Microfluidics: Advances in Fluid Dynamics and Engineered Swimming Systems

Chapter 1: Microfluidics

Microfluidics is the study of the behavior, precision control, and manipulation of fluids that are geometrically restricted to a tiny scale (usually sub-millimeter), at which point surface forces dominate volumetric forces. This kind of fluid geometry is often used in microfluidic devices. It is a discipline that draws on a wide range of academic specializations, including engineering, physics, chemistry, biochemistry, nanotechnology, and biotechnology. It may be used in the design of systems that process modest quantities of fluids to enable multiplexing, automation, and high-throughput screening, which are all practical uses of the technology. The field of microfluidics first appeared at the beginning of the 1980s and is now being used to the creation of technologies such as inkjet printheads, DNA chips, lab-on-a-chip technology, micro-propulsion, and micro-thermal devices.

In most cases, the term micro refers to one of the following characteristics:

Small volumes (μL, nL, pL, fL)

Small size

Low amounts of used energy

The impacts of microdomains

Microfluidic systems often transfer, mix, separate, or perform some other kind of processing on fluids. Several different applications depend on capillary forces for passive fluid management, which comes in the form of capillary flow modifying devices. These elements are analogous to flow resistors and flow accelerators. In some applications, extra mechanisms of external actuation are used in order to ensure the medium is transported in the desired direction. The use of rotary motors that produce centrifugal forces in order to facilitate fluid flow on passive chips is one example. The manipulation of the working fluid by active (micro) components such as micropumps or microvalves is what is meant by the term active microfluidics. Continuous delivery of fluids and accurate dosage are both possible with the use of micropumps. Microvalves are the components of a pump that control the flow of liquids and their direction of movement. Miniaturizing procedures that would ordinarily be carried out in a laboratory and placing them on a single chip not only improves efficiency and mobility but also cuts down on the amount of sample and reagent volume required.

When compared to the behavior of fluids at the macrofluidic scale, the behavior of fluids at the microfluidic size may be distinguished by the fact that elements such as surface tension, energy dissipation, and fluidic resistance begin to dominate the system. Microfluidics is the study of how these behaviors evolve over time and how they may be manipulated to serve a variety of new purposes or avoided altogether.

Microfluidic flows need only be confined by geometrical length scale; the modalities and techniques utilized to establish such a geometrical limitation are largely dependent on the desired application. Microfluidic flows may be constrained in a number of different ways.

Traditionally, microfluidic flows have been generated inside closed channels with the channel cross section being in the order of 10 μm x 10 μm.

Each of these approaches comes with its own set of related strategies that have been refined over the course of many years in order to maintain a strong flow of fluid.

Around 2005, new ground was broken in understanding how fluids behave and how to manipulate them in open microchannels.

Flow that is continuous The management of a steady state liquid flow via small channels or porous media is the primary focus of microfluidics, and this control is accomplished primarily by either hastening or stymieing the flow of fluid through capillary components.

Process monitoring capabilities in continuous-flow systems may be obtained using very sensitive microfluidic flow sensors based on MEMS technology. These sensors have resolutions in the nanoliter range and lower, making them ideal for use in continuous-flow systems.

In contrast to continuous microfluidics, the subfield of microfluidics known as droplet-based microfluidics has emerged in recent years.

The microfluidics technique known as droplet-based microfluidics involves the manipulation of discrete volumes of fluids in immiscible phases using flow regimes that have a low Reynolds number.

In recent decades, there has been a notable rise in the amount of attention paid to droplet-based microfluidics devices.

Microdroplets allow for handling miniature volumes (μl to fl) of fluids conveniently, give better blending, encapsulation, sorting, and sensing, These are suitable for investigations requiring a high throughput.

Novel open structures provide an alternative to closed-channel continuous-flow systems like the ones described above. In these structures, discrete droplets that are individually programmable are managed on a substrate by means of electrowetting. The term digital microfluidics was used to describe this methodology when it was shown to be analogous to digital microelectronics. The use of electrocapillary forces to move droplets along a digital track was pioneered by Le Pesant and colleagues, among others.

Paper-based microfluidic devices are filling a rising need in the medical diagnostic industry for systems that are portable, inexpensive, and easy to use. The phenomena of capillary penetration in porous media is essential to the operation of paper-based microfluidics. with the intention of reaching regions that are lacking in technologically sophisticated medical diagnostic equipment.

Particle detection in fluids is one application field that has seen substantial academic work as well as some commercial effort. This is because this is an important application area.

Particle detection of small fluid-borne particles down to about 1 μm in diameter is typically done using a Coulter counter, in which electrical signals are generated when a weakly-conducting fluid such as in saline water is passed through a small (~100 μm diameter) pore, in order to create an electrical signal that has a magnitude that is directly proportional to the ratio of the volume of the particle to the volume of the pore.

The science that underlies this is not too complicated, detailed in detail by DeBlois and Bean in their seminal study, This is the approach that is used to for example.

In order to do a conventional blood analysis, it is necessary to measure and count both erythrocytes (red blood cells) and leukocytes (white blood cells).

This technique is referred to by its general name, resistive pulse sensing, or RPS; The phrase coulter counting is protected under trademark law.

However, the RPS method does not work well for particles below 1 μm diameter, Whenever the signal-to-noise ratio drops below the limit of what can be safely detected, primarily determined by the dimensions of the pore through which the analyte travels and the amount of noise present at the input of the first-stage amplifier.

In conventional RPS Coulter counters, the maximum pore size that may be measured is determined by the process that is used to create the pores.

which, although being a well guarded secret, employs more conventional mechanical processes almost certainly.

Here is where the field of microfluidics has the potential to have an impact: The manufacturing of microfluidic devices by the use of lithography.

or, more likely, the development of reusable molds for the manufacture of microfluidic devices by the use of a molding technique.

is restricted to working with materials of significantly lower dimensions than conventional machining.

Critical dimensions down to 1 μm are easily fabricated, and at the cost of some more labor and money, Reliably patterning features with sizes less than 100 nm is also possible.

This makes it possible to produce holes integrated in a microfluidic circuit at a low cost, with pore diameters that may reach sizes on the order of one hundred nanometers, accompanied by a simultaneous shrinkage of the smallest particle sizes by a factor of many orders of magnitude.

As a direct consequence of this, there has been some development of microfluidic particle counting and sizing carried out at academic institutions, along with the concomitant commercialization of this technology. Microfluidic resistive pulse sensing is the name given to this particular technology (MRPS).

The separation and sorting of various kinds of fluids or cells is a significant area of application for microfluidic devices. Recent advancements in the area of microfluidics have led to the integration of magnetophoresis, which refers to the movement of particles in response to an applied magnetic field, into microfluidic systems. In the case of milk, quite a few of these metal pollutants display paramagnetism, which is really convenient. Therefore, in order to remove any metal impurities that may be present in the milk prior to packing, it may be possible to flow the milk through channels that have magnetic