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

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 ΔnD of ≥0.290, which enabled true optical zooming
capability.