Deflection of sliding droplets by dielectrophoresis force on a superhydrophobic surface.

In this study, we experimentally identify the effect of liquid dielectrophoresis (LDEP) force on a superhydrophobic surface in directing the trajectory of moving water droplets across designed interdigitated electrodes and show that this method is capable of rapidly selecting droplets at a high speed (200 mm/s). As the droplets traverse down the surface by the electric field, their deflection on the edge of these electrodes is achieved successively, allowing for the selective manipulation of discrete droplets. A series of experiments were conducted to validate the relationships among droplet deflections, applied electric fields, and dynamic contact angles. Our findings reveal that the principal driving force behind the droplet deflections is the LDEP force, which can provide instant manipulation of moving droplets rather than a variation in contact angles brought about by electrowetting. This study presents a proof-of-concept experiment utilizing LDEP for high-throughput droplet selection and also highlights the potential applications of this mechanism in high-speed digital microfluidics (DMF) and biological separation methodologies.

NanoMuscle: controllable contraction and extension of mechanically interlocked DNA origami.

Artificial molecular machines synthesized in supramolecular chemistry have attracted great interest over the past decades. DNA origami presents an alternative approach to construct nano-machines by directly designing its thermodynamically stable state by DNA sequences. Here, we construct a molecular device, named NanoMuscle, with mechanically interlocked DNA origami. NanoMuscle's configuration - either extended or contracted - can be controlled by adding specific DNA strands. We monitored NanoMuscle's multistep synthesis with gel electrophoresis, and verified that monomers of the NanoMuscle are interlocked at correct orientation with transmission electron microscopy (TEM). We then validated that NanoMuscle can switch between extended and contracted configuration. By converting binding energy from DNA hybridization and Brownian motion to mechanical movements, NanoMuscle may serve as a novel building block for future mesoscale machinery.

A Smartphone-based Diffusometric Immunoassay for Detecting C-Reactive Protein.

In this study, we developed a portable smartphone-based diffusometry for analyzing the C-reactive protein (CRP) concentration. An optimized fluorescence microscopic add-on system for a smartphone was used to image the 300 nm fluorescent beads. Sequential nanobead images were recorded for a period and the image data were used for fluorescence correlation spectrometric (FCS) analysis. Through the analysis, the nanobeads' diffusion coefficient was obtained. Further, the diffusion coefficients of the anti-CRP-coated nanobeads, which were suspended in the samples with various CRP concentrations, were estimated using smartphone-based diffusometry. After 10 min of reaction, the anti-CRP-coated nanobeads in a higher CRP concentration solution led to a lower diffusion coefficient. Based on the experiments, a linear sensing range of 1~8 µg/mL was found.

An electric-field-dependent drop selector.

Drop manipulation on hydrophobic surfaces is of importance in lab-on-a-chip applications. Recently, superhydrophobic surface-assisted lab-on-a-chips have attracted significant attention from researchers due to their advantages of contamination resistance and low adhesion between the drop and the surface during manipulation. However, control over both static and dynamic interactions between a drop and a superhydrophobic surface has been rarely achieved. In this study, we designed an electric-field-dependent liquid-dielectrophoresis force to manipulate a drop on a superhydrophobic surface. This type of control has been found to be fast in response, bio-friendly, convenient, repeatable, and energy efficient. Moreover, the adhesion force and rebounding for both the static and the dynamic interactions between the drop and the surface under an electric field have been explored. It was found that the adhesion force could be reversibly tuned three-fold without breaking the Cassie-Baxter state. Rebounding experiments showed a close to linear relation between energy dissipation and the applied voltage. This relation was used to tune the on-demand behaviors of a drop on a surface in a proof-of-concept experiment for drop sorting. This electric-field-dependent drop manipulation may have potential applications in digital microfluidics, micro-reactors and advanced lab-on-a-drop platforms.

Electrically Enhanced Self-Thermophoresis of Laser-Heated Janus Particles under a Rotating Electric Field.

The motion of a laser-heated Janus particle is experimentally measured under a rotating electric field. Directionally circular motions of the Janus particle following or countering the direction of the rotating electric field are observed in the low-frequency region (from 1 to 6 kHz) depending on the direction of electrorotation. In the higher frequency region (>10 kHz), only pure electrorotation and electrothermal flow are observed. By measuring the dependence of the frequency, voltage, and laser heating power, we propose that the tangential component of circular motion is caused by electric field enhanced self-thermophoresis, which is proportional to the laser heating power and the electric field. This result indicates that thermophoresis could be modified by the induced zeta potential of the Janus particle tuned by the applied electric fields. By this mechanism, the intrinsic thermophoresis can be enhanced several times at a relatively low applied voltage (~3 Volt). Electrically tunable thermophoresis of a particle may bring new insights to thermophoresis phenomenon and also open a new direction for tunable active materials.

