Page 58 - Optofluidics Fundamentals, Devices, and Applications

Optical Components Based on Dynamic Liquid-Liquid Interfaces 39

simplifies recycling, and facilitates closed-loop operation. Thermal
diffusivity in liquids is typically two orders of magnitude higher than
mass diffusivity of solute ions [6], however. A much higher rate of
flow is therefore necessary to maintain the contrast in refractive index
2
across the L interface.

3-4 Dynamic Liquid-Liquid Interfaces
in Microfluidic Systems
The interface between laminar streams in microfluidic systems is at
dynamic steady state: continuous flow is required to maintain the
interface between the streams. The use of this dynamic interface as
part of an optical component has advantages and disadvantages, as
discussed next.

2
3-4-1 L Interfaces Are Reconfigurable in Real Time
2
Liquids can be replaced and/or replenished continuously in L sys-
tems. This capability for replacement allows injection of liquids with
different properties (e.g., index of refraction, absorption, and fluores-
cence) to tune the optical output of the system in real time. The ability
to replenish liquids makes photobleaching and related phenomena
relatively unimportant, since the component that is bleached is
replaced continuously. This latter feature is especially important for
the operation of microfluidic dye lasers—without a continuous
replacement of solutions of dye, the lasing action would stop in a few
seconds when the dye is photobleached. The disadvantage here is the
need for constant supply of liquids. Microfluidic systems allow eco-
nomical use of solutions and reagents, however; the consumption of
fluids is therefore limited.
2
Another way to reconfigure the L interface is by manipulating
2
the flow conditions. The L interface is deformable: it is possible to
change the position or the shape of the liquid-liquid interface, and
therefore the path of light inside the optofluidic devices by changing
rates of flow (and other properties such as viscosity) of the fluids.
Changing the relative volumetric rates of flow between the streams of
2
liquids changes the position or the shape of the L interface. The L 2
lens, for example, can take up shapes varying from biconvex to plano-
convex to meniscus simply by changing the relative rates of flow
between the core and cladding streams.
The switching time of liquids in microchannels is on the order of
seconds. This time scale is limited by the time required for mass
transport of liquids in the microfluidic system. This value is much
longer than that in conventional optical systems. Nevertheless, the
liquid-liquid system should meet the demands of applications that
do not require fast switching, such as optical sensing and bioassays.