Active micromixers - Micromixers Fundamentals Design and Fabrication - page 242

The heat for bimorph actuators can come from any external source, such as a focused laser. For
compact designs, the heat is usually generated by a microheater, which is integrated between the two
solid materials or on one side of the bimorph. Because of the required deflection, most bimorph
structures are thin. Thus, the temperature gradient along the thickness is small. Therefore, the position
of the microheater is not significant. Bimetallic actuators offer an almost linear deflection. This
actuator type shares the same disadvantages of other thermal actuators, such as the high power
consumption and the slow response. Because of the linear relation and small hysteresis, feedback
control can be realized with an integrated temperature sensor.
Shape-memory alloys (SMA) are materials such as titanium/nickel alloy, which can return to their
original undeformed shape upon a change of temperature. The actuation concept is based on the phase
transformations of the alloy from a “soft” state (martensite) at low temperatures to a “hard” state
(austenite) at higher temperatures. Because the alloy structure is electrically conductive, heat can be
generated by passing a current directly through it. Titanium/nickel alloys are the most common SMA
commercially available.
Piezoelectricity is an effect that can convert electric energy directly into mechanical energy. An
applied electric field generates a mechanical strain in the material that can be translated into force or
displacement. In contrast to thermal concepts, the direct electrical-mechanical conversion prevents
losses and allows high energy efficiency. Piezoelectric actuators generally generate small strain
(usually less than 0.1%) and high stresses (several megapascals). Therefore, they are suitable for
applications that require large forces but small displacements. Large displacement can be realized by
stacking many piezoelectric layers in parallel. Common piezoelectric materials that are compatible to
microtechnology are polyvinylidene fluoride (PVDF), lead zirconate titanate (PZT), and zinc oxide
(ZnO). PZT offers high piezoelectric coefficients but is very difficult to deposit as a thin film. PVDF
and ZnO are often used in microfabrication. Thin-film piezoelectric actuators usually do not deliver
enough force for pressure disturbance in active micromixers. A more economic way is using external
actuators, such as piezostacks, bimorph piezocantilevers, or bimorph piezodiscs, are commercially
available. Piezostack actuators can deliver large force, while bimorph piezocantilevers allows large
displacement at the expense of small actuation forces.
Electrostatic actuators are based on the attractive force between two oppositely charged plates.
Similar to piezoelectric actuators, electrostatic actuators convert electrical energy directly into
mechanical energy. There are no thermal losses in this actuation concept. The simplest approximation for
electrostatic forces is the force between two plates with the overlapping plate area A , distance d , applied
voltage V , relative dielectric coefficient 3 r , and the permittivity of vacuum 3 0 ¼ 10 -12 F/m: 8.85418

2 3 r 3 0 A V

2

1

f ¼ : (7.7)

d

Themain advantage of electrostatic actuators is the fast response. However, the high voltage and small
deflection make them not suitable for active micromixers with dimensions on the order of millimeter.
Electromagnetic actuators are based on magnetic forces. The force can be created by electro-
magnets of permanent magnets that offer a large deflection. The vertical force of a magnetic flux B in
direction z acting on a magnet with its magnetization m m and volume V is given by:

m m Z d B

f ¼

d z d V

: (7.8)