@article{conrad_wu_tang_1999, title={Conductivity in electrorheology}, volume={13}, ISSN={["0217-9792"]}, DOI={10.1142/s0217979299001739}, abstractNote={ Analytical models for the quasi-static ER behavior of suspensions have been developed which take into consideration both the conductivity and dielectric mismatches between the components and the supra-ohmic behavior of the host liquid. Included are models for: (a) slightly conducting particles, (b) insulating particles with a conducting film and (c) highly conducting particles with an insulating film. Both dc and ac fields are treated. Good agreement between predicted and experimental results occur. The models indicate that promising ER suspensions for use over a wide frequency range are those which contain highly conducting particles coated with an "insulating" film which has a hihg dielectric constant and a high electrical breakdown strength. It is conceivable that with proper desigh a yield strength of 20–100 kPa may be obtained with an acceptable current density. }, number={14-16}, journal={INTERNATIONAL JOURNAL OF MODERN PHYSICS B}, author={Conrad, H and Wu, CW and Tang, X}, year={1999}, month={Jun}, pages={1729–1738} }
 @article{wu_conrad_1999, title={Electrorheology of suspensions of si particles with an oxide film in silicone oil}, volume={13}, ISSN={["0217-9792"]}, DOI={10.1142/s0217979299001715}, abstractNote={ The electrorheological (ER) behavior of suspensions of oxidized Si partices in silicone oil was determined with dc electric field E 0. The yield stress decreased with the thickness of the surface oxide film and was proportional to E01.7 when the surface film thickness δ=0.2 μ m . The current density and the field dependent exponent decreased with the surface film thickness. When the surface film thickness δ=0.05μ m , the current density was proportional to E001.3. The results are in accord with our recent non-ohmic conduction model for the influence of an oxide film on conducting particles on ER response. The ER response of highly conducting particles with a low conductivity coating with ac field is also discussed. }, number={14-16}, journal={INTERNATIONAL JOURNAL OF MODERN PHYSICS B}, author={Wu, CW and Conrad, H}, year={1999}, month={Jun}, pages={1713–1720} }
 @article{boissy_wu_fahmy_conrad_1999, title={Experimental study of the yield stress of electrorheological suspensions under AC field: Comparison with a theoretical model}, volume={13}, ISSN={["0217-9792"]}, DOI={10.1142/s0217979299001788}, abstractNote={ Recently a model describing the interaction between the particles of an electrorheological suspension and the resulting properties (yield stress, current density) has been proposed by Wu and Conrad. This model takes into account both the conductivity and the permittivity of each constituent of the suspension and predicts the behavior under DC and AC fields. The goal of the present work is to compare the predictions of this model with data available in the literature and with additional experiments using DC and AC fields at frequencies up to 2000 Hz. The ER fluids used in our experiments are suspensions of different ceramics particles ( Al 2 O 3, ZrO 2, TiO 2, CaTiO 3, BaTiO 3) in silicone oil. These particles cover a wide range of permittivity and conductivity, which allows us to study the ER effect with varying values of the conductivity and/or permittivity mismatch between the particles and the liquid. The model is in reasonable accord with experiment. }, number={14-16}, journal={INTERNATIONAL JOURNAL OF MODERN PHYSICS B}, author={Boissy, C and Wu, CW and Fahmy, Y and Conrad, H}, year={1999}, month={Jun}, pages={1775–1782} }
 @article{wu_conrad_1998, title={Electrical properties of electrorheological particle clusters}, volume={255}, ISSN={["0921-5093"]}, DOI={10.1016/s0921-5093(98)00777-1}, abstractNote={The electrical conductivity of static clusters of ∼220-μm diameter humidified glass beads in silicone oil arranged in a rectangular or cubic lattice was determined under d.c. electric field. The current density flowing through the cluster increased with field strength. It was independent of the number of glass spheres forming a chain in the cluster but increased with the number of chains comprising the cluster. The conductivity of a single chain was about twice the saturation conductivity of the silicone oil. Some possible reasons for the enhanced current density of a cluster of particles compared to an equal number of single-row chains are proposed.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Wu, CW and Conrad, H}, year={1998}, month={Oct}, pages={66–69} }