Preparation of Janus Particles and Alternating Current Electrokinetic Measurements with a Rapidly Fabricated Indium Tin Oxide Electrode Array.

This article provides a simple method to prepare partially or fully coated metallic particles and to perform the rapid fabrication of electrode arrays, which can facilitate electrical experiments in microfluidic devices. Janus particles are asymmetric particles that contain two different surface properties on their two sides. To prepare Janus particles, a monolayer of silica particles is prepared by a drying process. Gold (Au) is deposited on one side of each particle using a sputtering device. The fully coated metallic particles are completed after the second coating process. To analyze the electrical surface properties of Janus particles, alternating current (AC) electrokinetic measurements, such as dielectrophoresis (DEP) and electrorotation (EROT)- which require specifically designed electrode arrays in the experimental device- are performed. However, traditional methods to fabricate electrode arrays, such as the photolithographic technique, require a series of complicated procedures. Here, we introduce a flexible method to fabricate a designed electrode array. An indium tin oxide (ITO) glass is patterned by a fiber laser marking machine (1,064 nm, 20 W, 90 to 120 ns pulse-width, and 20 to 80 kHz pulse repetition frequency) to create a four-phase electrode array. To generate the four-phase electric field, the electrodes are connected to a 2-channel function generator and to two invertors. The phase shift between the adjacent electrodes is set at either 90° (for EROT) or 180° (for DEP). Representative results of AC electrokinetic measurements with a four-phase ITO electrode array are presented.

Particle concentrating and sorting under a rotating electric field by direct optical-liquid heating in a microfluidics chip.

We demonstrate a functional rotating electrothermal technique for rapidly concentrating and sorting a large number of particles on a microchip by the combination of particle dielectrophoresis (DEP) and inward rotating electrothermal (RET) flows. Different kinds of particles can be attracted (positive DEP) to or repelled (negative DEP) from electrode edges, and then the n-DEP responsive particles are further concentrated in the heated region by RET flows. The RET flows arise from the spatial inhomogeneous electric properties of fluid caused by direct infrared laser (1470 nm) heating of solution in a rotating electric field. The direction of the RET flows is radially inward to the heated region with a co-field (the same as the rotating electric field) rotation. Moreover, the velocity of the RET flows is proportional to the laser power and the square of the electric field strength. The RET flows are significant over a frequency range from 200 kHz to 5 MHz. The RET flows are generated by the simultaneous application of the infrared laser and the rotating electric field. Therefore, the location of particle concentrating can be controlled within the rotating electric field depending on the position of the laser spot. This multi-field technique can be operated in salt solutions and at higher frequency without external flow pressure, and thus it can avoid electrokinetic phenomena at low frequency to improve the manipulation accuracy for lab-on-chip applications.

Active motion of a Janus particle by self-thermophoresis in a defocused laser beam.

We study self-propulsion of a half-metal coated colloidal particle under laser irradiation. The motion is caused by self-thermophoresis: i.e., absorption of a laser at the metal-coated side of the particle creates local temperature gradient which in turn drives the particle by thermophoresis. To clarify the mechanism, temperature distribution and a thermal slip flow field around a microscale Janus particle are measured for the first time. With measured temperature drop across the particle, the speed of self-propulsion is corroborated with the prediction based on accessible parameters. As an application for driving a micromachine, a microrotor is demonstrated.

Manipulation of colloids by a nonequilibrium depletion force in a temperature gradient.

The nonequilibrium distribution of colloids in a polymer solution under a temperature gradient is studied experimentally. A slight increase of local temperature by a focused laser drives the colloids towards the hot region, resulting in the trapping of the colloids irrespective of their own thermophoretic properties. An amplification of the trapped colloid density with the polymer concentration is measured, and is quantitatively explained by hydrodynamic theory. The origin of the attraction is a migration of colloids driven by a nonuniform polymer distribution sustained by the polymer's thermophoresis. These results show how to control the thermophoretic properties of colloids.

Experimental test of a new equality: measuring heat dissipation in an optically driven colloidal system.

Measurement of energy dissipation in small nonequilibrium systems is generally a difficult task. Recently, Harada and Sasa [Phys. Rev. Lett. 95, 130602 (2005)] derived an equality relating the energy dissipation rate to experimentally accessible quantities in nonequilibrium steady states described by the Langevin equation. Here, we show an experimental test of this new relation in an optically driven colloidal system. We find that this equality is validated to a fairly good extent, thus the irreversible work of a small system is estimated from readily obtainable quantities.