Microfluidic Method for Synthesizing Cu2O Nanofluids

Abstract

NANOFLUIDS, fluid suspensions of nanometer-sized particles, have recently been demonstrated to have thermal conductivities far superior to that of the liquid alone [1–4]. This and their other distinctive features offer unprecedented potential for many applications in various fields, including energy, biological, pharmaceutical, chemical, electronic, environmental, material, medical, and thermal engineering industries [1–8]. Yet the functional outcomes of existing nanofluids have not been satisfactory because of the inadequacies of conventional synthesis approaches in engineering microstructures and properties of nanofluids [1–9]. For creating nanofluids by design, we have recently developed a one-step chemical solution method (CSM) that is capable of synthesizing nanofluids of various microstructures [9–11]. The method has been successfully applied to produce the nine kinds of nanofluids [9–11]. The nanofluids synthesized by theCSMhave both higher conductivity enhancement and better stability than those produced by other methods. The CSM is also distinguished from the others by its controllability. The nanofluid microstructure can be easily varied and manipulated by adjusting synthesis parameters such as temperature, acidity (pH), ultrasonic and microwave irradiation, types and concentrations of reactants and additives, and the order in which the additives are added to the solution [9–11]. Problems with the CSM come from that reactions take place in macroscale batch reactors such as beakers and flasks. The CSM uses a bottom-up approach to generate nanoparticles through chemical reactions in the liquid phase, and thereby it has the potential to manipulate atoms and molecules for synthesis of tailor-made nanofluids. However, the difficulty of controlling the microscale while operating at the macroscale is insuperable. Mixing in a macroscale batch reactor is usually achieved by stirring. In this case, the fluid entity is broken into fragments by circular motion. The last part of mixing takes place based on molecular diffusion. In the diffusion process, the mixing time t depends on the diffusion path d in the form of t / d=D, where D is the diffusion coefficient. Therefore, if the diffusion path becomes smaller, the mixing time becomes shorter. However, it is very difficult to make small-sized fragments by conventional stirring in solution phase. At the macroscale, therefore, mixing time is usually much larger than reaction time. The reaction rate is normally determined by the mixing time and is usually very low. Moreover, the longer mixing time and lack of effective ways to accurately control mixing also lead to poor product selectivity of competitive parallel reactions and competitive consecutive reactions, thereby yielding poor quality of synthesized nanofluids containing some undesired side products. Because of the work-up demand, a batch-model operation is not commercially economical. The repeatability of nanofluids’ structures is also poor with the batch-model operation. To resolve these critical issues, we propose to replace batch-based macroreactors in the CSM by continuous-flow microfluidic microreactors, allowing a continuous and scalable (simply by numberingup) synthesis of nanofluidswith amore accurate and effective control over particlemicrostructures such as the size, distribution, and shape. Here, we report such a microfluidic system for synthesizing Cu2O nanofluids: suspensions of Cu2O nanoparticles in water.