Switchable electrowetting of droplets on dual-scale structured surfaces
2D layout of micrometer and nanometer masks. (a) Layout of the micrometer test mask showing the 21 different micrometer die identifications. Each die is 22 × 26 mm2. (b) Layout of the nanometer test mask showing the 20 different device identifications.
Scanning electron microscope images of fabricated dual-scale structures. (a) 0.6 μm wide parallel corrugations (d/w = 1) patterned into the microscale checkerboard shape (checkerboard 20/20) b = 20 μm and a = 20 μm. (b) 0.4 μm posts (d/w = 1) patterned into the microscale checkerboard shape (checkerboard 20/20) b = 20 μm and a = 20 μm. (c) Isolated posts, 1 μm posts (d/w = 2) patterned into the microscale parallel corrugation shape (line 20/20) b = 20 μm and a = 20 μm. (d) 1 μm wide parallel corrugations (d/w = 1) patterned into the microscale bullseye shape (square bullseye 20/20) b = 20 μm and a = 20 μm. The height of the features was measured as h = 2 μm.
Fabrication process for creating dual-scale structures. (a) Deposit 500 nm of PECVD as a hard mask. (b) Pattern and etch nanometer features into hard mask. (c) Pattern and etch micrometer feature into hard mask. (d) Etch the dual nanometer and micrometer features into silicon. (e) Strip SiO2 hard mask. (f)Grow 50 nm of thermal oxide over silicon surface. (g) Coat surface with CYTOP using a surface grafting process.
(a) Configuration for electrowetting on a textured surface. (b) Schematic of the solid–liquid interaction with the surface showing the architecture of the silicon electrode and dielectric insulating layers.
(Color online) (a) Contact angles for 1-μm posts with a spacing-to-width ratio of d/w = 1, superimposed on a microscale checkerboard, with CYTOP grafting. Roll-off angles ϕ are indicated within each bar. Roll-off angle was determined by tilting the surface and recording the angle at which the drop started to move. (b) Scanning electron microscope images of 1-μm posts with varying microscale checkerboard geometries used to obtain contact angle measurements shown in water contact angle plot (a). A surface consisting of only 1-μm posts with a spacing-to-width ratio of d/w = 1 had contact angle of θ = 153°, and a roll-off angle ϕ = 16°.
Image showing a portion of the three phase contact line of a water drop on the micrometer parallel corrugation surface (line 10/10 b = 10 μm, a = 10 μm). It is clear that the interface is pinned along the edge of line feature. Image made by sandwiching the drop between the micrometer surface and a glass slide. Since the textured surface is more hydrophobic than the glass slide the three phase contact line at the surface can be directly viewed.
Plot of contact angle vs negative dc voltage for 1-μm posts having a spacing-to-width d/w = 1 patterned in micrometer checkerboard shape, checkerboard-20/20 with a spacing-to-width b/a = 1 and for 1-μm posts having a spacing-to-width d/w = 2 patterned in micrometer checkerboard shape, checkerboard-20/60 with a spacing-to-width b/a = 0.33. Included in the plot are the results for a nontextured planar surface.
(a) SEM image of the 1-μm wide corrugations with spacing-to-width ratio = 1:1 patterned into the checkerboard pattern b = 20 μm, a = 60 μm. (b) Side view of droplet on surface prior to electrowetting actuation. (c) View of droplet with −18 V dc applied. (d) View of droplet immediately after the potential has been removed. (e) Plot of contact angle vs negative dc voltage. Measurements parallel with corrugations.
Example of an electrowetting response of a 10-μL drop on the single-scale 0.6-μm-wide corrugations. (a) Voltage-off state. (b) Wetting state at −20 V dc. The drop is elongated in the direction of the corrugations.
Contact angle vs voltage comparing single-scale 1-μm wide corrugation and dual-scale 1-μm wide corrugation patterned into the 10-μm wide parallel corrugation shape. Included in the plot are the results for a nontextured, planar surface. Plot includes the contact angle for the voltage-on state (filled symbols) and corresponding voltage-off state (open symbols).
Features and sizes of the of the nanometer structures on the nanometer test mask. “Name” is the common name for the nanometer feature, w is the feature width in micrometers, d is the distance between features in micrometers, and Pitch is the sum of w and d.
Features and sizes of the of the micrometer structures on the micrometer test mask. “Name” is the common name for the micrometer feature, a is the feature width in micrometers, b is the distance between features in micrometers, and Pitch is the sum of a and b.
Reversibility range in degree of contact angle for the features categories. Micrometer only features are in column called “No Nano” and nanometer only features are in row called “No Micro.”
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