Transient response in ultra-high speed liquid lenses

Liquid lenses are appealing for applications requiring adaptive control of the focal length, but current methods depend on factors such as liquid inertia that limit their response time to tens of milliseconds. A tunable acoustic gradient index (TAG) lens uses sound energy to radially excite a fluid-filled cylindrical cavity and produce a continuous change in refractive power that, at steady state, enables rapid selection of the focal length on time scales shorter than 1 s. However, the time to reach steady state is a crucial parameter that is not fully understood. Here we characterize the dynamics of the TAG lens at the initial moments of operation as a function of frequency. Based on this understanding, we develop a model of the lens transients which incorporates driving frequency, fluid speed of sound and viscosity, and we show that is in good agreement with the experimental results providing a method to predict the lens behaviour at any given time.