Yihui Wu

Topology optimization of unsteady incompressible Navier-Stokes flows

This paper discusses the topology optimization of unsteady incompressible Navier-Stokes flows. An optimization problem is formulated by adding the artificial Darcy frictional force into the incompressible Navier-Stokes equations. The optimization procedure is implemented using the continuous adjoint method and the finite element method. The effects of dynamic inflow, Reynolds number and target flux on specified boundaries for the optimal topology of unsteady Navier-Stokes flows are presented. Numerical examples demonstrate the feasibility and necessity of this topology optimization method for unsteady Navier-Stokes flows.

Gold nanoparticle amplified optical microfiber evanescent wave absorption biosensor for cancer biomarker detection in serum

Sensitive and selective detection for cancer biomarkers is critical in cancer clinical diagnostics. In this work, we report a new optical microfiber (OMF) biosensor using gold nanoparticles (GNPs) as amplification labels for the detection of alpha-fetoprotein (AFP) in serum samples. By combining the unique optical property of OMFs and the strong optical absorption of GNPs, very high sensitivity and selectivity can be achieved. Critical parameters namely fiber diameter and GNP size were optimized for better performance. The limit of detection (LOD) of this sensor for AFP is 0.2 ng/mL in PBS and 2 ng/mL in bovine serum, which is comparable to conventional assays. The advantages of this biosensor are simple detection scheme, fast response time, and ease of miniaturization, which might make this biosensor a promising platform for clinical cancer diagnosis and prognosis.

Investigation of the NaBH4-induced aggregation of Au nanoparticles.

Colloidal Au nanoparticles (AuNPs) with a diameter of 17 nm were prepared by the reduction of HAuCl(4) with citrate trisodium. The addition of NaBH(4) to the prepared citrate-stabilized AuNP solutions not only induced a blue shift in the surface plasmon resonance peak (lambda(max)) because of the increased number of electrons in the NPs injected by NaBH(4) but also affected the stability of citrate adsorbed on AuNPs. The zeta potential of AuNPs after the addition of 6 mM NaBH(4) decreased (67%) but was restored (88%) after the discharge of the injected electrons. The effect of NaBH(4) treatment on the stability of citrate ions on AuNPs was investigated by X-ray photoelectron spectroscopy (XPS). The XPS data showed that citrate ions partially desorbed from the surfaces of AuNPs (67%) after NaBH(4) treatment but readsorbed onto the AuNPs (80%) after the discharge of the NPs; this result agrees well with the zeta potential data. The partial removal of citrate ions from AuNPs results in an anisotropic charge distribution around the AuNPs. By increasing the amount of NaBH(4) and the electrolyte concentration of the solution, non-close-packed aggregates of AuNPs can be formed, from monomers to small aggregates containing a few AuNPs and 3D network aggregates.

Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region.

A polarization-independent and omnidirectional nearly perfect absorber in the visible region has been proposed. The absorber is two-layer structure consisting of a subwavelength metal grating layer embedded in the high refractive index and lossless dielectric layer on the metal substrate. Extraordinary optical absorption with absorption peaks of over 99% can be achieved over the whole visible region for both TM and TE polarization. This absorption is attributed to cavity mode (CM) resonance caused by the coupled surface plasmon polaritons (SPP). Through adjusting the grating thickness, the absorption peak can be tuned linearly, which is highly advantageous to design various absorbers. Furthermore, the absorbance retains ultra-high over a wide angular range of incidence for both TM and TE polarization. This nearly perfect absorber offers great potential in the refractive index (RI) sensors, integrated photodetectors, solar cells and so on.

Investigation of Halide-Induced Aggregation of Au Nanoparticles into Spongelike Gold

We present a facile method for fabricating spongelike Au structures by halide-induced aggregation and fusion of gold nanoparticles (AuNPs). Halide ions (F(-), Cl(-), Br(-), and I(-)) showed distinctly different effects on the synthesized AuNPs, which were characterized by localized surface plasmon resonance (LSPR) and dynamic light scattering measurements. A noticeable red-shift in the LSPR peak was found after Br(-) and I(-) ion treatment, which indicates the adsorption of halide atoms or ions on the AuNPs. The surface potential of AuNPs varied by treatment with different types of halides; this finding indicates that different halide ions have different effects on the AuNPs. Br(-) and I(-) ions showed strong affinity toward the AuNPs. The different affinities of halide ions toward the AuNPs play an important role in controlling the formation process of spongelike gold. Citrate ions adsorbed on AuNPs were displaced by halide ions to different extents. Such displacement determined the aggregation and fusion behaviors of the AuNPs and eventually the formation of different spongelike structures.

