The microfluidic devices have drawn great attentions and applied to various research aspects such as micro-scale heat transfer analysis or biomedical engineering. The designs of microfluidic devices and their flow fields are important to the success of the applications for heat exchanger or drug delivery. The designs and flow fields of microfluidic devices can be examined and investigated using numerical method with commercial software; however, experimental approach are necessary to further validate or compare with the numerical data. Conventional experiment approaches utilize micro-machined pressure and temperature sensors, like micro-membrane pressure sensors or micro-thermocouples, to acquire pressure and temperature data. However, the micro-machined sensors have the limitation of numbers of sensor implantation and consequently discrete data.
In the past decade, a new experiment technique has been developed and it has been applied to various microfluidic devices for investigating the flow field. It is adapted from a luminescence-based pressure/temperature measurement technique known as pressure- and temperature-sensitive paint (PSP/TSP). The PSP/TSP sensor is prepared by selecting luminescent molecules and mixing with polymer binders, then coating on a glass slide. The PSP/TSP coated glass slide is used as cover glass to seal microfluidic devices made by PDMS. During the experiment, a UV light is used as excitation light source and the luminescence intensity emitted from luminescent molecules changes the based on the nearby oxygen concentration (as oxygen quenching) or temperature (as thermal quenching). A scientific grade CCD camera is used to collect the luminescence data in images. The luminescence intensity can be further calibrated and translated into pressure and temperature data. Detailed pressure and temperature profiles have been successfully acquired in microfluidic devices [1]. Heat transfer analysis inside the microchannel has been executed after the fluid and surface temperature obtained by TSP measurements. The oxygen and nitrogen gases mixing in a T-type micromixer can also be realized by using PSP sensors [2]. The experimental results obtained by PSP/TSP measurements have been compared with numerical simulation with ANSYS Fluent, and good agreement have been established [3]. In addition, the temperature evolution inside a microfluidic device with a recess microchannel has been recently measured by TSP measurement and it has been used for the design and improvement of the microfluidic device for microorganism cultivation.
The schematic of PSP/TSP measurement is illustrated in Figure 1. Figure 2 presents the detailed pressure distribution acquired by PSP sensor in a 90-degree bend microchannel with an air flow at Reynolds number of 274. The oxygen concentration profile has been successfully obtained by PSP sensor in a T-type micromixer at the Reynold number of 51.3, as shown in Figure 3. Figure 4 (a), (b), and (c) present the fluid temperature, surface temperature, and the local heat transfer evolution around a 90-degree bend microchannel measured by TSP sensor with liquid flow at Reynolds number is 67.4. Detailed heat transfer evolution has been obtained and higher Nusselt number has identified near the outside wall after the corner, the region where the liquid flow impinging at the wall.
