Fabrication of fully enclosed paper microfluidic devices using plasma deposition and etching.

A fully enclosed paper microfluidic device has been fabricated using pentafluoroethane (PFE) plasma deposition followed by O2 plasma etching. Structures with one and two non-interacting, fully enclosed hydrophilic channels were generated in a single paper sheet using metal masks. Furthermore, by performing an additional O2 plasma step with a secondary mask, pinholes were created at the reaction zones for reagent loading. Finally, to demonstrate the functionality of the device, a glucose assay was performed. Quantitative analysis of glucose assays showed that the device can be used for the clinically relevant range of glucose. To our knowledge, this is the first time that such structures have been fabricated without paper stacking. Multi-layer devices have enhanced functionality relative to a single channel device, because the design space for creating networks of channels within the paper substrate is greatly expanded. The fluid-filled channels in the fabricated device are isolated, thereby preventing contamination due to handling and environmental exposure. Fluid evaporation can be inhibited by sealing the device with adhesive tape without contaminating the enclosed channels. The method described is a dry process and compatible with roll-to-roll technology, thus facilitating large scale production. The novel method to fabricate enclosed µ-PADs overcomes many of the limitations experienced with current approaches and thus offers an alternative means to develop low-cost point-of-care diagnostics for resource-limited settings.

Affine and nonaffine motions in sheared polydisperse emulsions.

We study dense and highly polydisperse emulsions at droplet volume fractions ϕ≥0.65. We apply oscillatory shear and observe droplet motion using confocal microscopy. The presence of droplets with sizes several times the mean size dramatically changes the motion of smaller droplets. Both affine and nonaffine droplet motions are observed, with the more nonaffine motion exhibited by the smaller droplets which are pushed around by the larger droplets. Droplet motions are correlated over length scales from one to four times the mean droplet diameter, with larger length scales corresponding to higher strain amplitudes (up to strains of about 6%).

Measuring shear-induced self-diffusion in a counterrotating geometry.

The novel correlation method to measure shear-induced self-diffusion in concentrated suspensions of noncolloidal hard spheres which we developed recently [J. Fluid Mech. 375, 297 (1998)] has been applied in a dedicated counterrotating geometry. The counterrotating nature of the setup enables experiments over a wider range of well-controlled dimensionless time (gamma;Deltat in the range 0.03-3.5, compared to 0.05-0.6 in previous experiments; here gamma; denotes the shear rate and Deltat the correlation time). The accessible range of timescales made it possible to study the nature of the particle motion in a more detailed way. The wide radius geometry provides a well-defined flow field and was designed such that there is optical access from different directions. As a result, shear-induced self-diffusion coefficients could be determined as a function of particle volume fraction straight phi (0.20-0.50) in both the vorticity and velocity gradient direction. A transition could be observed to occur for gamma;Deltat of O(1), above which the particle motion is diffusive. The corresponding self-diffusion coefficients do not increase monotonically with particle volume fraction, as has been suggested by numerical calculations and theoretical modeling of Brady and Morris [J. Fluid Mech. 348, 103 (1997)]. After an exponential growth up to straight phi=0.35, the diffusion coefficients level off. The experiments even suggest the existence of a maximum around straight phi=0.40. The results are in good agreement with experimental literature data of Phan and Leighton [J. Fluid Mech. (submitted)], although these measurements were performed for much larger values of the dimensionless time gamma;Deltat.