dc.description.abstract | The focuses are to study PMMA mold fabrication for preparing PDMS micromodel, drying in micromodel, and drying impacts on mixing behaviors in micromodel. PDMS micromodels were successfully prepared by replica molding using PMMA molds built by CNC milling. The molds included Model 1 and Model 2 with the average throat sizes are ~120 µm and ~90 µm, respectively.
The micromodel was fully imbibed by a low concentration of dyed ethanol (the old liquid). Drainage was done by invading gas air and stopped at steady condition. Drying was conducted by changing the injection rate of gas air to be 0.2 µl/min and stopped when the dried old liquid reached certain saturations (So). Then, mixing was conducted by injecting a high concentration of dyed ethanol (the new liquid) into the micromodel with the same injection rate (Q) as in the imbibition and the drainage. There were two kinds of experiments, the second was conducted without drainage.
The liquid injection rates (Q) were varied based on capillary numbers (Ca) of 3x10-6, 3x10-5, 3x10-4 which were responsible Q of 1.25µl/min and 1.55µl/min (Model 1 and Model 2); 12.5µl/min and 15.5µL/min; 125µl/min and 155µL/min; respectively. The desired So were varied around So=10%, So=20%, and So=30%.
The dried liquid of So=10% has the longest drying times and the highest concentration than So=20% and So=30% due to the more aqueous phase is evaporated. Model 2 has faster drying rates than Model 1 because much small liquid cluster spreads in the pore that eases and accelerates evaporation.
In Model 1, finger of the new liquid is formed and mixing happened by dispersion. In Model 2, at So=30%, mixing is dominated by diffusion while advection dominates in So=10% and So=20%. The increase of Ca during the new liquid injection limits the diffusion and advances the advection. The higher So induces longer diffusion.
In Model 1, the air bubble is completely displaced during the new liquid injection with Ca=3x10-5 and Ca=3x10-4. At Ca=3x10-6, it remains in the middle of micromodel. In Model 2, the air bubbles trapped in some areas prevent mixing by blocking the new liquid to pass. The decrease of Ca reduces the air bubble trapping in Model 2.
The mixing areas in Model 2 are higher than those are in Model 1 due to disorder liquid channel in Model 2 that upsurge the chance of mixing; the increase of Ca rises the mixing areas because it improves the advection process; the higher So has the more mixing areas due to the more significant concentration gradient.
Model 2 (no drainage) needed much longer drying times than Model 2 (with drainage). After drying, In Model 2 (with drainage) has many disconnect liquid clusters spreading in micromodel. In Model 2 (no drainage), it has fewer disconnect liquid clusters and several big bulks of liquid. It influences mixing behaviors in Model 2. The air area in Model 2 (no drainage) is lower than it is in Model 2 (with drainage), thus in Model 2 (no drainage) has higher mixing area. | en_US |