Brightness of Microtrench Superhydrophobic Surfaces and Visual Detection of Intermediate Wetting States.

For a superhydrophobic (SHPo) surface under water, the dewetted or wetted states are easily distinguishable by the bright silvery plastron or lack of it, respectively. However, to detect an intermediate state between the two, where water partially intrudes the surface roughness, a special visualization technique has been needed. Focusing on SHPo surfaces of parallel microtrenches and considering drag reduction as a prominent application, we (i) show the reliance on surface brightness alone may seriously mislead the wetting state, (ii) theorize how the brightness is determined by water intrusion depth and viewing direction, (iii) support the theory experimentally with confocal microscopy and CCD cameras, (iv) present how to estimate the intrusion depth using optical images taken from different angles, and (v) showcase how to detect intermediate states slightly off the properly dewetted state by simply looking. The proposed method would allow monitoring SHPo trench surfaces without bulky instruments-especially useful for large samples and field tests.

Current commercialization status of electrowetting-on-dielectric (EWOD) digital microfluidics.

The emergence of electrowetting-on-dielectric (EWOD) in the early 2000s made the once-obscure electrowetting phenomenon practical and led to numerous activities over the last two decades. As an eloquent microscale liquid handling technology that gave birth to digital microfluidics, EWOD has served as the basis for many commercial products over two major application areas: optical, such as liquid lenses and reflective displays, and biomedical, such as DNA library preparation and molecular diagnostics. A number of research or start-up companies (e.g., Phillips Research, Varioptic, Liquavista, and Advanced Liquid Logic) led the early commercialization efforts and eventually attracted major companies from various industry sectors (e.g., Corning, Amazon, and Illumina). Although not all of the pioneering products became an instant success, the persistent growth of liquid lenses and the recent FDA approvals of biomedical analyzers proved that EWOD is a powerful tool that deserves a wider recognition and more aggressive exploration. This review presents the history around major EWOD products that hit the market to show their winding paths to commercialization and summarizes the current state of product development to peek into the future. In providing the readers with a big picture of commercializing EWOD and digital microfluidics technology, our goal is to inspire further research exploration and new entrepreneurial adventures.

Fluidic conduits for highly efficient purification of target species in EWOD-driven droplet microfluidics.

Due to the lack of continuous flows that would wash unwanted specifies and impurities off from a target location, droplet microfluidics commonly employs a long serial dilution process to purify target species. In this work, we achieve high-purity separation for the case of electrowetting-on-dielectric (EWOD) based droplet microfluidics by introducing a "fluidic conduit" between a sample droplet and a buffer droplet. The long and slender fluidic path minimizes the diffusion and fluidic mixing between the two droplets (thus eliminating non-specific transport) but provides a conduit between them for actively transported particles (thus allowing the specific transport). The conduit is purely fluidic, stabilized chemically (e.g. using surfactants) and controlled by EWOD. The effectiveness of the technique is demonstrated by eliminating approximately 97% non-magnetic beads in just one purification step, while maintaining high collection efficiency (>99%) of magnetic beads.

Direct-referencing Two-dimensional-array Digital Microfluidics Using Multi-layer Printed Circuit Board.

Digital (i.e. droplet-based) microfluidics, by the electrowetting-on-dielectric (EWOD) mechanism, has shown great potential for a wide range of applications, such as lab-on-a-chip. While most reported EWOD chips use a series of electrode pads essentially in one-dimensional line pattern designed for specific tasks, the desired universal chips allowing user-reconfigurable paths would require the electrode pads in two-dimensional pattern. However, to electrically access the electrode pads independently, conductive lines need to be fabricated underneath the pads in multiple layers, raising a cost issue especially for disposable chip applications. In this article, we report the building of digital microfluidic plates based on a printed-circuit-board (PCB), in which multilayer electrical access lines were created inexpensively using mature PCB technology. However, due to its surface topography and roughness and resulting high resistance against droplet movement, as-fabricated PCB surfaces require unacceptably high (~500 V) voltages unless coated with or immersed in oil. Our goal is EWOD operations of aqueous droplets not only on oil-covered but also on dry surfaces. To meet varying levels of performances, three types of gradually complex post-PCB microfabrication processes are developed and evaluated. By introducing land-grid-array (LGA) sockets in the packaging, a scalable digital microfluidics system with reconfigurable and low-cost chip is also demonstrated.

Concentration and binary separation of micro particles for droplet-based digital microfluidics.

