ELECTROCAVITATION IN NANOCHANNELS Kjeld G.H. Janssen 1* , Jan C.T. Eijkel 2 , Niels R. Tas 2 , Lennart J. de Vreede 2 , Thomas Hankemeier 1 and Heiko J. van der Linden 1 1 Department of Analytical Biosciences, University of Leiden, THE NETHERLANDS 2 Mesa+ Research Institute for Nanotechnology, University of Twente, THE NETHERLANDS ABSTRACT We present a fundamental nanofluidic phenomenon: The liquid column in a nanochannel filled with two adjacent solu- tions of different conductivity cavitates above a certain threshold voltage, visible as the sudden rapid formation of bubbles in the channel. Under a continued electric field application, the whole channel, 3.5 cm in length, would empty. This effect is relevant for the study of the physics of fluids since it provides a controllable cavitation mechanism, and important for the downscaling of electrophoretic techniques in nanochannels as it puts a fundamental limit on the applicable field strength. KEYWORDS: Nanofluidics, Electrocavitation, Isotachophoresis, Cavitation, Nanochannel INTRODUCTION In the field of the physics of fluids much interest exists for cavitation phenomena and nanobubbles [1]. Cavitation has many applications in biochemical engineering and biotechnology [2]. As a consequence there is a need for a flexible and re- liable method to generate cavitation. Cavitation is provoked by large stresses on the liquid. Here we introduce a new plat- form for cavitation studies based on the controlled generation of negative pressures by the application of an electrical field in a solution with a conductivity gradient. We call this new experimental method electrocavitation. Advantages include the flexible adjustment of experimental parameters influencing the cavitation (applied field, nanochannel height, fluid conductiv- ity) and the advantages of an enclosed chip-based platform (visual observation, interfacing to external equipment). THEORY When a nanochannel is filled with adjacent solutions of high and low conductivity (Figure 1), an applied electrical poten- tial will drop predominantly over the low-conductivity solution. A stronger electroosmotic flow (EOF) will therefore be in- duced in the low-conductivity solution. To satisfy the requirement of mass conservation a pressure gradient results, generat- ing liquid flow against the EOF in the low-conductivity part and with the EOF in the high conductivity part. high conductivity low conductivity L/2 L/2 0 V -500 V X position Pressure 0 ΔP P atm v EOF v pdf v pdf Figure 1. Schematic drawing of a nanochannel containing two adjacent electrolyte solutions, each occupying half of the channel. The difference in conductivity is assumed to be very large. By application of a potential difference between the channel ends, an EOF is induced in the positive direction (arrow) in the low-conductivity side. To assure an equal volume flow along the entire channel length (as imposed by mass conservation), a pressure gradient is generated as indicated at the bottom. It generates pressure-driven flow against the EOF at the right-hand side and with the EOF at the left-hand side (ar- rows). In the channel centre a negative pressure P results, while the exits of the channel are at atmospheric pressure, P atm . As an example of our model, assume that each electrolyte fills half of the nanochannel (Figure 1). Also assume for sim- plicity that the potential drop over the high-conductivity part can be neglected, it follows from the equations of pressure- driven flow and EOF [3] that the generated pressure drop, ΔP, from the reservoirs to the midpoint x in the channel is: 2 6 h V P    (1) 978-0-9798064-4-5/μTAS 2011/$20©11CBMS-0001 1755 15th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 2-6, 2011, Seattle, Washington, USA