|March 23/30, 2005|
|Molasses and dough
Shrinking pipes and test tubes to the scale of computer chips promises to dramatically improve how people diagnose illnesses, monitor the environment and make medicine. But it’s not enough to simply make tiny pipes. The minuscule amounts of fluid that flow through those pipes behave differently than the larger flows we’re used to seeing in the everyday world.
Viscosity, for instance, becomes a relatively large force, making pumping water more like pumping molasses. This happens because different portions of flowing liquid travel at different speeds. Fluid very close to pipe walls, for instance, moves more slowly than fluid at the center of the pipe, and the effect is amplified in smaller channels. Viscosity can be thought of as a type of friction between fluids traveling at different speeds.
And when viscosity is high, turbulence disappears. The phenomenon responsible for bumpy airplane rides also causes liquids to easily mix at the macro scale. A few swirls generates enough turbulence to thoroughly mix a teaspoon of cream into a cup of coffee, for instance. At the small scales that lack turbulence, however, mixing liquids becomes very like kneading dough.
Pumping and mixing
Several traditional pumping methods have been scaled down for use in microfluidics, including pneumatics, peristalsis and mechanics. Pneumatics methods release compressed air or another gas through tiny pins inserted into microfluidic channels. The pressurized gas expands in the channels, pushing fluids ahead of it.
Peristalsis, which is how the heart pumps blood and the intestines move food, involves compressing a chamber to expel the fluid it contains. A micromechanical actuator or a series of actuators can compress the walls of a channel to push fluids through.
Mechanical pumps are popular at the large-scale, but it has proven difficult to make microscale versions of them because they’re generally powered by motors.
The simplest methods of microfluidic mixing draw on geometry. The shapes and orientations of channels in microfluidics systems can induce the necessary kneading action. Just sending fluids around a sharp been increases the rate of mixing.
Microelectronics provide another avenue for moving and mixing fluids at the microscale. Electricity can move and mix fluids in several ways:
· Electrocapillary Pressure
Electroosmosis is a flow resulting from the combination of electric voltage and the chemical reaction between a solid surface and a liquid. At the boundary between a liquid and a solid, a negative charge builds up on one side and a positive charge on the other. Putting a pair of electrodes some distance apart in the fluid and applying a voltage causes the charged atoms, or ions, in the liquid to move in the direction of the oppositely charged electrode, and friction drags the rest of the fluid along. Using arrays of electrodes, researchers can make fluids flow in patterns that enhance mixing.
Magnetohydrodynamics is the interaction between electrically conductive fluids and magnetic fields. Applying a magnetic field to a portion of a channel causes a charged fluid to flow toward or away from that portion of the channel, depending on the polarities of the fluid’s electric field and the magnetic field. Causing fluids to flow in patterns can enhance mixing.
A network of electric circuits beneath a glass surface can move droplets in precise patterns and merge and split droplets. The key is surface tension, the effect responsible for droplets holding their shapes rather than spreading out.
Surface tension has to do with cohesive forces between liquid molecules. Molecules at the surface of a liquid behave differently than molecules in the bulk of the liquid because surface molecules do not have molecules on all sides of them.
Molecules are attracted to neighboring molecules on all sides. Because molecules on the surface are not subject to an attractive force above them, they are pulled into the interior. This makes the surface of the liquid rearrange until the least number of molecules possible are present on the surface. Liquid naturally forms into drops, or spheres because the shape has the minimum surface area for a given volume. The surface molecules are also pulled more closely together than the rest of the molecules in the liquid, to form a film-like surface.
Electric fields affect the forces between molecules and therefore can be used to manipulate liquids.
Electrowetting is the technique of using an electric field to modify the wetting behavior of a droplet. Applying an electric field to one side of a droplet lowers the surface tension on that side, causing the droplet to flow in that direction.
Electrocapillary pressure taps electricity to change the surface tension between the inner wall of a channel and the fluid within. Applying an electric charge to the surface reduces the surface tension. Reducing the surface tension in one portion of the channel causes the fluid to flow toward that portion of the channel.
Temperature and surfaces
Temperature also affects surface tension, and researchers have used tiny heating elements to move droplets across a surface.
Liquids can also be moved by using electricity to change the surface properties of a film from hydrophobic, or water repelling, to hydrophilic, or water-attracting. This process causes a droplet to bead or spread out on the surface.
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