This paper describes a concept of concentration and binary separation of particles and its experimental confirmations for digital microfluidics where droplets are driven by the mechanism of electrowetting-on-dielectric (EWOD). As a fundamental separation unit, a binary separation scheme is developed, separating two different types of particles in one droplet into two droplets, one type each. The separation scheme consists of three distinctive steps, each with their own challenges: (1) isolate two different types of particles by electrophoresis into two regions inside a mother droplet, (2) physically split the mother droplet into two daughter droplets by EWOD actuation so that each type of particle is concentrated in each daughter droplet, and (3) free the daughter droplets from the separation site by EWOD to ready them for follow-up microfluidic operations. By applying a similar procedure to a droplet containing only one type of particle, two daughter droplets of different particle concentrations can be created. Using negatively charged carboxylate modified latex (CML) particles, 83% of the total particles are concentrated in a daughter droplet. Successful binary separation is also demonstrated using negatively charged CML particles and no-charge-treated polystyrene particles. Despite the undesired vortex developed inside the mother droplet, about 70% of the total CML particles are concentrated in one daughter droplet while about 70% of the total polystyrene particles are concentrated in the other daughter droplet.

Contact Angle Measurement of Small Capillary Length Liquid in Super-repelled State.

The difficulty of measuring very large contact angles (>150 degrees) has become more relevant with the increased popularity of super-repellent surfaces. Measurement is more difficult for dynamic contact angles, for which theoretical profiles do not fit well, and small capillary length liquids, whose sessile droplets sag by gravity. Here, we expand the issue to the limit by investigating dynamic contact angles of liquids with an extremely small capillary length (<1.0 mm), empowered by the superomniphobic surface that can super-repel even fluorinated solvents, which highly wet all materials. Numerically simulating and experimentally testing 13 different liquids on the superomniphobic surface, we discover their dynamic contact angles can be measured with a consistent accuracy despite their vastly different capillary lengths if one keeps the lens magnification inversely proportional to the capillary length. Verifying the droplet equator height is a key parameter, we propose a new Bond number defined by the equator height and optical resolution to represent the measurement accuracy of large contact angles. Despite negligible improvement for most liquids today, the proposed approach teaches how to measure very large contact angles with consistent accuracy when any of the liquids in consideration has a capillary length below 1.0 mm.

An integrated digital microfluidic chip for multiplexed proteomic sample preparation and analysis by MALDI-MS.

To realize multiplexed sample preparation on a digital microfluidic chip for high-throughput Matrix Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS), several fluidic functions need to be integrated. These include the generation of multiple droplets from a reservoir and parallel in-line sample purification. In this paper, we develop two critical new functions in handling protein solutions and standard proteomic reagents with electrowetting-on-dielectric (EWOD) actuation, leading to an integrated chip for multiplexed sample preparation for MALDI-MS. The first is a voltage sequence designed to generate a series of droplets from each of the three reservoirs--proteomic sample, rinsing fluid, and MALDI reagents. It is the first time that proteomic reagents have been dispensed using EWOD in an air (as opposed to oil) environment. The second is a box-in-box electrode pattern developed to allow droplet passing over dried sample spots, making the process of in-line sample purification robust for parallel processing. As a result, parallel processing of multiple sample droplets is demonstrated on the integrated EWOD-MALDI-MS chip, an important step towards high-throughput MALDI-MS. The MS results, collected directly from the integrated devices, are of good quality, suggesting that the tedious process of sample preparation can be automated on-chip for MALDI-MS applications as well as other high-throughput proteomics applications.

Specific binding and magnetic concentration of CD8+ T-lymphocytes on electrowetting-on-dielectric platform.

In the quest to create a low-power portable lab-on-a-chip system, we demonstrate the specific binding and concentration of human CD8+ T-lymphocytes on an electrowetting-on-dielectric (EWOD)-based digital microfluidic platform using antibody-conjugated magnetic beads (MB-Abs). By using a small quantity of nonionic surfactant, we enable the human cell-based assays with selective magnetic binding on the EWOD device in an air environment. High binding efficiency (∼92%)of specific cells on MB-Abs is achieved due to the intimate contact between the cells and the magnetic beads (MBs) produced by the circulating flow within the small droplet. MBs have been used and cells manipulated in the droplets actuated by EWOD before; reported here is a cell assay of a clinical protocol on the EWOD device in air environment. The present technique can be further extended to capture other types of cells by suitable surface modification on the MBs.

Structured surfaces for a giant liquid slip.

We study experimentally how two key geometric parameters (pitch and gas fraction) of textured hydrophobic surfaces affect liquid slip. The two are independently controlled on precisely fabricated microstructures of posts and grates, and the slip length of water on each sample is measured using a rheometer system. The slip length increases linearly with the pitch but dramatically with the gas fraction above 90%, the latter trend being more pronounced on posts than on grates. Once the surfaces are designed for very large slips (>20 microm), however, further increase is not obtained in regular practice because the meniscus loses its stability. By developing near-perfect samples that delay the transition from a dewetted (Cassie) to a wetted (Wenzel) state until near the theoretical limit, we achieve giant slip lengths, as large as 185 microm.