Flow Visualization in Artificial Porous Media from Microfluidic PMMA Devices

Microfluidic polymethylmethacrylate (PMMA) devices for study of particle transport in artificial porous media were designed and microfabricated using hot embossing with a brass mold insert containing a microchannel network with eight layers. After thermal bonding to enclose the microchannel network, a process protocol was applied to successfully remove bubbles in the PMMA device. Characterization protocols were developed for study of fluorescent particle tracking, accumulation, and retention in these microfluidic chip artificial porous media. Particle accumulation and retention was observed throughout the microfluidic network domain and predominantly at the inlet section of the PMMA device due to entrance effects. Particle Image Velocimetry of the PMMA device allowed for generating velocity profiles in the chip microchannel networks. INTRODUCTION Visualization of the flow in the artificial porous media is important [1-5] because it allows for correlation of the observed flow patterns from the artificial porous media to physical and fluid properties of real reservoir materials including sandstone, carbonate, and fractured rocks. A common approach to make the artificial porous media is using glass beads packed in a confined space for experimental investigation of flow patterns [2], but it limits the control of flow geometries of the porous media such as convergingdiverging channels, splitting and rejoining channels, and a distribution of channels sizes. Microfabrication of silicon/glass [4,6] can be used for fabrication of artificial porous media through well-established techniques of metal deposition, photolithography, and etching for multiple layers, but this process carries the drawbacks of high material and process costs. In this paper, a 2.5D artificial porous medium was designed with random bifurcation and re-combination geometries and cross-sections, and microfabricated in polymethylmethacrylate (PMMA) using hot embossing. This process is relatively inexpensive compared to fabrication in silicon/glass and allows for production of multiple chips through replication at low cost after fabrication of a mold insert [7, 8]. The PMMA devices were used for development of characterization protocols to study particle tracking, accumulation, and retention with fluorescent nanoparticles as tracers in artificial micro-chip porous media. DESIGN The prototype design of a 2.5D artificial porous medium involved a network of 2,500 channels (throats) with different widths and depths, and was based on the design used by Crandall et al. [9] after applying anisotropic scaling. Eight different layers defining an equal number of different arbitrarily defined depths were deployed to provide a limited three dimensional component of the geometry. The resulting 2.5D geometry, although not entirely consistent with the heavily 3D pathways of actual rock samples, makes three dimensional motion of the fluid and the target nanoparticles possible. The prototype design for the artificial porous medium device (50 by 50 channels, square footprint of 101.6 mm by 101.6 mm, Figure 1) had eight different layers (Figure 1(b): LDG (dark gray layer) being the top layer in the polymer chip with LM (magenta layer), LB (blue layer), LC (cyan layer), LG (green layer), LY (yellow layer), LR (red layer), and LLG (light gray layer)) with a minimum channel width of 0.2 mm and maximum channel width of 1.0 mm and uniform distribution channel widths. The dimensions for each layer of the prototype were summarized in Table 1.