**1. Introduction**

Both microfluidic and nanofluids are rapidly growing and hugely potential technologies which emerged as multidisciplinary research fields. While microfluidic technology began with the development of the first lab-on-a chip in 1979 microfluidic research had its first step forward only when Manz introduced the idea of μ-TAS (Micro total analysis systems) in 1990 [1]. Other main timelines of the development of microfluidic technology include introduction of microfluidics in cell biology and biochemistry in 1994, employing PDMS (polydimethylsiloxane) in microchips production in 1998, introduction of digital microfluidics in 2000, investigations on microfluidics cell culture systems in 2004, development of organon-chip technology between 2005 and 2010, emergence of paper-based microfluidics in 2007, application of 3D printing in microfluidic technology in 2010 and the latest development of microfluidics for theranostics between 2012 and 2015 [2, 3]. Microfluidic technology deals with small amounts (i.e., microliter or nanoliter) of fluids (liquids and gases) in micron or sub-micron size devices/systems/geometries. Microfluidics is a very popular research field which can be evidenced from the rapid growth of numbers of publications on this topic (**Figure 1**). Year-wise publication data on microfluidics for the past two decades (data obtained by searching topic: "microfluidic" in Web of Sciences on 10th May 2021) are presented in **Figure 1**. The impact and importance of microfluidic technology can be manifested from its diverse real-world applications ranging from virus detection and bioanalytical, cell manipulation and separation, 3D printing, paper microfluidics to anticancer drug screening as highlighted in the following section. The advantages of microfluidics include very small quantity (e.g., microliter) of sample or reagent usage, contamination risk reduction, low cost (e.g., for analysis/diagnosis), automation, enhanced sensitivity, accuracy and reliability. Although different materials have been used, PDMS is currently the most widely used material for the fabrication of microfluidic devices and systems.

On the other hand, nanofluids, which is a new class of heat transfer fluids coined in early 1990's [4, 5] have also attracted tremendous attention from the researchers due to their enhanced thermophysical properties, potential benefits, and numerous applications [6–9]. **Figure 1** also shows publication records on nanofluids during the last two decades and the number of publications on nanofluids (search by the topic: Nanofluids) increases exponentialy. It is to note that nanofluid is a much smaller

**Figure 1.** *Microfluidics and nanofluids related publications from 2001 to 2020 (source: Web of science).*

field of research without any (noticeable) real-world applications compared to microfluidics. Nonetheless, these numbers indicate research activities and popularity of both research fields. Another important aspect of nanofluids is that having superior properties and nano-sized particles, they can be applied to microfluidic systems and devices which can result in improving performance and diversifying the applications of both nanofluids and microfluidic technologies [10, 11]. Nanofluids also span a wide range of potential applications starting from thermal management, energy conversion to nuclear reactor. However, this field is far from developing, exploring its benefits, and exploiting its real applications. The main challenges of nanofluids research are their inconsistent data, unidentified underlying mechanisms for the observed results and maintaining their long-term stability. The progress of the nanofluids research is briefly overviewed in the following section.

Since both microfluidics and nanofluids fields are well established and reported in the literature and textbooks [2, 3, 6–9, 12, 13], they will not be elaborated further. However, advances, applications and challenges of these technologies are highlighted in this chapter.
