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Complexity and Emergence
We are exploring complex and emergent phenomena in several dynamically self-assembling systems. Systems that we have studied include disks spinning at liquid/liquid and liquid/air interfaces, metal beads rolling on polymer surfaces, and components moving autonomously on the surface of a hydrogen peroxide solution using bubble-based propulsion. Our recent work in this area focuses on systems in which bubbles and droplets in microfluidic networks are the primary components.
Periodic and Chaotic Formation of Bubbles
We are exploring the formation of bubbles in a microfluidic flow-focusing device (Figure 1) in which the rate of flow of liquid and the pressure of gas are externally controllable. Over much of the flow rate/pressure phase space, the system produces monodisperse bubbles. We have shown that these bubbles can be used to generate flowing lattices and dynamically assembled foams (Figure 2). As one of the parameters is varied, however, the sizes of the bubbles produced become bi-disperse (Figure 3). Further variation of the parameter leads to periodic production of bubbles of four different sizes. The flow-focusing device can also be tuned to produce bubbles with a random size distribution. The system shows similar behavior to a dripping faucet, which also displays period-doubling bifurcations.
Stable, Periodic Behavior in a Bubble-Making System
We have extended the flow-focusing device to include five inlets for liquid on either side of the gaseous thread. In a simple flow-focusing device, the gaseous thread advances into the orifice region where it is squeezed closed by the buildup of pressure in the liquid around it. In the five-inlet system, as the gaseous thread advances through the orifice region, it blocks the orifices sequentially, thereby increasing the rate of flow of liquid through the unblocked orifices. The advancing gaseous thread thus creates a mechanism of feedback in the system. Bubbles are squeezed off by the downstream orifices as the thread is slowly squeezed at the most upstream orifice, leading to the production of bursts of bubbles (Movie 4). By varying the pressure of gas in the system, for a constant rate of flow of liquid, we can tune the number of bubbles produced by the device in each burst from one up to 40 and back down to ~10. We observe highly stable periodic behavior over a range of pressures in which 29 bubbles are produced per period (Figure 5).
Solving Mazes Using Bubbles in Microchannels
Previously, we have shown that an advancing front of ink in a microfluidic network can elucidate the paths through the network. We are extending this research to incorporate bubbles that move in a continuous flow into the microchannels. The use of bubbles increases the potential utility of these systems as models for complicated networks, such as traffic patterns in a busy city.
Select Publications:
1. Grzybowski, B. A., Stone, H. A. and Whitesides, G. M. Dynamics of self assembly of magnetized disks rotating at the liquid-air interface. Proceedings of the National Academy of Sciences of the United States of America 99, 4147-4151 (2002).
2. Garstecki, P., Gitlin, I., DiLuzio, W., Whitesides, G. M., Kumacheva E. and Stone, H. A. Formation of monodisperse bubbles in a microfluidic flow-focusing device. Applied Physics Letters 85, 2649-2651 (2004).
3. Wiles, J. A., Grzybowski, B. A., Winkleman, A. and Whitesides, G. M. A tool for studying contact electrification in systems comprising metals and insulating polymers. Analytical Chemistry 75, 4859-4867 (2003).
4. Fuerstman, M. J., Deschatelets, P., Kane, R., Schwartz, A., Kenis, P. J. A., Deutch, J. M. and Whitesides, G. M. Solving mazes using microfluidic networks. Langmuir 19, 4714-4722 (2003).
5. Grzybowski, B. A., Wiles, J. A., and Whitesides, G. M. Dynamic self assembly of rings of charged metallic spheres. Physical Review Letters 90, (2003).
6. Grzybowski, B. A. and Whitesides, G. M. Directed dynamic self-assembly of objects rotating on two parallel fluid interfaces. Journal of Chemical Physics 116, 8571-8577 (2002).
7. Grzybowski, B. A. and Whitesides, G. M. Dynamic aggregation of chiral spinners. Science 296, 718-721 (2002).
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