Optimization Design And Performance Study On Shear Thickening Fluid

Shear thickening fluid(STF)is a kind of dense suspensions consisting of nanospheres or microspheres and polar dispersing medium.Beyond a critical shear rate,the viscosity of STF may increase by orders of magnitude.Because of the excellent mechanical properties,STF has attracted considerable interests in the design of soft armor,dampers,impact absorbers and control devices.Over the past decades,researchers have made remarkable progress in material development,application and mechanism based on STF.However,there are still many problems to be solved.Optimization design and mechanical property research of STF are the aim of this work.First,STFs with high performance were developed.By adjusting the pH of STF,the onset of shear thickening could be controlled.After that,a kind of conductive STF(C-STF)was developed and the shear dependent electrical property was investigated.Then,the shear time dependent viscosity of PSt-EA based STF was studied.A structural kinetic model was proposed to illustrate the variation of viscosity and microstructure of STF.Finally,squeeze flow behavior of STF under constant volume was characterized.The details are as follows.1.Preparation of STF and pH effects on shear thickening behavior.First,several kinds of STFs with high performance were developed.The shear thickening effect of polystyrene-ethylacrylate(PSt-EA)based STF was optimized by adding acid or alkali solutions to the suspensions.Rheological test results indicated the shear thickening effect of STF was significantly dependent on pH.The mechanism of pH effects on the shear thickening behaviors was generally explained by the interparticle repulsive force.Zeta potential was used to characterize the particle interactions.It was found that the critical shear rate of STF was linearly related to the Zeta potential.The critical shear rate increased with the increase of Zeta potential(absolute value).When the pH value of STF got away from the isoelectric point,the interaction between particles was large.Thus,the formation of particle clusters was impeded and the shear thickening phenomenon was suppressed.2.The preparation and shear dependent electrical property of C-STF.C-STF was prepared by dispersing CNT into SiO2-EG suspensions.The shear thickening effect of C-STF was strengthened with the increasing CNT mass fraction.Different from the traditional STF,the as-prepared C-STF showed unique electrical property due to presence of the conductive doping CNT.The resistance could be critically decreased once accounting the unexpected shear and the decrements reached to as high as 90%.Meanwhile,impedance spectroscopies at different shear rates were analyzed and an equivalent circuit model was proposed to investigate the electrical property.The impedance spectroscopy response of C-STF was attributed to the microstructure evolution and interface behavior of the electrode-sample system.3.Shear time dependent viscosity of PSt-EA based STF.The influence of the shear rate and shear time on transient viscosity of PSt-EA based STF was investigated.If the shear rate was stepwise changed,it was found that both of the viscosity and critical shear rate were affected by the shear time.Above the critical shear rate,the viscosity of the STF with larger power law exponent(n)increased faster.However,long shear time would delay the onset of shear thickening and weaken the shear thickening effect.This phenomenon can be responded for the reversible structure buildup and broken-down process.An effective volume fraction(EVF)mechanism was proposed to analyze the shear time dependent viscosity and it was found that viscosity changed in proportion to EVF.To further clarify the structure evolution,a structural kinetic model was studied because the structural kinetic parameter(A)could describe the variation of the effective volume fraction.With this model,the change of viscosity and EVF could be speculated from the variation of λ and then the structure evolution was better illustrated.4.Squeeze flow behavior of STF under constant volume.STF was squeezed out radially between the parallel plate accessory of rheometer.Due to formation of particle chains and alignment of clusters,an obvious increase in normal stress was found in the squeeze procedure.The output stress of STF under squeeze was larger than that under shear.Moreover,it was found that normal stress was significantly enhanced at large squeeze velocity,large mass fraction and appropriate wall roughness.Meanwhile,shear viscosity under moving boundary was studied by applying a constant shear on STF during the squeeze test.Together with the shear effects,squeeze could change the inner particle distributions and affected the rheological property of STF.