Cheng-Pu Chiu

Liquid Lenses and Driving Mechanisms: A Review

Abstract In this paper, we discuss liquid lenses driven by various mechanisms. By properly designing the device structure and choosing the optimal materials, the liquid lenses offer great potential for practical uses. The driving mechanism dictates the application and performance of the liquid lenses. Here we categorize the driving mechanisms into mechanical and electrical ones. In general, mechanical driving with an elastic membrane and an external pump drives liquids in a cavity by controlling the hydraulic pressure. The mechanical driving method can be applied to most of the liquids, but the application of the electrical driving method would be limited by the conductivity or the permittivity of the liquids. Therefore, the properties of the different liquids, e.g., dielectric liquids, liquid crystal molecules, and conductive liquids, deeply affect the mechanism we may choose to realize a liquid lens. Among various electrical methods, dielectrophoresis (DEP), electrostatic forces, and electrowetting-on-dielectric (EWOD) are emphasized here for driving dielectric liquids, liquid crystal molecules, and conductive liquids, respectively. DEP deforms the liquid lenses when the permittivities are different between the liquid and the medium. Electrostatic force orients the liquid crystal molecules to follow the applied electric field. Electrowetting-driven liquid lenses change their focal lengths by altering the contact angle. Here we show the designs and the structures of liquid lenses to describe their mechanisms, performances and feasibilities. It is worth mentioning that the liquid lenses using electrowetting have been commercialized. No moving parts would be the most important reason to use electrical manipulations rather than mechanical ones.

Electrowetting on polymer dispersed liquid crystal

Polymer dispersed liquid crystal (PDLC) is used as a dielectric layer in electrowetting. By applying voltage between a liquid droplet and the electrode underlying PDLC, electrowetting occurs at the liquid/PDLC interface accompanied with electro-optic responses of the reoriented LC droplets embedded in PDLC. Two basic experiments investigating the electrowetting by sessile water droplets and the electro-optic effects through squeezed water droplets were design and performed. The basic functions of a liquid lens and droplet manipulations, including transporting, splitting, and merging, were demonstrated.

Transmittance tuning by particle chain polarization in electrowetting-driven droplets.

A tiny droplet containing nano∕microparticles commonly handled in digital microfluidic lab-on-a-chip is regarded as a micro-optical component with tunable transmittance at programmable positions for the application of micro-opto-fluidic-systems. Cross-scale electric manipulations of droplets on a millimeter scale as well as suspended particles on a micrometer scale are demonstrated by electrowetting-on-dielectric (EWOD) and particle chain polarization, respectively. By applying electric fields at proper frequency ranges, EWOD and polarization can be selectively achieved in designed and fabricated parallel plate devices. At low frequencies, the applied signal generates EWOD to pump suspension droplets. The evenly dispersed particles reflect and∕or absorb the incident light to exhibit a reflective or dark droplet. When sufficiently high frequencies are used on to the nonsegmented parallel electrodes, a uniform electric field is established across the liquid to polarize the dispersed neutral particles. The induced dipole moments attract the particles each other to form particle chains and increase the transmittance of the suspension, demonstrating a transmissive or bright droplet. In addition, the reflectance of the droplet is measured at various frequencies with different amplitudes.

Particle chain display – an optofluidic electronic paper

A particle-based display medium and a driving mechanism insensitive to the charge polarity of those particles, based on the transformation of particle chains, are developed for reflective electronic paper displays. Particle chains are formed by dipole-dipole interactions between polarized particles with an appropriate electric field applied across the tested display medium, i.e. the solution that regulates the light in the field of display technology, containing neutral polystyrene (PS) particles dispersed in water. Formation of the particle chains results in a large change in optical transmittance and reflectance of the display medium. The performance of the particle chain displays (PCD) was evaluated according to macroscopic (device), microscopic (particle) and optical (reflectance) points of view. A display medium (thickness 100 μm) containing colored PS particles (3 μm, 2.5% w/v) was polarized to display the fixed images of the directly driven electrodes and programmable images of arrayed (5 × 5) electrodes with electric fields (0.48 MV m(-1) and 0.09 MV m(-1), 500 kHz, respectively). The formation of particle chains under electric fields (0.2 MV m(-1) and 0.4 MV m(-1), 500 kHz) was observed in the microscopic images of a display medium (thickness 100 μm) with fluorescent PS particles (5 μm, 1%). Images recorded with a confocal microscope demonstrated the particle chains. The opacity, a common parameter serving to characterize a display medium, was derived by measuring the reflectance ratio of a black background to a white background of the display medium with varied thickness and particle concentration. The temporal response of a display medium (thickness 50 μm) with black PS particles (3 μm, 5%) was tested. When an electric field (0.6 MV m(-1), 500 kHz) was applied, the reflectance increased twice at the first data point in 0.7 s, attaining a contrast ratio of 2. Application of a voltage (20 s) yielded a contrast ratio of 10. The performance of a tested display medium, composed of simple PS particles and water and driven to form particle chains by polarization, is reported.

