Simple, robust storage of drops and fluids in a microfluidic device

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Keywords

Microfluidics, Coalescence, PDMS, Wettability and Contact Angle

Summary

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".

Device design

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.

Device with different fluids

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.

Creating multiple drops, then storing them.

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.

Store drops while creating them.

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.

Mass-production of the storage device.

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).

References

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



2nd Wiki Entry

AP 225 - Fall 2009


Keywords

Microfluidic device, drop processing, Laplace pressure.

Summary

In this paper, a microfluidic device is used to create and store stable droplets without the need to use any surfactants to prevent coalescence. Other existing methods of drop processing in microfluidic devices involve the use of multiplexed valves to mix drop contents and multiple, individual storage chambers. These methods allow the creation and storage of drops, however, the processing time is too long and the design/manufacturing of these microfluidic devices is intricate.

There are two drop processing methods that are proposed. One involves storing drops that are created upstream from the channel and is called the “create then store” method. The second is called the “store and create” method where drops are simultaneously created and stored. The “create then store” method consists of a main channel carrying drops that are created elsewhere upstream, and it also consists of a second channel at 90 degrees from the main channel called the “bypass” as seen in the first figure above. At one point, the main channel narrows by a factor of ten. The region between the by-pass channel and the channel narrowing is the storage well. When only water is introduced into the channel, the liquid takes the path of least resistance and flows through the much wider by-pass. However, when the water droplet is suspended in oil, none of it goes into the bypass. Instead, the drop is unable to pass through the narrower channel and as a result gets stored in the “well.” The “store and create” method consists of exactly the same channel design as described in the previous method. The only exception is that the entire device is first filled with oil and then the water is injected into tubing connected to the device. When the flow of oil is started the water drops are stored in the wells as shown in the fourth figure above.


Soft Matter Connection

The storage of drops in the wells can be attributed to surface tension forces. The drop cannot go through the narrow opening unless it is driven by higher pressures. In order for the drop to be forced through, the pressure across the drop must be greater than the Laplace pressure defined as:

<math>\Delta P = \frac{2 \gamma}{r}</math>

where <math>\gamma</math> is surface tension and r is the radius of the droplet.

By taking advantage of this, the drops can be created and stored in a very simple way without use of surfactants. Another great advantage of this system is that the extraction of drops is done by simply reversing the flow of oil. The device is also easy to use and cost-effective since the injection of water can be done with a hand-held syringe and the only materials needed are PDMS and oil. Some of the applications of this system can be cell sorting or protein analysis in the drops. Although when working with biological materials such as cells, one needs to consider contamination issues that might arise.