The four-roll mill has been used to investigate various flow behaviors of complex fluids, such as drop deformation and breakup, rheological behavior of polymeric liquids in the extensional flow field, flow-induced crystallization of polymers, flow-induced orientation in suspensions, and conformation change of macromolecules. Unlike the pressure-driven cross-slots apparatus or convergent-divergent flow, the four-roll mill can generate approximately homogeneous, two-dimensional extensional flow in the central region between the rollers. It can also generate other types of planar flow fields like rotational and simple shear flows. In addition, positioning the sample at desired locations is relatively easy through computer-controlled rollers. However, the conventional four-roll mill is very difficult to scale-down for micro/nanofluidics due to its moving parts (i.e. rollers) and the end effect from both top and bottom covers.
In this project, we intend to utilize electrokinetic force to generate desired flow patterns. Electrokinetic force has been widely used for moving fluid/analyte inside the microchannels. One of the advantages is that the movement of fluid/analyte is uniform with a plug-shaped velocity profile, which is substantially different from that using pressure-driven flow. Another is that the fluid /analyte movement will not be affected drastically by the channel dimensions. In addition, the velocity of fluid /analyte can be readily manipulated by varying either the applied electrical field, charge density of the channel surface, or solution properties. Figure 1 shows the stagnation (or elongational) flow pattern generated by electrokinetic force. One of the applications of this micro four-roll mill is to study the coil-stretch of single DNA molecules in different flow fields to provide better understanding of polymer physics at molecular scale. Several research groups have been conducting this type of experiments using various devices mainly based on pressure-driven flow [Perkins, et. al., 1997; Shrewsbury, et. al., 2001; Wong, et. al., 2003] or translation stage [Smith, et. al., 1999]. With our electrokinetics-based micro four-roll mill, the coil-stretch of single DNA molecules was observed near stagnation point shown in Figure 2. Different amount of DNA stretching can also be achieved by adjusting the applied electrical field, which is shown in Figure 3.
Figures:
Figure 1. Streamline of stagnation (or elongational) flow generated by electrokinetic force. Figure 2. Coil-stretch of single DNA molecules near stagnation point. Figure 3. l -DNA solution added with ~50% (w/w) glucose: (a) voltage = 0V; (b) voltage = 257 V; (c) voltage = 727 V. Arrow: flow direction.
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