER fluid is expected to facilitate its application in car clutches, fluid brakes, and vehicle shock absorbers, etc.