MİKROAKIŞKAN BİR SİSTEMDE METAL ORGANİK KAFES YAPILARININ SENTEZİNE YÖNELİK ÇEŞİTLİ SİLİKON YAĞI VİSKOZİTELERİ İÇİN AKIŞ KOŞULLARININ OPTİMİZASYONU

Abstract

Microfluidic systems offer a highly controlled environment for the synthesis of micro and nanoparticles with advanced properties. The extremely small volumes within these systems enable precise control over heat and mass transfer and reaction conditions. A droplet-based two-phase flow within a micro channel system can provide a rapid and controlled approach for synthesizing metal-organic framework (MOF) nanoparticles with a narrow size distribution. The diameter of monodisperse picoliter droplets defines the size of the micro-reactors. Droplet generation is influenced by the viscosity of the continuous phase and the interfacial tension between the two phases. The flow rates are limited due to the very small volumes that can pass through microchannels and the technological limitations related to optical imaging. Using low flow rates are a requirement to stay within the safe pressure limits of transparent materials, such as polydimethylsiloxane (PDMS), which are often used in microchannel fluidic device fabrication. Hence, selecting an oil with optimal viscosity is crucial for producing droplets, that is micro reaction vessels, of the desired size. N,n-dimethylformamide (DMF) is the common solvent in precursor solutions used for synthesizing MOF crystals derived from metal salts like Zr, Ti, and V. This study investigates the optimization of flow conditions for droplets containing DMF within a microfluidic system. Silicone oils with 100, 350, 500, and 1000 cSt viscosities were used in flow-focusing microfluidic systems with x-junction outlets of 50 and 100 µm. The results showed that as the viscosity and flow rate of the silicone oil increased, the droplet size decreased. For systems with x-junction outlets of 50 µm and 100 µm, the smallest droplet diameters of 15 µm and 35 µm, respectively, were obtained when the volumetric flow rate ratio of the dispersed phase (DMF) to the continuous phase (silicone oil), Qd/Qc, was 0.071 and the capillary number Ca was 0.35. Theoretically, droplet formation frequencies were calculated as 283 s-1 and 45 s-1 for these systems. Two-dimensional numerical simulations using COMSOL Multiphysics confirmed that for a system with a 100 µm x-junction outlet, the smallest achievable droplet diameter using a 1000 cSt silicone oil was 25 µm. The simulation results showed a very good agreement with the experimental data, with the lowest error rate of 6.5% observed for the 1000 cSt silicone oil. Based on the experimental and theoretical results, a microfluidic device with a 100 µm x-junction outlet and 1000 cSt silicone oil was selected for MOF particle synthesis. In the study with the microfluidic system, the production of zirconium (Zr) based UiO-66 nanoparticles, which have a stable structure, have a wide usage area, and whose synthesis is reproducible on a macro scale with the conventional solvothermal method, was investigated. The synthesis temperature and the retention time were determined as 100 ℃ and 30 minutes, respectively. Hydrochloric acid (HCl) and water were used as modulators. Droplets with a diameter of 50 µm and a volume of 262 picoL were used as microreactors and UiO-66 MOF nanocrystals with a size of 14±3 nm were obtained.