July 16, 2014: From Growth of Aerosol Nanoparticles to Step-Emulsification in Microfluidics, Dr. Maximilian Eggersdorfer


From Growth of Aerosol Nanoparticles to Step-Emulsification in Microfluidics

Wednesday, July 16, 2014, 16:15, ML H 44, ETH Zurich

Dr. Maximilian Eggersdorfer
Harvard School of Engineering and Applied Sciences, Cambridge, MA, USA
Gas-phase (aerosol) technology is used widely in manufacture of various nanostructured commodities at tons/hour today. The involved processes span 10 and 15 orders of magnitude in length and time scales, respectively, involving gases, liquids and solids. Aerosol particles are formed by gas and surface reaction, coagulation and sintering typically under turbulent conditions with Reynolds numbers up to 10^6. As chemical reactions in high temperature aerosol processes are completed much faster than particle dynamics, the detailed structure of these particles is largely determined by the interplay of particle collision and coalescence. Initially particles are formed in the free molecular regime obeying kinetic gas theory. During particle growth they experience a transition to the rarefied gas and finally continuum regime. This work focuses on meso- or nano-scale simulations of particle growth and their resulting structure as well as restructuring in shear flows. Such particles form fractal-like agglomerates. A power law is found between agglomerate mobility and primary particle diameter that is essentially independent of time, material properties or sintering mechanism. So the average primary particle diameter and specific surface area can be determined in almost real time as elucidated with mass-mobility measurements of fractal-like silica nanoparticles.
Such aerosol nanoparticles are commonly used for rubber reinforcement, e.g. in polydimethylsiloxane (PDMS) with up to 40 wt% of silica nanoparticles. The second part focuses on PDMS-based microfluidic devices for emulsification. Emulsions are ubiquitous in our daily life with a broad spectrum of applications, e.g. enhanced oil recovery, food, cosmetics etc. Typically emulsions are obtained by shearing two immiscible fluids with a surfactant. However this traditional approach lacks the fine control of droplet size and polydispersity that is crucial for many applications like drug delivery or carbon free copy paper. In contrast, almost monodisperse droplets of a given size can be formed with microfluidics. Nevertheless, scalability is challenging with typical flow rates in the order of a few hundred μl/h. Parallelization of droplet makers is a promising route to overcome this challenge but exhibits limitations as the droplet size is determined by the flow rates. Thus clogging of one channel affects the flow rates in all other channels hence changing the droplet size. Here, a new droplet break-up mechanism is discussed that is independent of the flow rates and allows to massively parallelize drop makers for robust operation at high flow rates.
Host: Dr. D. W. Meyer-Massetti, Prof. P. Jenny