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Friction Control of a Gel by Electric Field in Ionic Surfactant Solution
J. Phys. Soc. Jpn. 79 (2010) 063602
Control of Sliding Friction by Electric-Field Application
by Hiroshi Matsukawa (Department of Physics and Mathematics, Aoyama Gakuin University)
Published June 10, 2010
Friction is one of the most familiar physical phenomena and has been investigated from ancient age [1, 2]. It plays an important role in various industries as well as in our day-to-day life; however, numerous fundamental problems associated with the mechanism of friction remain unresolved even today. The reasons for this are as follows: i) lack of experimental techniques for the observation of the frictional phenomena occurring at a solid–solid interface, ii) lack of surface-controlling techniques, iii) absence of a general theory of nonequilibrium statistical mechanics, which is required for theoretical studies on kinetic friction, and so on. However, in the last few decades, considerable technological progress has been achieved in terms of experimental apparatuses, sample control techniques, computer simulations, etc.; this has facilitated the study of friction from a new perspective [1, 2].
Hydrogels have attracted considerable attention because of their strong affinity with biosystems, low frictional coefficient, and novel frictional properties . A hydrogel is a kind of gel that contains a large quantity of water as solvent. It is used as food, e.g, agar, as well as in contact lenses, artificial articular cartilage, and so on. The frictional force of a hydrogel on a substrate depends on its apparent contact area and is not proportional to the applied load. This property means that the Amontons–Coulomb’s law of friction, which holds well for various systems, does not hold for this system.
It is also known that the frictional force of hydrogels on a substrate depends on the charge on both surfaces . Based on this observation, Takata et al. successfully controlled the frictional force of the gel by applying an electric field [4, 5]. Such controlling of friction is important in industrial applications, wherein the adequate enhancement and/or reduction of frictional forces are desired, depending on the requirements of the application. It also provides important information regarding the mechanism of friction. Researchers reported the measurement of the frictional coefficient of an electrolyte gel, poly(sodium-2-acrylamido-2-methylpropanesulfonic acid) (NaAMPS) gel, on a SiO2 substrate by applying a dc electric voltage between the gel and the substrate . The frictional coefficient of NaAMPS in the absence of the electric voltage is quite low, approximately 0.02∼0.1. It increases by approximately an order of magnitude when a 70-V dc voltage is applied and returns to its original value after the voltage is switched off. The voltage dependence of the frictional force is quantitatively explained by a model that assumes that the strong adhesion of the polymer-containing SO3- group in NaAMPS is attributable to the positive charge on the substrate induced by the applied electric voltage.
|Fig. 1: Schematic view of the adsorption of ionic surfactant on the substrate by the application of an electric voltage between the gel and the substrate (reprinted from fig.5 in ref. 6 after editing).|
The success of this model lies in the possibility that a reduction in frictional force can be induced by the application of a negative voltage to the substrate. However, electrolysis prevents this from occurring. Therefore, researchers employed a neutral gel, acrylamide (AAm) gel, swollen with an ionic surfactant and successfully reduced the frictional force by applying an electric field . The ionic surfactant used is sodium dodecyl sulfate (SDS), which possesses a negatively charged group. The experimental setup is the same as that used in the previous work . The frictional coefficient of the AAm gel on the substrate decreases by a factor of approximately 3/4 when a dc voltage of 50 V is applied, and it returns to its original value when the electric voltage is switched off. It is considered that the applied electric voltage attracts SDS to the substrate, thereby lubricating the layer and reducing the frictional force between the gel and the substrate, as shown in Fig. 1.
Some previous studies investigated the control of sliding friction by electric-field application in macroscopic [6, 7] systems as well as in mesoscale  and nanoscale  systems. Kimura et al. controlled the frictional force between steel specimens having liquid crystals as a boundary lubricant by using a dc/ac electric field . The frictional coefficient reduced by approximately 25% when a dc electric field of 30 V was applied. This reduction is attributable to the structural changes in the liquid crystal. In another study, the friction between ceramic and metal plates was reported to exhibit electric-field dependence . The mechanisms that govern the frictional-force variations in macroscopic [6, 7] and mesoscale  systems are not completely clear; those reported in [4, 5] are clearer. Thus, it is possible to theoretically predict the magnitude of the variations in frictional force by the application of electric fields. These works also provides a deeper understanding of the phenomena reported in earlier studies [6–8]. Further studies on mechanisms for controlling sliding friction are essential.
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