The development and ultimate operation of a nanocomposite high-aspect-ratio photoinjection (HARP) device is presented in this work. The device makes use of a nanocomposite material as the optically active layer and the device achieves a large optical penetration depth with a high aspect ratio which provides a strong actuation force far away from the point of photoinjection. The nanocomposite material can be continuously illuminated and the position of the microdroplets can, therefore, be controlled to diffraction limited resolution. The nanocomposite HARP device shows great potential for future on-chip applications.
Digital (droplet-based) microfluidic systems apply electromagnetic characteristics as the fundamental fluid actuation mechanism. These systems are often implemented in two-dimensional architectures, overcoming one-dimensional continuous flow channel practical issues. The fundamental operation for digital microfluidics requires the creation of an electric field distribution to achieve desired fluid actuation. The electric field distribution is typically non-uniform, enabling creation of net dielectrophoresis (DEP) force. The DEP force magnitude is proportional to the difference between microdroplet and surrounding medium complex dielectric constants, and the gradient of the electric field magnitude squared. Force sign/direction can be manipulated to achieve a force towards higher (positive DEP) or lower (negative DEP) electrostatic energy by tailoring the relative difference between microdroplet and surrounding medium complex dielectric constants through careful selection of the devices fabrication materials. The DEP force magnitudes and directions are applied here for well-controlled and high-speed microdroplet actuation. Control and speed characteristics arise from significant differences in the microdroplet/medium conductivity and the use of a micropin architecture with strong electric field gradients. The implementation, referred to here as a DEP microjet, establishes especially strong axial propulsion forces. Single- and double-micropin topologies achieve strong axial propulsion force, but only the double-micropin topology creates transverse converging forces for stable and controlled microdroplet actuation. Electric field distributions for each topology are investigated and linked to axial and transverse forces. Experimental results are presented for both topologies. The double-micropin topology is tested with biological fluids. Microdroplet actuation speeds up to 25 cm/s are achieved—comparable to the fastest speeds to-date.
Microfluidics technologies have received great attention and appear in many bioanalyses applications. A recent microfluidics subset has appeared as droplet-based digital microfluidics (DMF). Here, microdroplets are manipulated in a two-dimensional on-chip plane using electric fields, contrasting the one-dimensional pressure-based channel flow of continuous flow microfluidics. These DMF systems fundamentally offer reconfigurability, whereby one device performs many bioanalysis tasks. A subset of DMF systems called optoelectrowetting is also of recent interest due to its ability for intricate microdroplet routing processes in the on-chip plane. For an optoelectrowetting chip, the DMF structure is modified with optically triggered electrodes with arrayed photoconductive switches. The arrayed photoconductive switches are optically-activated so microdroplets in the vicinity are routed to the illuminated switch. Unfortunately, such systems still require intricate electrode arrays, limiting microdroplet actuation resolution by the electrode size. This work proposes an on-chip optofluidic device with a continuous and planar semiconductor layer as the photoconductive mechanism. An illuminated section of the semiconductor layer acts as a localized electrode, with the photogenerated charge-carriers attracting nearby microdroplets. Given this planar topology, the illuminating beam is used to move the microdroplets continuously over the on-chip plane with precise optical control. The resolution for such a process is ultimately limited by charge-carrier diffusion, so an alternative material, a nanocomposite, is introduced to the on-chip device design. The nanocomposite consists of 20 nm semiconductor nanoparticles embedded in an insulative polymer host. This gives restricted diffusion length, being on the nanometer-scale of the nanoparticle diameter. Experimental device operation is demonstrated.