Chapter 1
Introduction

This chapter introduces the terminology related to microfluidics and its practical applications. The historical perspective of this emerging discipline is introduced first. Subsequently, different natural systems with microscale structure and transport phenomena are discussed and correlated to the microfluidic systems. Various practical examples of microfluidic devices are presented to illustrate the importance of studying transport processes in microscale. Scaling laws are used to demonstrate different flow physics of small-scale devices.

1.1 History

The year 1959 is considered as the beginning of microtechnologies and nanotechnologies. In December 1959, R.P. Feynman gave a visionary speech during the American Physical Society meeting at Caltech entitled "There is plenty of room at the bottom".

The beginning of the speech was as follows:

"I would like to describe a field, in which little has been done, but in which an enormous can be done in principle. This field is not quite the same as the others in that it will not tell us much of fundamental physics (in sense of "what are the strange particles?") but it is more like solid-state physics in the sense that it might tell us much of great interest about the strange phenomena that occur in complex situations. Furthermore, a point that is most important is that it would have an enormous number of technical applications."

Some other excerpt of his speech is:

"How many times when you are working on something frustratingly tiny like your wife's wrist watch, have you said to yourself, 'If I could only train an ant to do this!' What I would like to suggest is the possibility of training an ant to train a mite to do this. What are the possibilities of small but movable machines? They may or may not be useful, but they surely would be fun to make."

Feynman's suggestion didn't remain in fantasy world. The first microbeam was fabricated in 1982 and the first microspring was fabricated in 1988. Microfluidics emerged in the beginning of the 1980s and has been used in the development of ink-jet printheads, DNA chips, lab-on-a-chip technology, micropropulsion, andmicrothermal technologies. In 1995, the word IBM was spelled out using only few atoms. Microfluidics is a field associated with flows that are constrained to small geometries, where the characteristic dimensions are of the order of few hundred microns. It deals with the behavior, precise control, and manipulation of flows at submillimeter scale. It is a multidisciplinary field intersecting engineering, physics, chemistry, microtechnology, and biotechnology, with practical applications to the design of systems in which small volumes of fluids are used.

1.2 Definition

A fundamental question initially arises about the definition of microfluidics. The basic meaning of this terminology is the flow at small scales. The primary advantage is the utilization of breakdown phenomena in scaling laws for new effects and better performance. Hence, the importance is not the size of surrounding instrumentation and the material of the device but the space where the fluid is processed. The minimization of the entire system may be beneficial but is not a requirement of a microfluidic system. The key issue of microfluidics is the microscopic quantity of fluid in which small-scale causes change in fluid behavior.

There are different points of view regarding device size and fluid quantity for the definition of microfluidic device. The microelectromechanical systems (MEMS) terminology indicates that the device size must be smaller than 1 mm. Electrical and mechanical engineers are interested to work on microfluidics because of their fabrication capabilities using microtechnology. Their idea is to shrink the device size and thus define microfluidics in terms of size to take advantage of the new effects and better performance. The objective is to shrink down the pathway of the chemicals. Another preferred way to define microfluidics is based on fluid quantities. Figure 1.1(a,b) shows the size and volume characteristics of different microsystems.

Figure 1.1 (a) Size characteristics and (b) volume characteristics of different microsystems

Nanodevices are of size less than 1 m. Human hair is between 1 m and 1 mm, and has similar size as microsystem. Microneedle, micropumps, microanalysis system, and microreactor are best defined based on the volume of fluid handled. Microanalysis system handles fluid volume more than 1 l. Microneedle handles fluid volume between 1 pl and 1 l. Microreactors handle fluid volume between 1 pl and 1 ml.

1.3 Analogy of Microfluidics with Computing Technology

Many days/hours of computing are required to perform numerical simulation for weather forecasting and various computational fluid dynamics applications. The development of parallel architecture in modern computers has contributed significantly to speed up the computational speed for these applications. Similar to parallel computer, microfluidics can revolutionalize chemical screening power. Compared to manual and bench-top experiments, microfluidics can allow pharmaceutical industry to screen combinatorial libraries at high throughput. A microfluidic assay can have several hundreds to several thousands parallel processes in comparison to few hundreds parallel processor of parallel computer. This high-performance capability is important for DNA-based diagnostics in pharmaceutical and healthcare applications.

1.4 Interdisciplinary Aspects of Microfluidics

Owing to the rapid development of microfabrication technologies, it is now possible to miniaturize mechanical, fluidic, electromechanical, and thermal systems to micrometer sizes for various applications. This new development led to the creation of a new discipline known as microfluidics. Microfluidics is defined as the study of flows, which can be simple or complex, mono- or multiphasic circulating in artificial microsystems, that is, systems fabricated using new technologies, namely, etching, photolithography, and microimpression.

Research on microfluidics has become a truly multidisciplinary field representing almost all traditional engineering and scientific disciplines. Initially, microfluidicsdeveloped as a part of MEMS technology using the available infrastructure and technology of microelectronics. Electrical and mechanical engineers are interested in contributing to the fabrication technology related to microfluidics. Fluid mechanics researchers are interested to study the new phenomena in fluid flows. The flow physics in microfluidic devices is governed by a transitional regime between the continuum and molecular dominated regimes. There are a new class of fluid measurement tools for microscale flows using in situ microinstruments in addition to new analytical and computational models. Microfluidic tools allow the life scientists and chemists to explore new effects that are not possible in traditional devices.

1.4.1 Microfluidics in Nature

Microfluidics is not limited to "systems made by man." Nature also produces micrometric systems with impressive characteristics having controlled circulation of fluids.

One example is tree. The question is: how can a tree bring water and nutrients to the leaves? Nature used a complex network of capillaries to achieve this (see Figure 1.2). The trunk of the tree has microcapillaries of a hundred micrometers size and leaves have microchannels of size equal to several tens of nanometers. Air on leaf surfaces causes water to evaporate creating a pull in the water column. There is transport of water into the cells of the root by osmosis. There is simultaneous active transport of sucrose from leaf cells into the cells of the root or stem. The supply of food carrying liquids, that is, sap, is homogeneous despite the complexity of the network. The pressure drop in the complex network is significant, that is, several tens of bars, implying that the sap is subjected to negative pressure. The hydrodynamic of this system involves consideration of deformability of the channels under the effect of pressure.

Figure 1.2 (a) A banyan tree and (b) schematic of transport process inside the capillary network

Spider web is another example of micrometric flows appearing in nature (Figure 1.3). Spider produces a long silken thread by synthesis of protein in a gland mixed with a solution. The silken thread has exceptional mechanical properties. Individual threads of spider web look fragile. However, they are extremely strong, that is, more than that of steel. The spider silks may be useful for human applications such as making medical sutures and high-performance ropes or used as filling in bulletproof vests. There are many examples of microfluidic systems existing in nature. Man-made systems are far from being able to compete with the natural systems.

Figure 1.3 A picture of spider web having more strength than that of steel

1.4.2 Unit Systems in Small Scales

The development of various microsystems also requires new definition of unit system to describe these devices. Volume is associated with exponent equal to 3 of the length scale of a system that is, . If length scale reduces from centimeters to micrometers, the volume decreases by 12 orders of magnitude....