 @article{wu_chen_conrad_1998, title={Electrorheology of a zeolite silicone oil suspension with dc and ac fields}, volume={31}, ISSN={["0022-3727"]}, DOI={10.1088/0022-3727/31/8/006}, abstractNote={The electrorheology of a zeolite/silicone oil suspension with direct current (dc) and alternating current (ac) electric fields was determined at room temperature. The shear yield stress changed only slightly with the field frequency, but the current density increased considerably. Good agreement occurs between the experimental results and those predicted by our model for both the shear yield stress and the current density. This study shows that there is a significant electrorheological (ER) effect over a large frequency range when a suspension has both a high conductivity ratio and a high dielectric ratio of the particles to the host oil.}, number={8}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Wu, CW and Chen, Y and Conrad, H}, year={1998}, month={Apr}, pages={960–963} }
 @article{wu_conrad_1998, title={Influence of mixed particle size on electrorheological response}, volume={83}, ISSN={["0021-8979"]}, DOI={10.1063/1.366621}, abstractNote={Two sizes (6 and 100 μm average diameter) glass spheres suspended in silicone oil (volume fraction φ=0.3) were used to investigate the influence of particle size on electrorheological response at a shear rate 2 s−1. The larger particles gave a higher shear yield stress but a lower current density at zero shear than the smaller ones. When the two particle sizes were mixed together, the shear yield stress decreased, reaching a minimum when the volume fraction of the small particles equals that of the large particles. The minimum in the current density occurred when the ratio of the volume of small to large particles was ∼1/3.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wu, CW and Conrad, H}, year={1998}, month={Apr}, pages={3880–3884} }
 @article{wu_conrad_1998, title={Multi-coated spheres: recommended electrorheological particles}, volume={31}, ISSN={["0022-3727"]}, DOI={10.1088/0022-3727/31/22/021}, abstractNote={This paper considers the design of electrorheological (ER) particles. Multi-coated particles suspended in insulating (very weakly conducting) oil are recommended for obtaining high-performance ER suspensions. Only the outer two coatings determine the ER strength. The outermost coating should be a material with high dielectric constant, high electrical breakdown strength and a reasonable level of conductivity. The coating immediately below should be a highly conducting material. The inner coatings, including the core (which can be void), can be of any material having such a density that the composite particle has substantially the same density as the host liquid. Our analysis gives that multi-coated particles can have an ER shear strength as high as 29 kPa when the volume fraction of particles is 0.4 and the applied field is 5 kV . Results in the literature provide support for the concept and analysis.}, number={22}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Wu, CW and Conrad, H}, year={1998}, month={Nov}, pages={3312–3315} }
 @article{wu_conrad_1998, title={On the prediction of the shear stress of electrorheological suspensions}, volume={13}, ISSN={["0884-2914"]}, DOI={10.1557/JMR.1998.0449}, abstractNote={Several common methods used for calculating the shear stress and shear modulus of electrorheological (ER) suspensions are reviewed. It is found that when the mismatch ratio of the particle dielectric constant to that of the host liquid in an ac electric field or the ratio of the particle conductivity to that of the host fluid in a dc electric field is >300, no significant difference exists between the predictions of the various methods employed to-date. However, when Λ < 300 the several methods give different estimates for the shear yield stress and shear modulus. Empirical equations are given for a simple estimate of the shear yield stress and the shear modulus when Λ < 300.}, number={11}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Wu, CW and Conrad, H}, year={1998}, month={Nov}, pages={3299–3303} }
 @article{wu_conrad_1998, title={Shear strength of electrorheological particle clusters}, volume={248}, ISSN={["0921-5093"]}, DOI={10.1016/s0921-5093(98)00497-3}, abstractNote={The electrorheological shear strength of clusters of small, humidified glass spheres with an average diameter ∼220 μm in silicone oil forming a rectangular or cubic lattice was determined with d.c. electrical field. The arrangement, shearing, and the yield and fracture processes of the clusters were observed. The shear strength of the cluster increased with the number of chains in a cluster and with the applied electrical field. The shear modulus was essentially independent of the number of chains in a cluster, but increased with applied field.}, number={1-2}, journal={MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING}, author={Wu, CW and Conrad, H}, year={1998}, month={Jun}, pages={161–164} }
