George Y. Chen

Cleaning of Oil Fouling with Water Enabled by Zwitterionic Polyelectrolyte Coatings: Overcoming the Imperative Challenge of Oil–Water Separation Membranes

Herein we report a self-cleaning coating derived from zwitterionic poly(2-methacryloyloxylethyl phosphorylcholine) (PMPC) brushes grafted on a solid substrate. The PMPC surface not only exhibits complete oil repellency in a water-wetted state (i.e., underwater superoleophobicity), but also allows effective cleaning of oil fouled on dry surfaces by water alone. The PMPC surface was compared with typical underwater superoleophobic surfaces realized with the aid of surface roughening by applying hydrophilic nanostructures and those realized by applying smooth hydrophilic polyelectrolyte multilayers. We show that underwater superoleophobicity of a surface is not sufficient to enable water to clean up oil fouling on a dry surface, because the latter circumstance demands the surface to be able to strongly bond water not only in its pristine state but also in an oil-wetted state. The PMPC surface is unique with its described self-cleaning performance because the zwitterionic phosphorylcholine groups exhibit exceptional binding affinity to water even when they are already wetted by oil. Further, we show that applying this PMPC coating onto steel meshes produces oil-water separation membranes that are resilient to oil contamination with simply water rinsing. Consequently, we provide an effective solution to the oil contamination issue on the oil-water separation membranes, which is an imperative challenge in this field. Thanks to the self-cleaning effect of the PMPC surface, PMPC-coated steel meshes can not only separate oil from oil-water mixtures in a water-wetted state, but also can lift oil out from oil-water mixtures even in a dry state, which is a very promising technology for practical oil-spill remediation. In contrast, we show that oil contamination on conventional hydrophilic oil-water separation membranes would permanently induce the loss of oil-water separation function, and thus they have to be always used in a completely water-wetted state, which significantly restricts their application in practice.

A review of microfiber and nanofiber based optical sensors

Rapid advances in optical microfiber and nanofibers (MNF) based sensors have been driven by powerful industries such as automotive, biomedical and defense, with the increasing demand for highly-sensitive, selective, nonintrusive, fast-response, compact and robust sensors that can perform in-situ measurements at remote and harsh environments. A diverse range of MNF based sensors have been developed for measuring refractive index, bio-chemical, temperature, current, displacement, bend, surface, acceleration, force, rotation, acoustic, electric field and magnetic field. Given the growing interest for this exciting area of scientific research, new designs are emerging continuously at a fast pace. This paper attempts to provide a comprehensive review of all MNF based sensors reported to-date. Sensors are divided according to their morphology and measurand.

Resonantly Enhanced Faraday Rotation in an Microcoil Current Sensor

A proof-of-principle experimental demonstration with theoretical modeling is presented for resonantly enhanced Faraday rotation in a microcoil current sensor. The recirculation of resonant light within the coil gives rise to an accumulated polarization rotation and thus improved responsivity. According to simulations, microcoil resonators with sharper resonances could offer significantly larger enhancements. This new type of current sensor has the potential to be ultrafast, compact, and low-cost.

Theoretical and experimental demonstrations of a microfiber-based flexural disc accelerometer

The proof-of-concept demonstration of a microfiber-based flexural disc accelerometer is presented. The reduced microfiber size and bending radii give rise to high device compactness and responsivity. A flexural disc accelerometer manufactured from a 10 mm long microfiber showed a performance of ~2.2 rad/g, with the responsivity expected to increase proportionally with the microfiber length.

Compact optical microfiber phase modulator.

A compact optical microfiber phase modulator with MHz bandwidth is presented. A micrometer-diameter microfiber is wound on a millimeter-diameter piezoelectric ceramic rod with two electrodes. When a voltage is applied to the piezoelectric ceramic, the rod is strained, leading to a phase change along the microfiber; because of the small size, the optical microfiber phase modulator can have as high as a few MHz bandwidth response.

Ultrafast colorimetric humidity-sensitive polyelectrolyte coating for touchless control

Herein we report the visible colorimetric humidity-sensitive properties of the layer-by-layer assembled poly(diallyldimethylammonium) (PDDA)/poly(styrenesulfonate) (PSS) polyelectrolyte coatings and their application for a touchless control system that can be accessed by humidity change. The as-developed touchless control system enables humidity signals, such as those imparted by human breath or a close-proximity fingertip, to switch a light-emitting diode and trigger a computer to implement commands (e.g., making phone calls and playing music). The PDDA/PSS coatings exhibit different colors at different thicknesses, which results from the interference of visible light in the coating. Notably, they can undergo vivid and reversible color changes that span the whole visible spectrum at an ultrafast speed (ca. 35 ms) according to the local humidity change. Exploiting the ability of the coating to modulate light, the output optical signal (i.e., color change) was converted into an electrical signal by irradiating the coating with a beam of monochromatic light and collecting the reflected modulated light using a photodetector. The obtained electrical signal was subsequently processed and used as an input signal for touchless control that is accessible by a humidity signal. The as-developed touchless control system is applicable for the touchless interface of electronic devices, smart switches, automotive, smart buildings, and so forth.

Spun Optical Microfiber

The first report of spun optical microfiber (OM) is presented, with details on its fabrication method and a demonstration of its important role in making practical Faraday-based microcoil current sensors. These sensors have shown potential for significantly higher bandwidth and compactness compared to conventional fiber-optic current sensors. We demonstrate that spun OM exhibits superior resistance to bend- and packaging-induced linear birefringence, which can improve the responsivity and reproducibility of the sensor head.

Ultrafast pulse generation in a mode-locked Erbium chip waveguide laser

We report mode-locked ~1550 nm output of transform-limited ~180 fs pulses from a large mode-area (diameter ~50 μm) guided-wave erbium fluorozirconate glass laser. The passively mode-locked oscillator generates pulses with 25 nm bandwidth at 156 MHz repetition rate and peak-power of 260 W. Scalability to higher repetition rate is demonstrated by transform-limited 410 fs pulse output at 1.3 GHz. To understand the origins of the broad spectral output, the laser cavity is simulated by using a numerical solution to the Ginzburg-Landau equation. This paper reports the widest bandwidth and shortest pulses achieved from an ultra-fast laser inscribed waveguide laser.

Angle-Resolved Characterization and Ray-Optics Modeling of Fiber-Optic Sensors

A powerful angle-resolved method is presented for characterizing ray-group attenuation in cylindrical multimode waveguides that can be used for the design optimization of fiber-optic sensors. It is supported by an in-depth theoretical model that is accurate yet simple for describing the propagation of meridional and skew rays in passive cylindrical multimode waveguides. In particular, the transmitted power of different ray groups and the near- and far-field profiles can be simulated. Also, the individual parameters contributing to the attenuation of light can be extracted from the experimental results.

Enhanced responsivity with skew ray excitation of reflection- and transmission-type refractometric sensors

The responsivity of optical fibers to refractive index can be enhanced using high-order skew rays compared with using meridional rays. Skew rays can have a much higher number of reflections with increased interaction length along the core-cladding interface, which gives rise to stronger interactions with the external medium. Reflection/transmission-type refractometric sensors based on twin-coupled-core and multimode fibers showed one/two orders of magnitude increase in responsivity with skew ray excitation. The responsivity and sensitivity for the two types are ~2000%/RIU, ~1400%/RIU, and 4.9×10⁻⁵  RIU, 7.0×10⁻⁵  RIU, respectively.