Enhanced Droplet Mixer by LDEP on Spiral Microelectrodes

We demonstrated a micromixer in which a liquid column was pumped in a virtual channel without channel walls by LDEP. Two droplets were first transported and joined together by EWOD actuation. The small contact area at the interface of the merged droplet and the long diffusion distance caused long mixing time by merely diffusion. A spiral-shaped LDEP electrode was designed to draw a thin liquid column from the merged droplet at the interface in order to increase the contact area and reduce the diffusion distance. Since no channel walls were required for LDEP pumping, the spiral-shaped liquid column would merge together again to form a new droplet for further analyses.

Integrated Digital and Analog Microfluidics by EWOD and LDEP

EWOD (electrowetting-on-dielectric) and LDEP (liquid dielectrophoresis) are investigated to provide digital and analog microfluidic functions on an integrated chip respectively. By altering the frequency of the applied voltage and the surrounding medium of a EWOD device, we found that when using oil as surrounding medium and applying an AC signal 100 kHz, liquid column would be drawn and follow the thin connecting lines of the EWOD driving electrodes instead of remaining on top of the center of the EWOD driving electrodes. This new phenomenon is described in this paper and is regarded as a LDEP effect. Three fundamental tools for integrated digital and analog microfluidics are developed, including a digital-to-analog converter, an analog-to-digital converter, and a valve. Combining EWOD and LDEP effects, liquids can be pumped on a virtual channel (analog microfluidics), defined by energized thin LDEP electrode lines, continuously from liquid reservoir or from a digitized droplet (digital microfluidics). On the contrary, liquids on LDEP electrodes (analog microfluidics) can also be pumped on a EWOD electrode and be digitized in droplet forms with precise volumes (digital microfluidics). EWOD and LDEP can be selectively programmed on a single chip, making integrated digital and analog microfluidics a reality

45.2: Reflective Electronic Paper Display Utilizing Electric Polarized Particle Chains

We demonstrated a display technique by applying an AC field to polarize the originally evenly dispersed microparticles in liquid solution. The polarized particles would attract each other and form particle chains along the field lines. As a result, the light transmittance of the liquid can be regulated as a reflective display.

P-106: Characterization and Packaging of Electronic Paper Display Based on Particle Polarization

A particle-based reflective electronic paper has been characterized. Based on the particle polarization, we drive the neutral microspheres to form particle chains in a water solution by applying an AC electric field. The particle-chain transformation regulates the incident light and the reflectivity of the display. Here we developed the device packaging with hydrophilic surface treatments and UV sealing techniques. with a passive-matrix addressing scheme, a reflective electronic paper having 64×64 pixels by particle polarization is realized on a 4 inch by 4 inch panel.

Droplet Manipulation by Electrowetting on Polymer Dispersed Liquid Crystal

Droplets were successfully manipulated on an electrowetting-on-dielectric (EWOD) device whose dielectric layer was a polymer dispersed liquid crystal (PDLC) thin film. Dielectric layer is essential to EWOD since it would store charges and change the surface wettability. Therefore, most of the research on the dielectric material was focused on its dielectric constant and strength. However, besides serving as only a capacitor, many dielectric materials possess other physical or chemical characteristics including optical, thermal, and mechanical properties. In this paper, we introduced and investigated electrowetting on an electro-optic material PDLC. By applying a voltage across the PDLC film, dual phenomena, EWOD and electro-optic effects, were achieved. The proposed droplet manipulating device would be beneficial when light regulation is necessary.