Patterned Surfaces Using Varied Surface Tensions
Patterned surfaces in micro channels can be used to transport liquids from one reservoir to another along specified pathways, with multiple liquids flowing side-by-side without the need of actual physical walls between the liquids. Patterned surfaces are used to transport fluids in lab-on-a-chip, bioassays, microreactors and, chemical and biological sensing. In this case surface tension is used to manipulate fluid flows in micro channels to create patterned flows. Hydrophilic or hydrophobic behavior of a fluid on a solid surface is exploited to control the motion of multiple fluids through a microchannel. Fluid flow inside micro channels is laminar, meaning that multiple fluid streams (two in this case) can flow side-by-side without turbulent mixing. Because there are no physical walls on the sides of the fluid stream, the streams are confined by the so called virtual walls. These walls are basically the hydrophilic-hydrophobic boundaries between the two fluids.
The figure above shows the microchannel in the experiment. The middle strip of the central horizontal channel is hydrophilic, whereas the remaining channel along with the upper and lower vertical channels, have different degrees of hydrophobicity. They differ in their hydrophobicity only by a few degrees of the contact angle. Upper channels have a contact angle of 118o and the bottom channels have a contact angle of 112o. This small difference in contact angles however requires significantly different pressures for the fluid to flow into these regions.
Initially all channels are filled with another fluid (transparent). When the pink fluid is pushed into the horizontal channel, it takes the hydrophilic path in the central region (Phase A). As the pressure is increased, the fluid breaks the bottom hydrophilic-hydrophobic barrier and starts flowing into the bottom hydrophobic region (Phase B). On increasing the pressure even further, the fluid finally breaks the upper hydrophilic-hydrophobic barrier as well and starts flowing in upper region too (Phase C).
The numerical results above show reasonable comparability with the overall idea of patterned surface study in the experiment, given that there were important differences between the two. The numerical results shown above are in the transient state (pressure is continuously increasing), and therefore the fluid boundaries are not exactly similar to the experimental results. Similarly, the fluid properties are not exactly similar to the one used in the experiments. Notwithstanding, Fluid 1 goes through phases A, B and C as the pressure increases, just as in the experiment. In phase B, the transparent fluid continues to flow through the upper channels, but only the pink fluid flows in the bottom region. This is consistent with the experiment. What is interesting to see is the bubble formation shown in phase C. Revelation and study of interesting physics like bubble formation in phase C could be crucial to the design and fabrication process of microfluidic devices.
The animation below shows the simulation results of FLOW-3D for the experiment shown above. Fluid 1 (light blue) is equivalent to the pink fluid in the experiment. Initially the whole domain is filled with Fluid 2 (transparent fluid). The pressure is increased in a stepwise manner and all the three phases can be seen as the simulation evolves.
Evolution of fluid flow with increasing pressure in patterned micro channels created by varying contact angles.
Ref: Bin Zhao, Jeffrey S. Moore, David J. Beebe, Surface-Directed Liquid Flow Inside Microchannels, Science 291, 1023 (2001)