THE MODIFIED ELECTRODES FOR THE APPLICATION OF POLAR MOLECULE DOMINATED ELECTRORHEOLOGICAL (PM-ER) FLUIDS

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1 Internatinal Jurnal f Mdern Physics B ^fcfik \«i J e Vl. 21, Ns. 28 & 29 (2007) WS Wrld Scientific x ' « Wrld Scientific Publishing Cmpany THE MODIFIED ELECTRODES FOR THE PPLICTION OF POLR MOLECULE DOMINTED ELECTRORHEOLOGICL (PM-ER) FLUIDS XUEZHO WNG, RONG SHEN and GNG SUN Beijing Natinal Labratry fr Cndensed Matter Physics,Institute f Physics, Chinese cademy f Sciences, Beijing, China by CHINESE CDEMY OF BEIJING n 07/08/15. Fr persnal use nly. WEUI WEN Department f Physics, The Hng Kng University f Science and Technlgy,Hng Kng, China KUNQUN LU Beijing Natinal Labratry fr Cndensed Matter Physics,Institute f Physics, Chinese cademy f Sciences, Beijing, China aphy. iphy. ac.cn Received 31 ugust 2007 Fr plar mlecule dminated electrrhelgical (PM-ER) fluids [1], the surfaces f rdinary metallic electrdes cannt satisfy the cnditin f plar mlecules' aligning and interactin. Slide must ccur at the interface between PM-ER fluids and electrdes s that the measured shear stress is much lwer than the true value. Several mdified electrdes are develped and tested fr increasing the adhesin f PM-ER fluids t the electrdes and weakening the slide in shearing. The mdified electrdes can nt nly imprve the measured shear stress t apprach the intrinsic ne, but als greatly reduce the current density f PM-ER fluids. Using the mdified electrdes is certainly necessary fr measuring and applying the PM-ER fluids. 1. Intrductin Electrrhelgical (ER) fluids are cmplex suspensins cnsisting f nanmeter r micrn sized slid particles mixed with insulating il, whse shear stress and yield stress can be cntinuusly tunable with an applied external electric field. While the electric field strength is high enugh, suspensins can be changed int a slid-like state. The reversible transfrmatin f sft t hard state can ccur in abut millisecnd. Thse characters f ER fluids pen up the brad prspects fr their applicatin in industry and technlgy. Hwever, the shear stress f the cnventinal dielectric ER fluids, which are based n the interactin f plarized particles 2 " 6, is quite lw (usually less than lokpa). 2 The lw shear stress f available ER fluids has blcked their applicatin widely. new type f ER fluids ' with high yield stress has been develped in recent years, which are defined as plar mlecule dminated electrrhelgical (PM-ER) fluids. The yield stress f PM-ER fluids can reach hundreds f kpa which is rders higher than that f the cnventinal ER fluids, that wes t the plar mlecules adsrbed n the particles. mdel based n the interactin f plar mlecule-charge in between the particles is prpsed t explain the PM-ER effect. 1 The plar mlecules in the gap turn t the field directin due t the high lcal field and interact with the plarized charges f the neighbring particles. The attractive frce f the plar mlecule-charge can be rders higher than that f pure plarized particles in cnventinal ER fluids. 4940

Electrdes fr pplicatin f PM-ER Fluids 4941 by CHINESE CDEMY OF SCIENCES @ BEIJING n 07/08/15. Fr persnal use nly.

