Active control of electroosmotic ¯ow in microchannels using light Jaisree Moorthy a,b , Christopher Khoury a,b , Jeffrey S. Moore a , David J. Beebe a,b,* a Beckman Institute for Advanced Science and Technology, N. Mathews Ave., Urbana, IL 61801, USA b Department of Biomedical Engineering, 1410 Engineering Dr., Madison, WI 53703, USA Accepted 26 January 2001 Abstract The ability to control bulk electroosmotic ¯ow in microchannels via light modulation is demonstrated. Microchannels 1000 mm wide, 20 mmdeep)werefabricatedandaTiO 2 ®lmwasdepositedontheinnerwallsofthechannel.TiO 2 is a direct semiconductor that exhibits a changeinsurfacechargeuponradiationwithUVlight;leadingtovariationsinthezetapotentialandsubsequent¯owmodulation.Thebulk velocity was measured using the current monitoring technique in the presence and absence of UV radiation. Phosphate buffer solutions of different pH values 2.2, 4.7, 8.4) were tested. The magnitude of the change in velocity due to irradiation varied from 7 to 242 mm/s depending on the pH of the buffer. The largest velocity change was observed at pH 4.7, with smaller changes observed at higher and lower pH values. Parameters in¯uencing ¯ow velocity are also discussed. # 2001 Published by Elsevier Science B.V. Keywords: Zeta potential; TiO 2 ®lm; Flow modulation; UV radiation 1. Introduction Inordertorealizemicrosystemscapableofrapidanalysis of ¯uid samples, multiple functionality is desired. Two examples are capillary electrophoresis CE) coupled with nuclear magnetic resonance [1] and CE coupled with poly- merase chain reaction [2]. In a multifunction microsystem, ¯uid samples must typically be moved from one location to another or routed in different directions for analysis to be completed. Sample routing requires bulk modulation of the ¯ow direction and/or velocity. Also, for carrying out che- mical reactions or diluting samples, mixing different ¯uid streams is required, which can be achieved by modulating the local ¯ow velocity. To incorporate these functions in a microsystem,bothspatialandtemporalcontrolof¯uid¯ow isdesired.Forspatialcontrol,the¯uidvelocityismodulated at different points in the channel. Temporal control allows the ¯ow control to be switched on or off when required. Coating the channel walls with surfactants [3] or polymers e.g. PEG) [4] have been used to reduce the electroosmotic ¯ow EOF) velocity during CE separations. Recently, Abbott and coworkers [5] have demonstrated temporal controlbycoatingthesurfacewitharedoxsurfactant.When electric ®eld is applied, the surfactant is oxidized, changing thesurfaceenergy,whichmodulatesthe¯uiddirection.Lee et al. [6], Hayes and Ewing [7] and Poppe et al. [8] have investigatedradialelectric®eldtocontroltheEOFvelocity. Recently, van den Berg and coworkers [9] have shown that ¯ow switching is possible with radial ®elds. Pulse [10] and AC [11] ®elds are being investigated for mixing ¯ow streams. Although temporal control by radial and AC ®elds has been achieved, fringing effects make it dif®cult to control the velocity spatially [12]. Spatial control of EOF wasdemonstratedrecently[13]wherematerialsofdifferent zeta potentials were patterned inside a channel. To allow both spatial and temporal control we have explored the use of a direct semiconductor whose surface charge can be modulatedwithlight[14].Themodulationofsurfacecharge in¯uences the zeta potential and thus, the velocity of the ¯uid.Flowmodulationvialightcansimplifydevicefabrica- tion by reducing the electrical complexity i.e. reduced lead count) and operation by allowing precise spatial and tem- poral control of the ¯ow. In this paper, the active control of the bulk EOF velocity with light is demonstrated in micro- channels. Direct semiconductors like TiO 2 exhibit a change in the surface charge upon irradiation with light of appropriate wavelength.EarlierreportsbyBardandcoworkers[15]and Boxall and Kelsall [16] have shown that the electrophoretic mobilityofTiO 2 particlesisaffectedbyirradiationwithUV light.MotivatedbytheirworkandthefactthatTiO 2 isstable in aqueous solutions [17], we have used TiO 2 ®lms to coat Sensors and Actuators B 75 2001) 223±229 * Corresponding author. Tel.: 1-608-262-2260; fax: 1-608-265-9239. E-mail address: dbeebe@engr.wisc.edu D.J. Beebe). 0925-4005/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII:S0925-400501)00557-3