Active micromixers - Micromixers Fundamentals Design and Fabrication - page 245

FIGURE 7.2

Model of a disc spinning at an angular velocity of u. The liquid plug starts at the radial position R i and ends at

R o . The average radial distance between the plug and the disc center is R ¼ ( R o - R i )/2. The width of the

plug is W.

The above relation shows that transversal transport may dominate over radial transport is the angular
velocity if high enough.
Biological actuators are autonomous actuators at nanometer scale using motors from biological
systems [3,4] . These systems can work in an aqueous environment, which exists in many microfluidic
applications. For instance, bacterial actuation can be achieved with biomolecular motors from flag-
ellated bacteria, such as Escherichia coli ( E. coli ) or Serratia marcescens. These bacteria provide
regulatory hooks to build and control flagella. The biomolecular motors can be switched on, and their
direction as well as duration can be controlled.
The bacterial flagellar motor is about 50 nm in diameter and consists of about 20 different parts.
The motor can spin at about 100 Hz in both clockwise and counterclockwise directions. The motors
drive long thin helical filaments that allow cells to swim. Peritrichously flagellated cells, such as
Escherichia coli, search for food using sensors near the surface of the cell. The sensors count
molecules of interest, such as sugars and amino acids, and control the direction of the motor. An E.
coli cell is about 1 m long. Each cell has on average four helical flagellar

filaments. Each filament is driven by a rotary motor at its base. The motors switch the rotational
direction randomly. It's likely that the switch is triggered by a signaling protein. The motors are
powered by protons moving down an electrochemical gradient. This type of bacterial motor can
be used for stirring motion in the nanometer scale and improving transversal

m m in diameter by 2 m

transport in a

micromixer.