Research
K. H. Kang	Research	Publications	Life Style
Electrohydrodynamic instability of two-layered miscible fluids with different concentrations in microchannels Kwan Hyoung Kang, Jae Wan Park, In Seok Kang,† and Kang Y. Huh Department of Mechanical Engineering and Department of Chemical Engineering,† Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, Republic of Korea (To be submitted to the Phys. Review E) An interfacial instability has been observed for the DC- and AC-powered electroosmotic flow of the two miscible electrolytes layers having different concentrations in microchannels [1,2]. It is rather contrary to our common belief that the flow inside a microchannel is generally stable due to the dominant role of the viscous dissipation. This kind of instability may be disastrous in many microfluidics systems requiring stable transport and little dispersion of species. Sometimes, however, it can be beneficially utilized to augment the mixing of heterogeneous fluids in the microsystems. There is no rational explanation for even the origin of the instability. In this work, the mechanism of the interfacial instability is elucidated numerically within the framework of the Nernst–Planck framework of the transport equations. The numerical simulation reproduces the important features of the interfacial waves observed in the experiment. Figure 1 (a) shows the temporal evolution of concentration distribution together with the velocity vector fields. Note on the similarity of the calculated concentration profile with the photographic images in Chen and Santiago [1]. The crest of wave seems to be draggled due to the convective diffusion by counterclockwise vortex. After examining the influence of the factors such as the velocity gradient and the liquid junction potential, it is concluded that the polarization due to concentration gradient in an imposed electric field is the main source of the flow instability. The time evolution of the instability will be presented. (a) (b) Fig. 1 (a) Temporal evolution of the concentration field in which tc is defined as d/uc, d is the channel width, and uc is Helmholtz-Smoluchowski slip velocity; (b) photographic image taken by Chen and Santiago in case of V= 2.5 kV [1]. The numerical simulation reproduces the important features of the interfacial waves observed in the experiment. Fig. 1 (a) shows the temporal evolution of concentration distribution together with the velocity vector fields. Note on the similarity of the calculated concentration profile with the photographic images in Chen and Santiago. The crest of wave seems to be draggled due to the convective diffusion by counterclockwise vortex. References [1] C. H. Chen and J. G. Santiago (2002) “Electrokinetic flow instability in high concentration gradient microflows,” Proc. 2002 Int’l Mech. Eng. Cong. and Exp., New Orleans, LA, CD vol.1, Paper No. 33563. [2] S. M. Shin, I. S. Kang, Y.-K. Cho, and G. Im (2003) “Instability of electroosmotic flow under time-periodic electric fields,” Anal. Chem. (submitted). Lastly Modified in October 13, 2003