We describe the results of a numerical investigation of the dynamics of breakup of streams of immiscible fluids in the confined geometry of a microfluidic T-junction. We identify three distinct regimes of formation of droplets: squeezing, dripping and jetting, providing a unifying picture of emulsification processes typical for microfluidic systems. The squeezing mechanism of breakup is particular to microfluidic systems, since the physical confinement of the fluids has pronounced effects on the interfacial dynamics. In this regime, the breakup process is driven chiefly by the buildup of pressure upstream of an emerging droplet and both the dynamics of breakup and the scaling of the sizes of droplets are influenced only very weakly by the value of the capillary number. The dripping regime, while apparently homologous to the unbounded case, is also significantly influenced by the constrained geometry; these effects modify the scaling law for the size of the droplets derived from the balance of interfacial and viscous stresses. Finally, the jetting regime sets in only at very high flow rates, or with low interfacial tension, i.e. higher values of the capillary number, similar to the unbounded case.

Flows of droplets through networks of microchannels differ significantly from the flow of simple fluids. Our report focuses on the paths of individual droplets through the simplest possible network: a channel that splits into two arms that subsequently recombine. This simple system exhibits complex patterns of flow: both periodic and irregular, depending on the frequency at which the drops are fed into the "loop." A numerical model explains these results and shows regions of regular patterns separated by regions of high complexity. Our results elicit new questions regarding the dynamics of flow of discrete elements of fluids through networks, and point to potential opportunities and difficulties in the design of integrated mini-laboratories operating on droplets.

Articles you may be interested inReorientational and translational dynamics of benzene in zeolite NaY as studied by one-and two-dimensional exchange spectroscopy and static-field-gradient nuclear magnetic resonanceWe calculated transition state theory and exact rate coefficients for benzene jumps in Na-Y zeolite between 150 and 500 K. This is the first exact flux correlation function rate calculation for a non-spherical molecule inside a zeolite. We calculated rates for jumps between S II and W sites, located near Na ions in 6-rings and in 12-rings windows, respectively. Partition function ratios were calculated using Voter's displacement vector method. A general Arrhenius behavior is observed over the whole temperature range for all processes. The activation energies are close to the difference between the minimum energies in the sites, and between the sites and the transition states. The calculated prefactors present reasonable values around 10 12 -10 13 s Ϫ1 , in good agreement with nuclear magnetic resonance relaxation experiments. We were not able to decompose the prefactors into simple vibrational and entropic components, and therefore a complete calculation of the rate constant seems necessary to obtain reliable values. In three of the four types of motions investigated, the transition state theory rate constant is approximately equal to the more exact correlation function rate constant. However, in the case of the W→W jump, transition state theory is qualitatively wrong. This is due to the fact that the minimum energy path from one W site to another is very unstable and intersects the S II →S II minimum energy pathway, so a slight perturbation sends the molecule to a S II site instead of the W site. As a consequence, the prefactor for the W→W jump is found to be almost one order of magnitude smaller than the prefactor for the W→ S II jump, although the activation energies are similar.

The simplified kinetic scheme presented categorizes the volatile compounds into 4 classes: pyrroles and other nitrogen-containing heterocyclic compounds (PY), furans and other oxygen-containing heterocyclic compounds (FU), carbonyls (C), and pyrazines (PZ). The scheme comprises 11 reaction steps. The order of magnitude of the reaction rate for some of these steps could be determined from literature sources. Using these rates, the scheme was able to correlate the pseudo-zero-order rate of generation of FU and PZ (from the literature) to the initial temperature and concentration of reactants. It also reproduces the kinetics of volatile formation in a model glucose-alanine system in a glycerol matrix, as measured for 4 temperatures by GC-MS.

We have developed a new analytical theory for activated diffusion in zeolites at finite loadings, including the effect of adsorbate-adsorbate interactions. Excellent qualitative agreement is obtained comparing our new theory to kinetic Monte Carlo simulations. We have applied this theory to benzene diffusion in faujasite, to help resolve discrepancies among different experiments. Our results are in qualitative agreement with pulsed field gradient NMR, and in qualitative disagreement with tracer zerolength column data. [S0031-9007(98)06457-6]