WHAT IS MICROFLUIDICS
Microfluidics- A Technology of next generation
Microfluidics is the science and technology that manipulate and control fluidin the range of microliters (10-6) to picoliters (10-12), in the networks of channels with the lowest dimensions from tens to hundreds micrometers.
When the term micro is incorporated in fluidics, following features become an implicit part of the systems.
- Low bond number
- Laminar flow
- High surface to volume ratio
- Predominance of surface effects over volume effects
Why choose a microfluidic device?
Microfluidics is a very attractive technology for both academic researchers and industrials since it considerably:
- Decreases sample and reagent consumptions
- Shortens time of experiments
- Smaller amount of laboratory spaces
- Accuracy and parallelization
- Ultra sensitive detection
- Higher integration
- Reduces the overall costs of applications
Lab on Chip (LOC) are miniaturized devices which are replacing cumbersome equipment by implementing, all function into a few centimeter or less substrate. Microfluidics based lab-on-chip represents alternative to conventional laboratory techniques as it allows achieving complete laboratory protocols on a single chip of few square centimeters. Thus they are able to provide ultra-sensitive detection at significantly lower costs per assay than traditional methods and in a significantly smaller amount of laboratory space. Microfluidics-based systems, capable of continuous sampling and real-time testing of air/water samples for biochemical toxins and other dangerous pathogens.Microfluidics systems are able to perform in impressive number of tasks on a small chip, such as mixing, separating, analyzing, detecting molecules.Such devices allow applications in many areas such as medicine, biology, chemistry and physics.
Different aspects of microfluidics:
1. Reynolds Number : It is a ratio of inertial force to viscous force.
ρ = density of fluid, Dh = Hydraulic diameter
v = mean velocity of fluid, μ = viscosity of fluid
A = Cross sectional area, Pwet = wetted perimeter
Re < 2300, flow is always laminar. When Re > 2300, flow can be turbulent. Due to the small dimensions of microchannels, Re is usually between 1 and 100.
2. BondNumber : It is measures the strength of gravity with respect to surface tension. If the bond number less than unity the effect of gravity neglected and behavior of droplet define by surface tension alone.
3. Surface effects : As a system reduces in size, its surface area to volume ratio increases. Typically, for a micro device, this ratio turns out to be of the order of 106 m, resulting in the dominance of surface effects over volumetric effects.
These technologies are based on the manipulation of continuous liquid flow through microfabricated channels. Actuation of is implemented either by external pressure using micro pumps, or by combinations of capillary forces and electro kinetic.Continuous-flow microfluidic operation is the mainstream approach because it is easy to implement and less sensitive to protein fouling problems. Continuous-flow devices are adequate for many well-defined and simple biochemical applications, and for certain tasks such as chemical separation, but they are less suitable for tasks requiring a high degree of flexibility or ineffect fluid manipulations. Process monitoring capabilities in continuous-flow systems can be achieved with highly sensitive microfluidic flow sensors based on MEMS technology which offer resolutions down to the nanoliter range.
Continuous microfluidic chips are the devices used in microfluidics in which a micro-channels network has been molded and patterned using soft lithography techniques. Such devices allow applications in many areas such as medicine, biology, chemistry and physics. Three types of materials are commonly used to create microfluidic chips: silicon, glass, and polymers.
Typical continuous flow microfluidics experiments/applications involve external pumps, tubes with connectors, camera and a microchip under test.
Alternatives to the above closed-channel continuous-flow systems include novel open structures, where discrete, independently controllable droplets are manipulated on a substrate using electrowetting. Following the analogy of digital microelectronics, this approach is referred to as digital microfluidic. By using discrete unit-volume droplets, a microfluidic function can be reduced to a set of repeated basic operations, i.e., moving one unit of fluid over one unit of distance. Therefore, digital microfluidics offers a flexible and scalable system architecture as well as high fault-tolerance capability.One common actuation method for digital microfluidics is electrowetting-on-dielectric (EWOD). Many lab-on-a-chip applications have been demonstrated within the digital microfluidics paradigm using electrowetting. However, recently other techniques for droplet manipulation have also been demonstrated using surface acoustic waves, mechanical actuation, optoelectrowetting, etc.
Move and control the drop using EWOD
Electrowetting on Dielectrics (EWOD) involves droplet manipulation rather than continuous fluid flow through the microchannels. A drop of liquid, sandwiched between two electrodes, is moved along a defined “electrical” path by applying a potential difference to them when an electrical potential is applied to the droplet, charges accumulate on either side of the dielectric. These charges serve as electrostatic handles that can be used to control droplet position, and by biasing a sequence of electrodes in series, droplets can be made to dispense, move, merge, mix or split on the dielectric surface. EWOD is used to modulate the contact angle of the dielectric. Contact angle modulation together with proper electrode position and design can be used to achieve the elementary droplet processing operations.
As electricity is involved in this process, liquids like water will undergo electrolysis and would disappear after a few probing steps. Hence we need to remember a few points (assuming our working liquid is water):
a)It is crucial to make the electrode surface non-conducting by coating with a kind of material called dielectric.
b)In order to keep the liquid from spreading on the surface and to maintain its shape as a drop, there needs to be another layer on top of the dielectric which makes the surface hydrophobic and will stop smearing the liquid atop.
How EWOD works?
Electrowetting is essentially the phenomenon whereby an electric field can modify the wetting behavior of a droplet in contact with an insulated electrode. If an electric field is applied non-uniformly then a surface energy gradient is created which can be used to manipulate a droplet. Applying an electrical voltage to the electrodes causes the surface tension between solid and liquid interface to decrease, thus lowering the contact which is shown below.
The basic structure of EWOD in open configuration (a)without change of contact angleand (b)with change of contact angle based on an application of voltage between the droplet and electrode