Control of the Shape of Liquid Lenses on a Modified Gold Surface Using as Applied Electrical Potential across a Self-Assembled Monolayer
Title: Control of the Shape of Liquid Lenses on a Modified Gold Surface Using as Applied Electrical Potential across a Self-Assembled Monolayer
Authors: C.B. Gorman, H.A. Biebuyck, G.M. Whitesides
Journal: Langmuir 11 (1995) pgs. 2242-2246
The focus of this paper is describing how the contact angle of drops of liquid can be controlled and varied using an applied electrical potential. The drops could act as lenses and could be used an an optical switch which focuses/ defocuses light.
The electrical potential used to control the contact angles is applied across a self-assembled monolayer (SAM) on a gold surface. The drops described in this paper are hexadecanethiol (HDT). When the gold was transparent, the drops acted as lenses for transmitting light. The liquid HDT drops resting on a gold surface and surrounded by aqueous electrolyte - act like lenses whose shape can be quickly, reproducibly and reversibly changed by applying a potential to the substrate supporting the drops.
A drop of HDT was placed on a gold surface under an aqueous electrolyte solution (see figure below).
The drop spread reactively on gold at 0 V (relative to the silver electrode) and formed a hydrophobic monolayer under the drop. the advancing contact angle of this drop reached a value of ~37 degrees (see figure 1 below). When the potential of the gold was switched to -1.7V, the SAM underwent electrodesorption and the drop retracted. The contact angle of the drop as it receded was ~ 128 degrees. As the hydrophobic SAM desorbed and water wet the resulting charged gold surface, the drop retracted on the surface. When the drop retracted, this also reduced the interfacial area between the drop and the water.
The drop's contact angle changed continuously as a function of the potential between these two limiting values (see figure 2). Figure 2 shows hysteresis in the contact angles of the spreading/retraction which did not change with the thickness of the gold or with the chain length of the thiol.
The switching speeds for drops were determined by changing the potential of the gold (-1.7 V for retraction of drop and 0V for spreading of the drop) and then observing he time required for the shape to change by taking a video.
A drop of HDT on transparent gold was shown to be able to behave as a planar convex lens. A drop on the surface at -1.7 V will focus light transmitted through it (see figure 4 below).
The images in figure 4 show that the drop can act as an optical switch that will focus and defocus light or an image in the far field as a function of an applied electrical potential. In Figure 4c - the drop could be electrochemically perturbed to bring an image repeatedly into and out of focus.
The focal length of the planar convex lens (that is formed by the drop) is proportional to its radius of curvature such that f= r/ <math>\delta</math> n where r is the radius of curvature and <math>\delta</math> n is the difference in refractive indices between the two media
The size and shape of the drops under the aqueous electrolyte can be controlled with specific types of spatially patterned self-assembled monolayers. The gold surface is patterned with HDT using "microcontact printing". The patterned surface is moved through a think layer of HDT on top of aqueous electrolyte which causes the drops of HDT to self-assemble on the hydrophobic regions of the patterned SAM. By lowering the potential of the substrate - the shape of the drops changes and the focal point of light transmitted through each microlens changes.
Self-assembly of drops on surfaces can be directed by patterned SAM's. When the electrochemical potential is changed, the entire pattern of SAM's are desorbed reductively from the surface. Drops were seen to remain on the regions of the surface that were originally patterned with HDT. The experiments conducted in this paper show how an electrical potential can be used to influence the shape of a drop in a reversible way. This has applications in making liquid lenses and arrays of lenses whose focal lengths can be changed by changing the potential. Patterned self-assembly can be used to make arrays of drops of specific size, shape and organization. Drops of HDT could be used an electroactive switches to control the coupling of light to a detector.