Simple, robust storage of drops and fluids in a microfluidic device
Microfluidics, Coalescence, PDMS, Wettability and Contact Angle
Drop storage is of paramount importantce in microfluidic systems to manipulate and perform operations on drops of fluid. In this paper, the authors propose a new device design for storing drops that is simple, does not require user intervention, and avoids any problems that come from drop to drop contact. The device was made using conventional methods for microfluidic devices on PDMS. It has two operational functions, the first christened "created, then store", the second "store and create". The following image shows the fundamental design of the device and its function of "create, then store".
In (a) we see a schematic illustrating how a drop that is first created at a different location is flowed into the device. The width of the main channel and well are 150<math>\mu</math>m, the bypass is 75<math>\mu</math>m, and the restriction is 15<math>\mu</math>m. The height of the channels (the direction coming out of the image) is 110<math>\mu</math>m. During the operation of the device, there is a steady flow of oil. When a drop of water approaches the junction, a combination of capillary forces and hydrodynamic resistance determines its subsequent behavior. The authors observed that for the system of oil and water, the incoming water drop will first enter the well, and only head into the bypass when the well has been completely filled. The next images illustrate how the system behaves differently depending on factors such as flow composition and wettability.
The first image on the left shows how the device behaves when only water is flowed through. In the main channel, the top 48% of incoming water has been colored with a dye. When passing through the device, 70% of the water going through the bypass is dyed. This means that 48/70, or about 70%, of the water flows through the bypass and about 30% through the the restriction. The second image shows the effect when oil is flowed through the device followed by a water. In this case, the water all goes into the well until it is filled, then it goes into the bypass. The pair of images on the right shows the effect of wettability on the behavior of the drop in the storage device. In (a), a fluorinated oil is used so that the water wets the PDMS interface, but in (b) 2% surfactant is added to the oil so that the water does not wet the surface and instead forms a rounded drop. In the wetting case, the oil is able to drain away from the water-PDMS interface.
As the figure above shows, this device works for multiple drops. If we first create multiple drops, then flow them into these storage devices in series, the first drop will occupy the first storage well, then second drop will bypass the first well and enter the second well, etc. This is the function of "create, then store". The image below shows the "store and create" function.
In this functionality, the device is able to create drops using the property described above that a stream of water entering after the oil will first enter the well, then when the well is full, enter the bypass. This way, by having lots of devices in series and flowing in a plug of water after the oil, we can form a stored drop in each of the wells. The next issue then becomes whether the devices can be mass-produced and in series and still be operational. Since PDMS design is done using photolithography (just like semiconductor electronics), it is easy to mass-produce these storage bins on a single chip. The following images show high density fabrication of this device and its workability.
The top image shows high-density fabrication of the storage wells. There are 90 wells per cm<math>^2</math>. Each well is 200<math>\mu</math>m wide and with 25nL volume. The bottom images shows these wells filled with oil and dyed water. Lastly is the issue of how to recover the stored droplets. According to the paper, extraction would be done by reversing the flow of the oil.
Applications and Relevance to Soft Matter
Microfluidics has very far-reaching applications, which is why it is such a hot topic of research today. Scientists envision a "lab-on-a-chip" future, where we are able to use microfluidic chips to manipulate tiny drops of fluids however we wish, mixing them, separating them, translating and rotating them, and perform analysis on them such as spectroscopy or imaging. Perhaps, then, it would be best to focus on the importance of drop storage. The paper cited drop convalescence as a problem in drop storage, because if multiple drops contact one another, they may or may not combine or mix. The proposed storage device is able to separately store multiple drops in their own wells, precluding any problems from convalescence. Previously, many other storage devices have been proposed, but one strength of this setup is that it is passive, meaning that aside from a steady flow of oil, there is no other energy source or action needed to store the drops in their own containers. To me, the downside of this is that extraction can only be done through reversing the flow of the system, which may or may not be that difficult (I could see the storage of drops as a separate subsystem to the main microfluidic network such that it is relatively easy to reverse the flow of just that subsystem). The dual functionality of "create, then store" and "store and create" is also potentially useful in creating assays out of a main sample. One of the main strengths of microfluidics after all is to be able to use a small sample and perform many different tests on it, which requires breaking the sample up quickly into many smaller samples. This method to create drops, unlike many others, has no waste because all of the sample is converted into equal sized droplets (except perhaps the last one).
Soft matter is very relevant in microfluidics, because we are concerned heavily with ideas such as wettability, contact angle (and the related hysteresis, which was discussed in the paper), stability, flows, pressures and forces across interfaces, emulsions, convalescence, etc. Microfluidics itself also presents new problems for soft matter (e.g. getting things to mix on such a small scale where interfacial forces dominate).
H. Boukellal, S. Selimovic, Y. Jia, G. Cristobal and S. Fraden. "Simple, robust storage of drops and fluids in a microfluidic device." Lab on a Chip 9, 331–338 (2009) Movies! Lab on a Chip Supplementary Information