Michael Curt Elwenspoek

Transient nanobubbles in short-time electrolysis

Water electrolysis in a microsystem is observed and analyzed on a short-time scale of ∼10xa0μs. The very unusual properties of the process are stressed. An extremely high current density is observed because the process is not limited by the diffusion of electroactive species. The high current is accompanied by a high relative supersaturation, Sxa0>xa01000, that results in homogeneous nucleation of bubbles. On the short-time scale only nanobubbles can be formed. These nanobubbles densely cover the electrodes and aggregate at a later time to microbubbles. The effect is significantly intensified with a small increase of temperature. Application of alternating polarity voltage pulses produces bubbles containing a mixture of hydrogen and oxygen. Spontaneous reaction between gases is observed for stoichiometric bubbles with sizes smaller than ∼150xa0nm. Such bubbles disintegrate violently affecting the surfaces of the electrodes.

Chemically anisotropic single-crystalline silicon nanotetrahedra

We describe a method based on silicon micromachining to machine single-crystalline silicon nanoparticles bounded by (111) faces in the form of tetrahedra. The technology allows the fabrication of tetrahedra in a size range from 20 to 1000 nm side length, and gives the possibility to chemically modify sites (faces, edges and/or tips) within certain limits. The chemical modification is anticipated to facilitate the self-assembly into new supermaterials such as photonic crystals in the diamond lattice.

Optimized comb-drive finger shape for shock-resistant actuation

This work presents the analytical solution, finite-element analysis, realization and measurement of comb drives with finger shapes optimized for shock-resistant actuation. The available force for actuating an external load determines how large shock forces can be compensated for. The optimized finger shape provides much more available force than the standard straight finger shape, especially at large displacements. A graphical method is presented to determine whether stable voltage control is possible for a given available force curve. An analytical expression is presented for the finger shape that provides a constant large available force over the actuation range. The new finger shape is asymmetric, and the unit-cell width is equal to the unit-cell width of standard straight fingers that are commonly used, and can be used in all applications where a large force is required. Because the unit-cell width is not increased, straight fingers can be replaced by the new finger shape without changing the rest of the design. It is especially suited for shock-resistant positioning and for applications where a constant force is desired.

A modular assembly method of a feed and thruster system for Cubesats

A modular assembly method for devices based on micro system technology is presented. The assembly method forms the foundation for a miniaturized feed and thruster system as part of a micro propulsion unit working as a simple blow-down system of a rocket engine. The micro rocket is designed to be used for constellation maintenance of Cubesats, which measure 10 × 10 × 10 cm and have a mass less than 1 kg. The feed and thruster system contains an active valve, control electronics, a particle filter and an axisymmetric converging–diverging nozzle, all fabricated as separate modules. A novel method is used to integrate these modules by placing them on or in a glass tube package. The assembly method is shown to be a valid method but the valve module needs to be improved considerably.

Comb-drives: Versatile micro-structures for capacitive sensing and electrostatic actuation

A comb-drive is a structure of interdigitated conductive fingers from which one part is able to move. Electrostatic forces allow the structure to function as an actuator whereas capacitance changes give way to use the structure as a sensor. Such structures became feasible on micrometer scale with the advent of surface micromachining in the late eighties. Since then there has been an overwhelming number of applications of comb-drives both in micro-actuation and sensing. This paper gives an overview of the basic operation of a comb-drive and its applications in sensing and actuation.