Microfluidic devices containing reversibly pinned droplet samples and methods
A microfluidic device comprising: (a) a plate comprising a substrate, a plurality of electrodes, and a first layer of hydrophobic material applied over the plurality of electrodes; (b) a processing unit operably programmed to perform a method of pinning an aqueous droplet within the microfluidic device; and (c) a controller operably connected to a power source, the processing unit, and the plurality of electrodes. The method of pinning an aqueous droplet comprises: applying an electric field of a first polarity to an aqueous droplet located on the surface of the layer of hydrophobic material and having a first contact angle, to cause the droplet to maintain a second contact angle in the absence of the electric field, wherein the aqueous droplet contains a surfactant and the second contact angle is less than the first contact angle.
1. A method of pinning an aqueous droplet containing a surfactant within a microfluidic device, the microfluidic device comprising a plate comprising a plurality of electrodes and a layer of hydrophobic material completely covering the plurality of electrodes, the method of pinning the aqueous droplet comprising:
introducing the aqueous droplet to a surface of the layer of hydrophobic material, the aqueous droplet having a first contact angle; and
applying an electric field of a first polarity to the aqueous droplet for a first time at a first place on the surface of the layer of hydrophobic material, thereby causing the aqueous droplet to maintain a second contact angle in the absence of the electric field after the first time, whereby the aqueous droplet remains in the first place and does not drift from the first place during the method of pinning the aqueous droplet, and the second contact angle is less than the first contact angle, wherein the first contact angle is greater than or equal to 90 degrees and the second contact angle is less than 90 degrees.
2. The method of claim 1 , wherein the first contact angle is greater than or equal to 90 degrees and the second contact angle is less than 75 degrees.
3. The method of claim 1 , wherein the first polarity is negative.
4. The method of claim 1 further comprising the step of applying an electric field of a second polarity opposite to the first polarity to the aqueous droplet during a second time causing the aqueous droplet to maintain a third contact angle in the absence of an electric field after the first and second times, and the third contact angle is greater than the second contact angle.
5. The method of claim 1 , wherein the surfactant is a non-ionic surfactant.
6. The method of claim 1 , wherein the surfactant comprises polyethylene oxide.
7. The method of claim 1 , wherein the hydrophobic material is Teflon® AF.
8. A method of pinning an aqueous droplet containing a surfactant within a microfluidic device, the microfluidic device comprising:
a top plate including a top substrate and a first layer of hydrophobic material applied to a surface of the top substrate,
a continuous conductor between the first layer of hydrophobic material and the top substrate,
a bottom plate comprising a bottom substrate,
a plurality of electrodes,
a second layer of hydrophobic material completely covering the plurality of electrodes, and
a gap between the first and second layers of hydrophobic material, the method of pinning the aqueous droplet comprising:
introducing the aqueous droplet into the gap, the aqueous droplet having a diameter corresponding to a longest straight line segment between two points on the aqueous droplet surface; and
applying an electric field of a first polarity to the aqueous droplet during a first time at a first place, thereby causing the diameter to increase to establish an increased diameter and maintain the increased diameter in the absence of the electric field after the first time, whereby the aqueous droplet remains in the first place on the surface of the second layer of hydrophobic material and does not drift from the first place during the method of pinning the aqueous droplet.
9. The method of claim 8 , wherein the first polarity is negative.
10. The method of claim 8 , wherein the microfluidic device includes a controller and the controller is configured to apply an electric field of a second polarity opposite to the first polarity to the aqueous droplet during a second time, thereby causing the increased diameter of the aqueous droplet to decrease to establish a decreased diameter, and maintain the decreased diameter in the absence of the electric field after the first and second times.
11. The method of claim 8 , wherein the surfactant is a non-ionic surfactant.
12. The method of claim 8 , wherein the surfactant comprises polyethylene oxide.
13. The method of claim 8 , wherein the hydrophobic layer on the top substrate and the hydrophobic layer on the bottom substrate are the same composition.
14. The method of claim 13 , wherein the hydrophobic material is Teflon® AF.