The effects of fluid elasticity and channel dimension on polymeric droplet formation in the presence of a flowing continuous Newtonian phase are investigated systematically by using different molecular weight (MW) poly(ethylene oxide) (PEO) solutions and varying microchannel dimensions with constant orifice width (w) to depth (h) ratio (w∕h=1∕2) and w=25μm, 50μm, 100μm, and 1mm. The flow rate is varied so that the mean shear rate is practically identical for all cases considered. Relevant times scales include inertia-capillary Rayleigh time τR=(Rmax3ρ∕σ)1∕2, viscocapillary Tomotika time τT=η0Rmax∕σ, and the polymer relaxation time λ, where ρ is the fluid density of the dispersed phase, σ is the interfacial tension, η0 is the zero shear viscosity of the dispersed polymer phase, and Rmax is the maximum filament radius. Dimensionless numbers include the elasticity number E=λν∕Rmax2, elastocapillary number Ec=λ∕τT, and Deborah number, De=λ∕τR, where ν=η0∕ρ is the kinematic shear viscosity of the fluids. Experiments show that higher MW Boger fluids possessing longer relaxation times and larger extensional viscosities exhibit longer thread lengths and longer pinch-off times (tp). The polymer filament dynamics are controlled primarily by an elastocapillary mechanism with increasing elasticity effect at smaller length scales (larger E and Ec). However, with weaker elastic effects (i.e., larger w and lower MW), pinch-off is initiated by inertia-capillary mechanisms, followed by an elastocapillary regime. A high degree of correlation exists between the dimensionless pinch-off times and the elasticity numbers. We also observe that higher elasticity number E yields smaller effective λ. Based on the estimates of polymer scission probabilities predicted by Brownian dynamics simulations for uniaxial extensional flows, polymer chain scission is likely to occur for ultrasmall orifices and high MW fluids, yielding smaller λ. Finally, the inhibition of bead-on-a-string formation is observed only for flows with large Deborah number (De⪢1).

Conventional studies of the optimum growth conditions for methanogens (methane-producing, obligate anaerobic archaea) are typically conducted with serum bottles or bioreactors. The use of microfluidics to culture methanogens allows direct microscopic observations of the time-integrated response of growth. Here, we developed a microbioreactor (BR) with ϳ1-l microchannels to study some optimum growth conditions for the methanogen Methanosaeta concilii. The BR is contained in an anaerobic chamber specifically designed to place it directly onto an inverted light microscope stage while maintaining a N 2 -CO 2 environment. The methanogen was cultured for months inside microchannels of different widths. Channel width was manipulated to create various fluid velocities, allowing the direct study of the behavior and responses of M. concilii to various shear stresses and revealing an optimum shear level of ϳ20 to 35 Pa. Gradients in a single microchannel were then used to find an optimum pH level of 7.6 and an optimum total NH 4 -N concentration of less than 1,100 mg/liter (<47 mg/liter as free NH 3 -N) for M. concilii under conditions of the previously determined ideal shear stress and pH and at a temperature of 35°C.Microfluidic networks have recently gained importance for their wide variety of microbial applications. For example, microchannels were used by DiLuzio et al. (11) to examine the swimming behavior of Escherichia coli. DiLuzio et al. showed that E. coli sensed the presence of channel walls at distances of up to 10 m. Balagaddé et al. (5) built a microfluidic bioreactor containing a feedback control loop, which was able to correlate sustained oscillation in the cellular density of planktonic E. coli with morphological changes. The microfluidic networks used in these studies allowed researchers to directly observe the responses of microbial cells to various stimuli and provided new and unique insights into the growth and behavior of these cells. The use of microfluidics has become practicable because of the development of an inexpensive, biocompatible, and transparent but readily diffusive polymeric material (i.e., polydimethylsiloxane [PDMS]), which is used to construct micron-scale fluid networks in virtually any two-dimensional configuration (12, 23). Due to the extensive gas permeability of PDMS and the elevated cost of nondiffusive materials to construct microchannels, the application of microfluidics to the study of the growth and behavior of anaerobic microorganisms has been hindered.Conventional studies of the behavior of anaerobes, their responses to various stimuli, and their attachment have been performed with medium bottles or bioreactors ranging anywhere from several milliliters to several liters in size (2, 3, 6, 24). These systems serve to provide the anaerobic conditions necessary for growth. They do not, however, allow for any type of direct observation (without sampling disturbance) of microbe behavior, morphology, or the ability to attach to a matrix during growth. By utilizing microfluidics, ...

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When a droplet approaches a solid surface, the thin liquid film between the droplet and the surface drains until an instability forms and then ruptures. In this study, we utilize microfluidics to investigate the effects of film thickness on the time to film rupture for water droplets in a flowing continuous phase of silicone oil deposited on solid poly(dimethylsiloxane) (PDMS) surfaces. The water droplets ranged in size from millimeters to micrometers, resulting in estimated values of the film thickness at rupture ranging from 600 nm down to 6 nm. The Stefan-Reynolds equation is used to model film drainage beneath both millimeter- and micrometer-scale droplets. For millimeter-scale droplets, the experimental and analytical film rupture times agree well, whereas large differences are observed for micrometer-scale droplets. We speculate that the differences in the micrometer-scale data result from the increases in the local thin film viscosity due to confinement-induced molecular structure changes in the silicone oil. A modified Stefan-Reynolds equation is used to account for the increased thin film viscosity of the micrometer-scale droplet drainage case.