Comment on “preparation and electrorheology of new mesoporous polypyrrole/MCM-41 suspensions”

Abstract

Electrorheological (ER) fluid, typically composed of polarizable particles and an insulating liquid, is a kind of smart materials which can be characterized by a reversible change from a liquid-like to a solid-like state without or with an electric field, [1–9] along with its magnetically analogous magnetorheological suspensions under an external magnetic field [10–12]. Rheological properties of this ER material such as shear stress and shear viscosity are also altered and tuned according to the electric field. Therefore, many investigators put a focus on this material and dedicate to their potential applications [13, 14]. Recently, Cheng et al. [15] reported a new type of anhydrous ER fluid prepared by dispersing nanocomposite particles (PPy/MCM-41) of conducting polypyrrole (PPy) confined in mesoporous silica (MCM-41) in silicone oil. This ER material was synthesized via a polymerization of pyrrole, which was introduced to the MCM-41 channels prior to the reaction [16, 17], in an aqueous solution of FeCl3 H2O. The PPy/MCM-41 nanocomposite based ER fluid showed Newtonian behavior in the absence of an electric field. However, when an electric field was applied, it was reported to behave like a Bingham fluid with a nonvanishing yield stress (sy) owing to the formation of fibrillar structures [18, 19] of the dispersed particles caused by the inter-particle forces (polarization forces) [20–22]. In this letter, we replotted an original flow curve of the ER fluid based on the PPy/MCM-41 under different electric fields as shown in Fig. 4 of Ref. [8], and then analyzed it using Bingham fluid and De Kee–Turcotte models [23] as well as recently proposed Cho–Choi–Jhon model [24, 25], aiming to have better explanation on flow properties of the novel ER fluid. Compared to the Bingham fluid and De Kee–Turcotte models, the Cho–Choi–Jhon model was found to fit the flow curves much better. Although several reports have focused on this study, it still needs further detailed investigations. A Bingham fluid equation is the simplest model which can be applied to analyze flow behaviors of non-Newtonian fluids, taking the ER fluid as an example here. It is characterized by a yield stress followed by Newtonian behavior, and its relation between shear stress (s) and shear rate ( _ c) is as follows: