Dynamics of Electrically Modulated Colloidal Droplet Transport

Tuning the electrowetting behavior of quantum dot nanofluids

HYPOTHESIS

The electrowetting behavior of droplets can be altered by the inclusion of salts, surfactants, or nanoparticles. We propose that varying the properties of cadmium selenide/zinc sulfide quantum dots will affect the electrowetting behavior of fluorescent nanofluids. Information gathered will allow for greater control of fluid properties when designing a colloidal system in an electrowetting environment.

EXPERIMENTS

Aqueous-based quantum dots were functionalized with mercaptocarboxylic acid ligands of various chain length and binding motifs by a room temperature phase transfer method. The size and concentration of the quantum dot were varied, and droplets of the resulting nanofluids were exposed to increasing amounts of voltage. The change in contact angle was evaluated and correlated to the surface chemistry, size, and concentration of the quantum dots.

FINDINGS

Quantum dot nanofluids with longer alkyl chains have the most pronounced change in contact angle and were the most stable under applied voltage. The size of the nanoparticles does not significantly impact the electrowetting behavior at low concentration (3 µM), but nanofluids containing smaller diameter quantum dots show enhanced electrowetting behavior at higher concentration (27 µM). The fluorescent properties of the QD nanofluids studied were not affected after repeated electrowetting cycles.

Rapid determination of erythrocyte sedimentation rate (ESR) by an electrically driven blood droplet biosensor

In healthcare practice, the sedimentation rate of red blood cells (erythrocytes) is a widely used clinical parameter for screening of several ailments such as stroke, infectious diseases, and malignancy. In a traditional pathological setting, the total time taken for evaluating this parameter varies typically from 1 to 2 h. Furthermore, the volume of human blood to be drawn for each test, following a gold standard laboratory technique (alternatively known as the Westergren method), varies from 4 to 5 ml. Circumventing the above constraints, here we propose a rapid (∼1 min) and highly energy efficient method for the simultaneous determination of hematocrit and erythrocyte sedimentation rate (ESR) on a microfluidic chip, deploying electrically driven spreading of a tiny drop of blood sample (∼8 μl). Our unique approach estimates these parameters by correlating the same with the time taken by the droplet to spread over a given radius, reproducing the results from more elaborate laboratory settings to a satisfactory extent. Our novel methodology is equally applicable for determining higher ranges of ESR such as high concentration of bilirubin and samples corresponding to patients with anemia and patients with some severe inflammation. Furthermore, the minimal fabrication steps involved in the process, along with the rapidity and inexpensiveness of the test, render the suitability of the strategy in extreme point-of-care settings.

Electrodynamic-contact-line-lithography with nematic liquid crystals for template-less E-writing of mesopatterns on soft surfaces

We report the development of a single-step, template-less and fast pathway, namely, Electrodynamic-Contact-Line-Lithography (ECLL), to write micro to nanopatterns on the surface of a soft polymer film. As a model system, a layer of nematic liquid crystals (NLC), resting on a polymer thin film, was sandwiched between a pair of electrodes emulating the electrowetting on a dielectric (EWOD) setup. Upon the application of electric field, the Maxwell stresses thus generated at the NLC-polymer interface due to the high dielectric contrast stimulated an unprecedented fingering instability at the advancing NLC-polymer-air contact line. In the process, the advancing electrospreading front of NLC left the footprint of an array of micro to nanoscale wells on the polymer surface with a long-range ordering thus unveiling a pathway for maskless patterning of a soft elastic film. Unlike the conventional electric field induced lithography (EFL), the meso-scale morphology was found to follow the short wavelength-scales as the periodicity of the patterns (λ_c) varied linearly with the thickness of the film (h), (λ_c∝h). The high dielectric contrast at the NLC-polymer interface and the local fluctuation of the NLC directors ensured a time scale much faster than the same observed for the polymer-air systems.

Electrowetting Behavior and Digital Microfluidic Applications of Fluorescent, Polymer-Encapsulated Quantum Dot Nanofluids

Digital microfluidics is a liquid-handling technology capable of rapidly and autonomously controlling multiple discrete droplets across an array of electrodes and has seen continual growth in the fields of chemistry, biology, and optics. This technology is enabled by rapidly switching the wettability of a surface through the application of an electric field: a phenomenon known as electrowetting-on-dielectric. The results reported here elucidate the wetting behavior of fluorescent quantum dot nanofluids by varying the aqueous-solubilizing polymers, changing the size of the nanocrystals, and the addition of surfactants. Nanofluid droplets were demonstrated to have very large changes in contact angle (>100°) by employing alternating current voltage to aqueous droplets within a dodecane medium. The stability of quantum dot nanofluids is also evaluated within a digital microfluidics platform, and the optical properties are not perturbed even under high voltages (250 V). Multiple fluorescent droplets with varying emission can be simultaneously actuated and rapidly mixed (<10 s) to generate a new nanofluid with optical properties different from the parent solutions.

An enhanced charge-driven intranasal delivery of nicardipine attenuates brain injury after intracerebral hemorrhage

Intranasal drug delivery provided an alternative and effective approach for the intervention of an intracerebral hemorrhage (ICH). However, the short retention time at the absorption site and slow drug transport in intranasal gel influence the drug bioavailability and outcome of ICH. Herein, we fabricated a novel intranasal gel with oriented drug migration utilizing a charge-driven strategy to attenuate brain injury after ICH. Nicardipine hydrochloride (NCD) was entrapped in chitosan nanoparticles (CS NPs) and dispersed in an HAMC gel. Subsequently, one side of the gel was coated with a positively charged film. The oriented migration of CS NPs in the HAMC gel was determined, and the drug bioavailability was also enhanced. Furthermore, a blood-induced ICH rat model was established to evaluate the therapeutic effect of CS NPs + HAMC composites. Intranasal administration of the CS NPs + HAMC (+) composite showed a stronger neuroprotective effect in terms of brain edema reduction and neural apoptosis inhibition compared to the CS NPs + HAMC composite. These results suggested that the oriented and rapid drug transport from nose to brain can be achieved using the charge-driven strategy, and this intranasal drug delivery system has the potential to provide a new therapeutic strategy for the treatment of ICH.

Microfluidic Schottky-junction photovoltaics with superior efficiency stimulated by plasmonic nanoparticles and streaming potential

A droplet energy harvester (DEH) composed of aqueous salt solution could generate electrical energy from light when placed on a metal-semiconductor Schottky-junction emulating the principles of electrochemical photovoltaics (ECPV). The maximum potential difference generated was ∼95 mV under sun, which was enhanced by ∼1.5 times after the addition of gold nanoparticles (AuNPs) in the droplet because of the generation of additional charge carriers from the localized surface plasmon resonance (LSPR). Focusing the solar illumination through a bi-convex lens on five such droplets increased the voltage to ∼320 mV with a power density of ∼0.25 mW cm^-2. When the DEH was converted to a microfluidic energy harvester (MEH) by flowing the AuNP laden salt solution through a microchannel integrated with an array of Schottky-junction electrodes, at an optimal flow rate, another two-fold increase in the power density was observed. In the MEH, because the ECPV aided by the LSPR converted the solar energy into electrical energy, the streaming potential (SP) generated across the electrodes because of the fluid flow converted the mechanical energy into electrical energy. Increase in the number of electrode pairs improved the voltage generation, which suggested that the MEH had potential for microscale-very-large-scale-integration (μ-VLSI). The combined effects of ECPV, LSPR, and SP in the MEH could show an efficiency ∼2.5%, which was one of the highest ones reported, for Schottky-junction energy harvesters. This study shows some simple and efficient pathways to harvest high-density electrical power using microchannels and droplets from the naturally abundant solar or hydroelectric (hydel) energy resources.

Magnetic field induced push–pull motility of liquibots

Self-propelling liquibots as transport and delivery vehicles.