Poly(dimethylsiloxane) is currently the material of choice for rapidly fabricating microfluidic devices. As the size of these devices decreases, a significant hydrodynamic flow is generated due to permeation of fluid through the channel walls. We develop a theoretical model verified by single bead tracking experiments, which demonstrates that large flow rates (>10 m͞s) can be passively generated in a straight microchannel filled with water. Realizing that this flow may be unwanted in some applications, we present a method to eliminate it by inhibiting mass transfer of water into the poly(dimethylsiloxane) walls. Furthermore, we explore applications to harness this passively generated flow inside a microfluidic device such as bead stacking, chemical concentration, and passive pumping.fluid dynamics ͉ soft lithography O ver the last 10 years, there has been a burst of activity in the field of microfluidics and lab-on-chip devices (1). Much of this growth can be attributed to the pioneering work of Effenhauser (2) and Whitesides (3), who showed that inexpensive fluidic devices could be easily fabricated in poly(dimethylsiloxane) (PDMS) by replica molding from a microfabricated template. Consequently, PDMS has become the material of choice for making microfluidic devices (4-6).Although the use of PDMS in microf luidics is relatively new, this material has been used extensively in applications that require gas-or vapor-permeable membranes (7). PDMS is highly permeable to organic solvents, making solvent-PDMS compatibility one of the main drawbacks of using PDMS for f luidic devices (8). Permeation of solvent into the PDMS channel walls becomes increasingly more important as device size decreases because the surface area to volume ratio increases. Current research on cells and single molecules requires creation of devices that are only a few microns to hundreds of nanometers in height [e.g., the DNA cytometry device of Chou et al. (9) has height of 3 m]. The resulting microf luidic f low from the permeation f lux will be important in these thin devices, but it has not been studied. However, some have observed other solvent permeation effects in microchannels. In one example, protein crystallization in aqueous droplets has been observed at unexpected initial concentrations because the permeation of water through the PDMS increases the concentration in the droplets over time (10). Until now, water permeability into the PDMS has been considered negligible for most microf luidic applications; however, we will show that this small but finite permeability generates a significant f low in thin (O[m]) channels.For nanoscale applications that require sealed microchannel walls, this permeation-driven flow clearly must be eliminated. Conversely, the permeation-driven flow provides an alternative approach to passive flow applications. Several groups have previously demonstrated passive flow processes by filling one reservoir with liquid and leaving the other empty and open to air so that the liquid evaporates at the pinned conta...

Using single molecule fluorescence microscopy, we study the dynamics of an electric-field-driven DNA molecule colliding with a single stationary post. The radius of the obstacle is small compared to the contour length of the molecules. Molecules that achieve hooked configurations which span the obstacle were chosen for study. Four different types of hooked configurations were found: symmetric hairpins with constant extension during unhooking, asymmetric hairpins with constant extension during unhooking, asymmetric hairpins with increasing extension during unhooking, and rare multiply looped entangled configurations. The important physics describing the unhooking dynamics for each classification differ and models are proposed to predict unhooking times. Surprisingly, we find that most collisions do not follow classic rope-on-pulley motion but instead form hairpins with increasing total extension during the unhooking process (called X collisions). Last, we show that unraveling to form a hairpin and center-of-mass motion during unhooking affect the overall center of mass holdup time during a collision process.

Molecular bottle-brushes are highly branched macromolecules with side chains densely grafted to a long polymer backbone. The brush-like architecture allows focusing of the side-chain tension to the backbone and its amplification from the picoNewton to nanoNewton range. The backbone tension depends on the overall molecular conformation and the surrounding environment. Here we study the relation between the tension and conformation of the molecular brushes in solutions, melts, and on substrates. In solutions, we find that the backbone tension in dense brushes with side chains attached to every backbone monomer is on the order of f 0 N 3/8 in athermal solvents, f 0 N 1/3 in θ-solvents, and f 0 in poor solvents and melts, where N is the degree of polymerization of side chains, f 0 ≃ k B T/b is the maximum tension in side chains, b is the Kuhn length, k B is Boltzmann constant, and T is absolute temperature. Depending on the side chain length and solvent quality, molecular brushes in solutions develop tension on the order of 10-100 picoNewtons, which is sufficient to break hydrogen bonds. Significant amplification of tension occurs upon adsorption of brushes onto a substrate. On a strongly attractive substrate, maximum tension in the brush backbone is ~ f 0 N, reaching values on the order of several nanoNewtons which exceed the strength of a typical covalent bond. At low grafting density and high spreading parameter the cross-sectional profile of adsorbed molecular brush is approximately rectangular with thicknes , where A is the Hamaker constant and S is the spreading parameter. At a very high spreading parameter (S > A), the brush thickness saturates at monolayer ~ b. At a low spreading parameter, the cross-sectional profile of adsorbed molecular brush has triangular tent-like shape. In the cross-over between these two opposite cases, covering a wide range of parameter space, the adsorbed molecular brush consists of two layers. Side chains in the lower layer gain surface energy due to the direct interaction with the substrate, while the second layer spreads on the top of the first layer. Scaling theory predicts that this second layer has a triangular cross-section with width R ~ N 3/5 and height h ~ N 2/5 . Using self-consistent field theory we calculate the cap profile y (x) = h (1 − x 2 /R 2 ) 2 , where x is the transverse distance from the backbone. The predicted cap shape is in excellent agreement with both computer simulation and experiment.

We present a kinematic analysis and experimental study of DNA deformation in electric field gradients. Specifically, we investigate deformation near a large insulating cylinder with single molecule fluorescence videomicroscopy. Because the electrophoretic velocity field is a potential field, a kinematic analysis shows that local deformation of DNA in any electric field gradient is pure elongation, quantified by a strain rate and orthogonal axes of extension and compression. From the kinematics, we construct the electrophoretic Deborah number relating the competing effects of deformation in the field and the polymer elasticity. We report highly configuration-sensitive stretching at the front of the obstacle and affine compression in the region near the back stagnation point. Furthermore, the DNA also can extend both "pre-impact" and "post-impact" in this inhomogeneous extensional field. We find that field gradient induced deformation offers a simple way to extend and quickly compress DNA near surfaces in microdevices.

We study the dynamics of single DNA molecules driven by an electric field into a stationary obstacle. These collisions are broadly classified as "hook" and "roll-off" events. We show that obstacle-induced electric field gradients stretch impacting DNA and thus greatly influence the hooking probability. Consequently, in addition to collision geometry, determination of the hooking probability depends on the Deborah number (De) for 0.5