Sensors and Actuators A 123–124 (2005) 531–539 Thermal actuator with optimized heater for liquid drop ejectors Antonio Cabal a,∗ , David S. Ross a,b , John A. Lebens c , David P. Trauernicht c a The Archimedes Project, Kaiser Permanente, 848 Bridle Lane, Webster, NY 14580, USA b Department of Mathematics and Statistics, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA c Research & Development Laboratories, Eastman Kodak Company, 1999 Lake Avenue, Rochester, NY 14650, USA Received 20 August 2004; received in revised form 23 May 2005; accepted 25 May 2005 Available online 5 July 2005 Abstract In this paper, we present a novel liquid drop emitter driven by a bimorph cantilever. We describe our experimental and theoretical investigation of the multilayer thermal microactuator as the active component of our microfluidic ejector. We discuss the mathematical models used and compare the predictions of the models with experimental data. We use the models to optimize the performance of our droplet generator. We found the optimal heater length to be 67% of the total length of the microactuator. We compare our full 3D coupled-multiphysics numerical simulations with the performance of optimized real devices. © 2005 Elsevier B.V. All rights reserved. Keywords: Multilayer microfluidic ejector; Bimorph cantilever 1. Introduction Cantilevered structures that use geometrical amplification of electrothermal expansion to generate motion are mainstays of MEMS technology. Such MEMS actuators are subdivided into single layer structures called heatuators [1] and bimetal- lic, or effectively bimetallic, structures [2,3]. We investigated microbeams, like the one shown schemat- ically in Fig. 1. These beams consist of a thin layer of a metal—a titanium/aluminum alloy in the beams that have been built—on a thicker layer of oxide, anchored to the sil- icon wall at one end. Current is run through the metal in order to heat it. Because the metal’s coefficient of thermal expansion is much larger than the oxide’s, heating the beam produces a thermal moment that bends the beam at its free end. The problem of the motion of a microbeam, like the one described above, in fluid, is a difficult one. A full model requires coupling the elasticity equations with the Navier–Stokes equations, the heat equation and the electric ∗ Corresponding author. Fax: +1 585 588 5987. E-mail address: antonio.cabal@kp.org (A. Cabal). potential equation. We devised simple mathematical equilib- rium and dynamic models that describe the essential aspects of this problem. We present the equilibrium model in Section 2. In Section 3, we discuss an embellished dynamic model. In Section 4, we describe the fabrication of the microbeams and the fluid chambers of our drop ejectors. In Section 5, we discuss the experiments and compare the experimental results with the predictions of our 1D dynamic model and a 3D coupled-multiphysics model. In Section 6, we present conclusions about our optimized, fully functioning devices. 2. Equilibrium model We developed and built liquid drop emitters that consist of a multilayer microactuator inside a chamber with an orifice or nozzle. Fluid droplets are pushed out of the nozzle by the motion of the microactuator. The free end of the microac- tuator has a wider circular shape, so that the microactuator resembles a lollypop; we refer to this circular piece as the paddle. This circular region at the free end of the microac- tuator is tightly encased within the chamber to ensure that, when the microactuator is activated, more than 50% of the fluid displaced by its motion is ejected through the nozzle. 0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.05.015