Development of Biphasic Formulations for Use in Electrowetting-Based Liquid Lenses with a High Refractive Index Difference

Abstract

Commercial electrowetting-based liquid lenses are optical devices containing two immiscible liquids as an optical medium. The first phase is a droplet of a high refractive index oil phase placed in a ring-shaped chassis. The second phase is electrically conductive and has a similar density over a wide temperature range. Droplet curvature and refractive index difference of two liquids determine the optical strength of the lens. Liquid lenses take advantage of the electrowetting effect, which induces a change of the interface's curvature by applying a voltage, thereby providing a variable focal that is useful in autofocus applications. The first generation of lens modules were highly reliable, but the optical strength and application scope was limited by a low refractive index difference between the oil and conductive phase. Described herein is an effort to increase the refractive index difference between both phases, while maintaining other critical application characteristics of the liquids, including a low freezing point, viscosity, phase miscibility, and turbidity after thermal shock. An important challenge was the requirement that both phases have to have matching densities and hence had to be optimized simultaneously. Using high throughput experimentation in conjunction with statistical design of experiments (DOE), we have developed a series of empirical models to predict multiple physicochemical properties of both phases and derived ideal locations within the formulation space. This approach enabled the development of reliable liquid lenses with a previously unavailable refractive index difference of Δ n of ≥0.290, which enabled true optical zooming capability.

Taming Electrowetting Using Highly Concentrated Aqueous Solutions

“…Electrowetting at the liquid|liquid interface has found notable applications in commercial optofluidic devices, such as liquid lenses. , The latter are optical devices containing two immiscible liquids which serve as an optical medium. In most cases, the electrolytically conducting phase is an aqueous electrolyte and the insulating phase is an organic solvent.…”

Non-aqueous organic solution based on a large-aperture spherical electrowetting liquid lens with a wide tunable focal length range

“…This may substantially increase the lens response time. To address this issue, we formulated a glycol conductive liquid with the same weight fraction concentration as the conductive liquid mentioned in the article (the propane-1,3-diol containing 1 w% TBAC) and found that the viscosity could be reduced to 18.43 cP, which is not yet commercially required (<10 cP), 56 but the reduction in viscosity is significant. Using glycol formulation as a conductive liquid is expected to improve the response time.…”

“…For formulating conductive and insulating liquids for electrowetting liquid lenses, we have some requirements for the properties of these biphasic liquids. 56 The biphasic liquids must be immiscible, and their densities need to match. At the same time, the higher the difference between their refractive indices, the better.…”

“…[51][52][53][54][55] However, the existence of water in the conductive liquid inevitably implies the problem of evaporation, hydrolysis bubbles, or the tendency toward a higher freezing point. 56,57 For the insulating liquid, the most basic requirement is to form two clear immiscible phases with conductive solution. Optical oils, such as silicone oil, germanium oil, or laser liquid, have been used as an insulating liquid.…”

“…51–55 However, the existence of water in the conductive liquid inevitably implies the problem of evaporation, hydrolysis bubbles, or the tendency toward a higher freezing point. 56,57…”