Electric field induced fluid flow on microelectrodes: the effect of illumination

The electrokinetic manipulation of particles suspended in a fluid medium is accomplished using microelectrodes that generate non-uniform fields of significant strength from low applied potentials. The high strength fields produce not only forces on the particles but also on the fluid medium used for suspension. This paper presents qualitative and semi-quantitative observations of the movement of the fluid at applied field frequencies of the order of 1MHz and higher. The importance of the illumination in generating the fluid flow is described, the flow depending on both the intensity of illumination and the applied electric field. The theory of electrothermally induced fluid flow is briefly described and compared with the experimental observations. Reasonable agreement is found between the experiments and the theory, with the light generating temperature gradients, and therefore gradients in fluid permittivity and conductivity, and the electric field responsible for the motive force. (Some figures in this article appear in colour in the electronic version; see www.iop.org) Microelectrode structures, such as those used for dielectrophoresis (DEP) [1], generate high strength, non-uniform ac electric fields. Microelectrodes have been used for the dielectrophoretic manipulation of particles in solution over a range of sizes, from cells (∼10μm diameter) down to viruses (∼100 nm diameter) [2–5]. However, the strong electric fields can also interact with the suspending fluid medium, to produce forces on the fluid and hence flow. The study of this interaction is referred to as electrohydrodynamics (EHD) [6–8]. There are a number of mechanisms through which an electric field can interact with a fluid to produce a force. Recently, we reported a new type of fluid flow occurring in microelectrodes at frequencies below 100 kHz [9–11] arising from the interaction of the non-uniform field and the induced charges in the electrical double layer at the electrode-solution interface. This flow, referred to as ac electro-osmosis, is not fully understood and has been characterized and discussed in other publications [9–11]. This communication is concerned with fluid flow patterns observed in microelectrodes at frequencies around 1 MHz and above [5], where electrode polarization and ac electro-osmosis are negligible. The mechanism has been § Corresponding author. postulated to be electrothermal [7], where the electric field acts on gradients in permittivity and conductivity produced by non-uniform heating of the fluid [6]. For small changes in temperature, where the relative increments in permittivity 1ε/ε and conductivity 1σ/σ are much smaller than 1, the force on the fluid per unit volume is [7]: 〈fE〉 = 1 2 Re [( (σ∇ε − ε∇σ) ·E σ + iωε ) E∗ − 1 2 |E|2 ∇ε ] (1) where E is the applied electric field and i = √−1. The first term on the right-hand side represents the Coulomb force and the second the dielectric force. The gradients of the permittivity and conductivity are related to the temperature gradient by the expressions, ∇ε = (∂ε/∂T )∇T and ∇σ = (∂σ/∂T )∇T . The volume force is dominated by the dielectric force at high frequencies (ω σ/ε). At low frequencies (ω σ/ε), the Coulomb force dominates since, for an aqueous solution, the relative change in the conductivity is greater than that for the permittivity. In this paper, experimental observations and preliminary measurements of fluid flow on microelectrodes at frequencies around the charge relaxation frequency of the medium (ω ∼ σ/ε) are presented and discussed. The previously unreported importance of the intensity of the illumination 0022-3727/00/020013+05$30.00 © 2000 IOP Publishing Ltd L13