Integrated Coulter counter based on 2-dimensional liquid aperture control

In this paper, we present an integrated Coulter counter that applies a liquid aperture. The Coulter aperture is defined by a non-coaxial sheath of a conductive sample liquid that is surrounded on three sides by a non-conductive sheath liquid. The main advantage of this approach is that it prevents channel blocking, since the channel's physical dimensions can be much larger than the Coulter aperture. Furthermore, the dimensions of the aperture can be dynamically adapted to the size of the particle by changing the flow-rates of the sample liquid and the sheath liquid. Both the sample and the sheath liquid are aqueous to prevent unstable flow conditions due to surface tension effects at the interface of the sheath liquid and the sample liquid. Diffusion of ions from the sample liquid into the sheath liquid is limited as long as a minimum flow-rate of 0.5 mu1/s is used. The maximum flow-rate is determined by the occurrence of instable flow behaviour at flow-rates higher than 10 mu1/s. The device can be made in a simple process requiring only four lithographic steps. Measurement results show that a high sensitivity can be obtained in a channel with relatively large physical dimensions. (C) 2003 Elsevier B.V. All rights reserved.