---

Research Subject: Poly(dimethylsiloxane) (PDMS) is a silicone polymer that nowadays despite unique characteristics and high application potential of its microparticles, their preparation via bulk emulsification methods is a main challenge due to the limitations in mixing process, high viscosity and low surface energy of PDMS that make impossible accurate  control of final obtained particles. In the present work, size-controlled PDMS microparticles were prepared from a high-viscosity material.
Research Approach: PDMS microparticles were obtained by using glass capillary co-flow microfluidic device. The designed microfluidic device is facile, inexpensive and reusable and facilitated preparation of the high-viscosity PDMS microdroplets. Stabilizing the oil-in-water emulsion was obtained by optimizing the bath components and curing process that resulted in monodisperse and spherical PDMS microparicles. Effect of the some important adjustable parameters such as microchannel diameter and flow rate on the flow regimes and microparticles polydispersity were investigated by means of optical microscopy and scanning electron microscopy.
Main Results: Results showed a dripping regime for producing monodisperse microparticles at low flow rates of the continuous phase and monodisperse microparticles from it. On the contrary, microparticles obtained from jetting regime are more polydisperse and smaller in comparison with dripping regime. By reducing the diameter of inner microchannel, microparticles with a diameter of 1.83 µm were obtained. Using the designed technology, uniform nanocomposite PDMS/ZnO microparticles 318 µm in diameter containing 15% ZnO were obtained from an oil phase viscosity of 7550 mPa.s. Therefore by an optimized and facile method, size-controllable uniform microparticles can be prepared that are proposed for various applications including drug delivery, bioengineering and electronic industry.

---

Polymeric foams have a cellular structure composed of a polymeric matrix with gaseous cells which achieved by expansion of a blowing agent in polymer melt matrix during a foaming process. In the present study, the bubble expansion step in Polystyrene/CO2 batch foaming process was simulated and compared to the reported experimental results. A single spherical bubble surrounded by an incompressible viscoelastic fluid (upper-convected Maxwell model) was considered. To calculate concentration profile in the shell, mass diffusion equations were solved using finite element method, potential function definition and integral methods. The predicted results show that when the gas concentration profile obtained by finite element method and the concentration gradient near the bubble-shell interface was used to calculate the pressure inside the bubble, the predicted results were in good agreement with the experimental ones which there was less than 1% error at each foaming time. The effects of the thermo-physical and rheological properties on the bubble growth dynamics were also studied and It was found out that increasing the diffusivity coefficient by factor of 10 would increase the bubble size up to 1.5 times, whereas increasing the viscosity by 3 folds would only change the bubble size about 2% showing that the bubble growth step in foaming process was a mass transfer controlled process.