Color-switchable, emission-enhanced fluorescence realized by engineering C-dot@C-dot nanoparticles

This paper reports the preparation and properties of color-switchable fluorescent carbon nanodots (C-dots). C-dots that emit dark turquoise and green-yellow fluorescence under 365 nm UV illumination were obtained from the hydrothermal decomposition of citric acid. Dark green fluorescent C-dots were obtained by conjugating prepared C-dots to form C-dot@C-dot nanoparticles. After successful conjugation of the C-dots, the fluorescence emission undergoes a blue-shift of nearly 20 nm (∼0.15 eV) under UV excitation at 370 nm. The C-dots emit goldenrod, green-yellow, and gold light under excitation at 455 nm, which shows that the prepared C-dots are color-switchable. Furthermore, conjugation of the C-dots results in enhanced, red-shifted absorption of the π-π* transition of the aromatic sp(2) domains due to the conjugated π-electron system. N incorporation in the carbon structure leads to a degree of dipoles for all the aromatic sp(2) bonds. The enhanced absorption in a wide range from 226 to 601 nm indicates extended conjugation in the C-dot@C-dot structure. The time-resolved average lifetimes for the three different types of C-dots prepared in this study are 7.10, 7.65, and 4.07 ns. The radiative rate (reduced decay lifetime) increases when the C-dots are conjugated in the C-dot@C-dot nanoparticles, leading to the enhanced fluorescence emission. The fluorescence emission of the C-dot@C-dot nanoparticles can be used in applications such as flow cytometry and cell imaging.

Large‐Scale Horizontally Aligned ZnO Microrod Arrays with Controlled Orientation, Periodic Distribution as Building Blocks for Chip‐in Piezo‐Phototronic LEDs

A novel method of fabricating large-scale horizontally aligned ZnO microrod arrays with controlled orientation and periodic distribution via combing technology is introduced. Horizontally aligned ZnO microrod arrays with uniform orientation and periodic distribution can be realized based on the conventional bottom-up method prepared vertically aligned ZnO microrod matrix via the combing method. When the combing parameters are changed, the orientation of horizontally aligned ZnO microrod arrays can be adjusted (θ = 90° or 45°) in a plane and a misalignment angle of the microrods (0.3° to 2.3°) with low-growth density can be obtained. To explore the potential applications based on the vertically and horizontally aligned ZnO microrods on p-GaN layer, piezo-phototronic devices such as heterojunction LEDs are built. Electroluminescence (EL) emission patterns can be adjusted for the vertically and horizontally aligned ZnO microrods/p-GaN heterojunction LEDs by applying forward bias. Moreover, the emission color from UV-blue to yellow-green can be tuned by investigating the piezoelectric properties of the materials. The EL emission mechanisms of the LEDs are discussed in terms of band diagrams of the heterojunctions and carrier recombination processes.

A flexible layout design method for passive micromixers

This paper discusses a flexible layout design method of passive micromixers based on the topology optimization of fluidic flows. Being different from the trial and error method, this method obtains the detailed layout of a passive micromixer according to the desired mixing performance by solving a topology optimization problem. Therefore, the dependence on the experience of the designer is weaken, when this method is used to design a passive micromixer with acceptable mixing performance. Several design disciplines for the passive micromixers are considered to demonstrate the flexibility of the layout design method for passive micromixers. These design disciplines include the approximation of the real 3D micromixer, the manufacturing feasibility, the spacial periodic design, and effects of the Péclet number and Reynolds number on the designs obtained by this layout design method. The capability of this design method is validated by several comparisons performed between the obtained layouts and the optimized designs in the recently published literatures, where the values of the mixing measurement is improved up to 40.4% for one cycle of the micromixer.

Experimental investigation of passive micromixers conceptual design using the layout optimization method

This paper presents an experimental investigation of the novel efficient passive micromixers conceptual design using the flexible layout optimization method. Utilizing the layout optimization method when designing passive micromixers results in decreased reliance on the experience and intuition of designers. The detailed layout of passive micromixers is obtained by solving a variational optimization problem, in which the manufacturability and periodicity of passive micromixers can be considered by adding the corresponding design constraints. The obtained micromixers are fabricated by using polydimethylsiloxane soft photolithography techniques. The mixing performance is evaluated by stereoscopic and confocal microscopes. The effectiveness of the layout optimization method is confirmed by a comparison of the numerical and experimental results.

Optimization of no-moving part fluidic resistance microvalves with low reynolds number

This paper reports an effective optimization procedure for designing no-moving part microvalves where the diodicity of the optimized valves has better performance when compared with the typical Tesla valve. The detailed layout of microvalves is obtained by minimizing the fluidic resistance of fluidic channels for the forward flow under a user-specified design constraint about the fluidic work diodicity between the forward and reverse flows. A couple of novel valves which have different layout with the Tesla valve are presented. The numerical value of the fluidic resistance diodicity for a designed periodic valve is verified by experiments.