2 Electrdes fr pplicatin f PM-ER Fluids 4941 by CHINESE CDEMY OF BEIJING n 07/08/15. Fr persnal use nly. The essential difference between the PM-ER and cnventinal ER fluids bth in principle and characters results in the difference n measuring methd. Fr cnventinal ER fluids, the interactin between the electrdes and ER fluids is abut the same as inside the ER fluid because f the image effect f the particles at the bundary f metallic electrdes. Therefre the rdinary metallic electrdes can be used fr the measurement and applicatin f the cnventinal ER fluids in principle, althugh sme related experiments abut effect f electrde surfaces n this type f ER fluids were studied. I0 ' u Fr PM-ER fluids, hwever, because the cnditin at the surfaces f metallic electrdes is nt the same as in the gaps between the particles, where the interactin is weak bviusly, the slide f the fluids n the electrdes will ccur when the interactin f plar mlecule-charge is strng enugh in particles' chains. Then the measured yield stress must be much lwer than the exact value f PM-ER fluids. This prblem will als restrain the applicatin f PM-ER fluids. lthugh by rughing the surfaces f metallic electrdes the slide can be weaken, the break dwn is easy t ccur n the surfaces at high electric field. In this paper, several mdified electrdes substituting fr the rdinary metallic nes will be presented. The mdified electrdes can basically avid the slide by increasing the adhesin f material t electrdes. With the applicatin f the mdified electrdes nt nly the measured shear stress is clse t the intrinsic value, but als the current density is reduced essentially fr PM-ER fluids. The principle and thse mdified electrdes can be used in all kinds f devices f PM-ER fluids fr their applicatin. 2. Experimental The calcium titanate (CTO) nanpartical PM-ER Fluid was used in the experiment. Spheral nan-cto particles f 50 nm in diameter were synthesized by means f cprecipitatin methd withut adding any special additives t the particles. 8 Infrared spectrum demnstrated that there were C=0, O-H plar grups cntained in Ca-Ti-O nan-particles. The cntents f C=0, O-H plar grups in the particles were regulated in the washing prcedure during the material manufacture. The CTO particles were suspended in silicne il with the vlume fractin (j) ~ 35%. The yield stresses f CTO PM-ER fluid under different electric field were measured with a parallel-plate rtating rhemeter at shear rate f 0.2s" 1, which were determined apprximately by chsing the maximums n the curves f stress vs. strain. Fr cmparing the surface effect f the electrdes in the yield stress measurement five different kinds f electrdes were used. () Smth metallic electrdes, f which the surfaces were plished carefully. (B) Rugh metallic electrdes, f which the surfaces were grund with a sand paper. (C) The surfaces f the metallic electrdes were cvered with a layer f duble sticking tape 70 (xm thick. (D) The surfaces f the metallic electrdes were cated with alumina. By using plasma spraying technique 1 \mi sized alumina particles were sprayed n the surfaces f electrdes t frm a rugh insulating layer 20 (J.m thick. (E) The surfaces f the metallic electrdes were plated with diamnd grains, f which the grain size was 100 0,m and the layer thickness was 20 u.m. 3. Results and Discussin Because the surfaces f metallic electrdes can nt well satisfy the cnditin f plar mlecules' aligning and interactin in the PM-ER fluids, the slide at the interface between fluids and electrdes must ccur inevitably and be a hindrance in the applicatin and

4942 X. Wang et al. by CHINESE CDEMY OF SCIENCES @ BEIJING n 07/08/15. Fr persnal use nly. measurement fr PM-ER fluids.

3 4942 X. Wang et al. by CHINESE CDEMY OF BEIJING n 07/08/15. Fr persnal use nly. measurement fr PM-ER fluids. By using the abve 5 pairs f electrdes respectively t measure a CTO PM-ER fluid, we fund that the surface effect f electrdes n the measurement f the yield stress is bvius as shwn in the Fig. 1. In rder t btain the exact yield stress f PM-ER fluids an apparatus has been develped, which can measure the intrinsic rhelgical prperty and avid the influence f the electrdes. The measured data with this methd are als shwn in Fig. 1 fr a cmparisn. Under the same electric field strength 5000V/mm, the measured yield stress with smth metallic electrdes (data ) is less than 30kPa, which is nly 1/6 f real value f yield stress (data F). By using rugh metallic electrdes the measured yield stress is 60 kpa (data B) which is duble f that measured with smth metallic electrdes. With ther surface mdified electrdes the measured yield stresses can be much imprved. Especially, by using the electrdes cated with diamnd and alumina grains the measured yield stresses is abut 120 kpa (data D and E), which are abut 4 times f that measured with smth metallic electrdes and can reach 2/3 f the real ne at 5000V/mm. t the rugh surfaces cnsisted b O m B C D li F: 3 Smth melal Rugh metal Duble sticking lumina grains Diamnd grains Exact result. S» lape X W V Electric field (V/mm) Fig. 1. The variatin f yield stress vs. electric field measured with different electrdes. f diamnd and alumina grains the nan-cto particles can fill int the clearances between the grains. The salients f the grains resist the fluid flwing t create a shear stress, which is much higher than that caused by a flat surface and can be estimated t be abut 2/3 f the real shear stress. It can be seen that the values f the measured yield stress in data -E f Fig. 1 deviate frm the exact ne (data F) mre seriusly at high electric field. This phenmenn may cme frm the mechanic prperty f the ER fluid at different electric field. The ER fluid is sticky at lw electric field and is slid at high electric field. The slide f the slid ER fluid n the electrdes can easier ccur. In the dynamic shear stress measurement by using the mdified electrdes cated with diamnd grains, the dynamic shear stress at shear rate f 250 s" 1 can reach abut 70 kpa as the electric field is 3 kv/mm. detailed descriptin will appear elsewhere. Hwever the dynamic shear stress is unable t be measured successfully when the electrdes are made f smth metal due t the slide f the ER fluid n the electrdes. Besides aviding slide, the mdified electrdes can als reduce the electric current thrugh the sample greatly as shwn in Fig. 2. The current density f ur prepared ER fluid is very lw, less than 1 I^/cm. The current density with rugh metallic electrdes is the largest ne. It is easy t cause breaking dwn by using rugh metallic electrdes, althugh applying a file-like metallic electrde can als greatly increase measured yield m T T

