Chau-Chyun Chen

Microfluidic production of size-tunable hexadecane-in-water emulsions: Effect of droplet size on destabilization of two-dimensional emulsions due to partial coalescence

HYPOTHESIS Despite numerous studies, the mechanism of destabilization of oil-in-water emulsions during cooling-heating cycles is unclear due to indirect measurements and lack of direct control over the droplet size. It is hypothesized that emulsions with a smaller droplet size are more resistant to destabilization than emulsions containing larger droplets since the probability of initiating partial coalescence and forming large-scale aggregates is lower for small droplets. EXPERIMENTS Monodisperse hexadecane-in-water emulsions with controlled droplet sizes were produced using a microfluidic valve-based flow-focusing device and varying the system parameters. A unique approach was developed to create a two-dimensional (2D) array of droplets enabling visualization of the destabilization process due to temperature cycling. The influence of droplet size on partial coalescence and destabilization was investigated. FINDINGS In the 2D emulsion, destabilization proceeds through a combination of spontaneous coalescence events that yield small-scale structures followed by formation of large-scale structures by coalescence propagation. We find that emulsion destabilization increases with droplet size. Quantifying the frequency of n-body coalescence events reveals that in emulsions with small droplets coalescence propagation is hindered. Phenomena involving restructuring, growth and cross-linking of droplet aggregates are identified as the key features of the emulsion destabilization mechanism.

Prediction of χ Parameter of Polymer Blends by Combining Molecular Simulations and Integral Equation Theory

A combination of molecular simulations and integral equation theory is applied to predict the χ parameter for polymer blends. The inter- and intramolecular structures of the polymer blends are obtained from molecular dynamics (MD) simulations with atomistic models, which, in turn, are used to calculate the χ parameter using the integral equation theory (χI). This approach was employed to determine the temperature and concentration dependence of χI in the binary blends of atactic polypropylene (aPP)-head-to-head polypropylene (hhPP) and polyethylene (PE)-isotactic polypropylene (iPP), respectively. The χ parameter calculated from this approach (χI) is compared with the χ parameter estimated in the literature from phase equilibrium simulation data for aPP-hhPP blends. In the case of PE-iPP blends, χI is compared with the χ parameter obtained from fitting the structure factor to the random phase approximation. Our approach for calculating χ does not require any fitting, and the only input required for the approach is the radial distribution function which can be calculated from MD simulations. Thus, using this approach in conjunction with atomistic models provides a general methodology for predicting χ parameter of polymeric systems of any chemistry.