 @article{wu_conrad_1998, title={The temperature dependence of the conductivity of silicone oil and related electrorheology of suspensions containing zeolite particles}, volume={31}, ISSN={["0022-3727"]}, DOI={10.1088/0022-3727/31/23/015}, abstractNote={The effect of temperature (22-) on the conductivity of silicone oil was determined employing three heating-cooling schedules. In all cases the conductivity was super-ohmic with a good fit to the equation based on Onsager's dissociation theory, namely where is the conductivity at low electric field and A and are constants which define the field-enhanced ionic dissociation. The parameters , A and varied with temperature; however, the major effect of the heating-cooling schedules was on the parameter A. An analysis of the dissociation process gave that the valence of the conducting ions was in the range 1-2. The observed anomalous effect of temperature on the conductivity of the silicone oil was attributed to the gain and loss of water which occurred during a given heating-cooling schedule. We found good agreement between the measured anomalous temperature dependence of the yield stress (in dc field) of a zeolite-silicone oil suspension and that predicted by the conduction model when the variation of the conductivity of the oil with temperature was taken into account.}, number={23}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Wu, CW and Conrad, H}, year={1998}, month={Dec}, pages={3403–3409} }
 @article{wu_conrad_1997, title={Dielectric and conduction effects in ohmic electrorheological fluids}, volume={30}, ISSN={["0022-3727"]}, DOI={10.1088/0022-3727/30/18/019}, abstractNote={Dielectric and conduction effects in electrorheological (ER) fluids with ohmic conductivity of the host liquids and with an applied AC electric field are considered. It is found that the conductivity ratio , the dielectric constant ratio and the applied field frequency f are three important parameters which determine the ER effect. If , a stronger ER response is obtained with a low applied field frequency than with a high frequency. If , the opposite occurs. Empirical equations are given for the attractive force between the particles in an ER fluid and for the shear yield stress. The calculated current density is independent of the applied field frequency when the frequency is smaller than a critical value, but becomes a linear function of the frequency beyond the critical value. The predicted attractive force of two nearly touching polymer spheres in mineral oil is in good agreement with that measured; also the predicted shear yield stress of some common ER suspensions is in accord with measurements.}, number={18}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Wu, CW and Conrad, H}, year={1997}, month={Sep}, pages={2634–2642} }
 @article{wu_conrad_1997, title={Influence of a surface film on conducting particles on the electrorheological response with alternating current fields}, volume={81}, ISSN={["0021-8979"]}, DOI={10.1063/1.365412}, abstractNote={A method was developed for the frequency dependence of the electrorheological (ER) response of suspensions of highly conducting particles with a low conducting surface film. At dc or low frequency ac field, the shear yield stress of the ER fluids considered is determined by the conductivity ratio σI/σf of the film to the host oil; at high frequency ac field it is determined by the permittivity ratio εI/εf. The critical frequency separating the conduction domain from the dielectric domain is proportional to σf/εf, the ratio of the conductivity to the permittivity of the host oil. To obtain a high shear yield stress at high frequency ac field, a high ratio of the permittivity of the surface film to the oil is desired and a reasonably thin surface film. Too thin a film increases the possibility of electrical breakdown in the film especially at a small ratio of εI/εf. An effective method for overcoming electrical breakdown in the surface film is to increase the permittivity of the film. Good agreement exists between the measured shear yield stress, current density, and the applied breakdown field for oxidized aluminum particles suspended in silicone oil and those predicted by the present model. Recommendations are given for the design of ER fluids with high yield strength and reasonable current density.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wu, CW and Conrad, H}, year={1997}, month={Jun}, pages={8057–8063} }
 @article{wu_conrad_1997, title={Influence of a surface film on the particles on the electrorheological response}, volume={81}, ISSN={["0021-8979"]}, DOI={10.1063/1.364123}, abstractNote={A conduction model is developed for the dc electrorheological (ER) response of highly conducting particles (e.g., metal particles) suspended in a weakly conducting oil. The numerical analyses show that a surface film with some conductivity is desired, but not a completely insulating film as previously proposed. Increasing the film conductivity leads to an increase in the ER yield stress. However, too high a conductivity will give an unacceptable level of current density. The film should also have an intermediate thickness. A small thickness increases the possibility of electrical breakdown in the film; too large a thickness decreases the ER effect. Good agreement exists between the yield stress and the current density predicted by our model and those measured.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wu, CW and Conrad, H}, year={1997}, month={Jan}, pages={383–389} }