4 Electrdes fr pplicatin f PM-ER Fluids 4943 stress. The current densities are reduced greatly, even less than O^/cm, in using the mdified electrdes with cating alumina r diamnd grains because f their insulating prperty. V : Smth metal B: Rugh metal C: Duble sticking tape D: lumina grains li: Diamnd grains by CHINESE CDEMY OF BEIJING n 07/08/15. Fr persnal use nly. s 6c O$O Electric field (V/mm) Fig. 2. The variatin f current density vs. electric field measured with different electrdes. ccrding t the abve results, electrdes play an imprtant rle in the measurement f PM-ER fluids and their applicatin as well. ctually the rdinary metallic electrdes are unsuited t be used in the measurement and applicatin f PM-ER fluids. In principle, if there are the particles with plar mlecules cntained n the electrdes, the bundary cnditin at the surface f electrdes can be satisfied fr the PM-ER effect. Therefre the mst cnvenient way is t glue thse particles n the electrdes. Using the duble sticking tapes is an example. Hwever thse electrdes can nly be used fr few times because f their less firmness. lthugh duble sticking tape is nt strng enugh t be used in applied devices fr a lng time, it's cnvenient t be used fr testing the material. Diamnd and alumina are bth durable, insulated and stable in the range f wrking temperature C, which are suitable fr mdifying the electrdes in the measurement and applicatin f PM-ER fluids. 4. Cnclusin PM-ER fluids based n the interactin f plar mlecule-charge in between the particles are newly develped. The slide f the fluids inevitably exists at the surfaces f metallic electrdes because the interfaces cannt satisfy the bundary cnditin f plar mlecules' aligning and interactin. Cmparing the influence f different electrdes n the ER effect and electric current density f PM-ER fluids, we find that the electrdes are ne f the critical factrs in their measurement. The mdified electrdes cated with diamnd and alumina grains nt nly avid the slide effectively t imprve the measured stress, but als reduce the current density greatly, which are favrable fr measuring and applying the PM-ER fluids. cknwledgment The authrs wuld thank Prf. S.Z.Yang and B.R.Zha fr technical help in the mdificatin f electrdes. This wrk was supprted by the Natinal Natural Science

5 4944 X. Wang et al. Fundatin f China (Grant N ), the Natinal Basic Research Prgram f China (N 2004CB619005), the Knwledge Innvatin Prject and Outstanding Overseas Chinese Schlars Fund f Chinese cademy f Sciences and NSFC. by CHINESE CDEMY OF BEIJING n 07/08/15. Fr persnal use nly. References 1. K. Q. Lu, R. Shen, X. Z. Wang, G. Sun, W. J. Wen and J. X. Liu, in this prceedings. 2. H. R. Ma, W. J. Wen, W Y. Tarn and P. Shen, Phys. Rev. Lett. 77, 2499 (1996). 3. T. C. Halsey, Electrrhelgical fluids, Science 258, 761 (1992). 4. R. Ta and J. M. Sun, Phys. Rev. Lett. 67, 398 (1991). 5. Y. Chen,.R Sprecher, H. Cnrad, Electrstatic particle-particle interactins in electrrhelgical fluids, J. ppl. Phys. 70, 6796 (1991). 6. L. C. Davis, J. ppl. Phys. 81, 1985 (1997). 7. R. Shen, X. Z. Wang, W J. Wen, K. Q. Lu, Internatinal J. Mdern Phys. B 19, 1104 (2005). 8. X. Z. Wang, R. Shen, W J. Wen, K. Q. Lu, Internatinal J. Mdern Phys. B 19, 1110 (2005). 9. R. Shen et al, in this prceedings. 10. C. Y. Lee, K. L. Jw, Material & Design 22, 277 (2001). 11. R. Hanaka, M. Murakum, H. nzai, K. Sakurai, Transactins n Dielectrics and Electrical Insulatin 9, 10 (2002).

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