Yongshun Liu

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.

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.

Combination of topology optimization and optimal control method

This paper presents the combination of topology optimization and optimal control method to find the optimal match between the material topology and control. In the presented method, the material topology is determined using the SIMP (Solid Isotropic Material with Penalization) method, which has been popularly used in topology optimization. In the SIMP method, the design variable is relaxed and bounded in the interval 0 , 1 ] , and the evolution of the design variable is usually implemented by the method of moving asymptotes (MMA), which can be used to deal with optimization problem with multiple integral constraints and bound constraint of the design variable. In the combination of topology optimization and optimal control method, the control variable appears along with the design variable. In order to evolve the control variable and design variable using MMA simultaneously, the control variable is regularized using a bound constraint and the corresponding bound constraint is projected onto the interval 0 , 1 ] , which is the same as the bound constraint of the design variable. The optimization problem is analyzed using the adjoint method to obtain the adjoint sensitivity. During the optimization procedure, the design and control variables are filtered by the Helmholtz filters to ensure the smoothness of the distribution. To ensure the minimum scale length and remove the gray area in the material topology, the filtered design variable is projected by the threshold method. The feasibility and robustness of the combination of these two methods are demonstrated by several test problems, including heat transfer, fluid flow and compliance minimization.

Polymeric microlens array fabricated with PDMS mold-based hot embossing

This study presents a simple, flexible and cost-effective process to fabricate microlens arrays. The polymeric microlens arrays are fabricated using a polydimethylsiloxane (PDMS) mold-based hot embossing process. The desired profile of the lens is achieved with the use of air pressure to deform the PDMS membrane. The deformation of the PDMS membrane is determined by numerical simulation. Simulation results show that the sag height of the PDMS membrane varies nearly linearly along with the change of the negative pressure. The shape of the PDMS membrane is transferred to the PDMS mold with UV curing and casting processes. Then, PDMS is used as a mold insert, and polycarbonate microlens arrays with different sag heights are fabricated with the hot embossing technique. The surface profile of the fabricated microlens keeps spherical with the variation of the sag height induced by the negative pressure. For the negative pressure -3600 and -5900 Pa, sag heights with 40 and 65 mu m are obtained and the corresponding focal lengths are changed from 1.0 to 0.6 mm. Good uniformity and imaging quality of the microlenses is confirmed by the experimentally evaluated and measured optical properties of the replica.

Topology Optimization-Based Computational Design Methodology for Surface Plasmon Polaritons

This paper presents the topology optimization-based computational design methodology for nanostructures in surface plasmon polaritons. Using the proposed method, nanostructures can be designed solely based on the user’s desired performance specification for the surface plasmon polaritons. This topology optimization-based computational design methodology is implemented based on the material interpolation with hybrid formulation of logarithmic and power law approaches, to mimic the metal surface with exponential decay of the electromagnetic field. The constructed computational design problem is analyzed using the continuous adjoint method, and the filter and projection techniques are utilized to ensure the minimum length scale in the obtained nanostructures. The outlined design methodology is used to investigate the nanostructures for localized surface plasmonic resonances, extraordinary optical transmission, and surface plasmonic cloaking, respectively. For localized surface plasmonic resonances and extraordinary optical transmission, the metallic nanostructures are designed with spectra peaks at the prescribed wavelengths and the shift of the spectra peak is controlled by solving the computational design problem corresponding to a different incident wavelength; for surface plasmonic cloaking, the cloak covered at a curved metal-dielectric interface is designed to bound the surface plasmon polariton at the interface and remove the radiation, where the conventional simple isotropic dielectric readily available in nature is used instead of the material possessing gradient electromagnetic properties with challenges on realization for optical frequencies.

Hydrodynamic particle focusing design using fluid-particle interaction.

For passive sheathless particles focusing in microfluidics, the equilibrium positions of particles are typically controlled by micro channels with a V-shaped obstacle array (VOA). The design of the obstacles is mainly based on the distribution of flow streamlines without considering the existence of particles. We report an experimentally verified particle trajectory simulation using the arbitrary Lagrangian-Eulerian (ALE) fluid-particle interaction method. The particle trajectory which is strongly influenced by the interaction between the particle and channel wall is systematically analyzed. The numerical experiments show that the streamline is a good approximation of particle trajectory only when the particle locates on the center of the channel in depth. As the advantage of fluid-particle interaction method is achieved at a high computational cost and the streamline analysis is complex, a heuristic dimensionless design objective based on the Faxens law is proposed to optimize the VOA devices. The optimized performance of particle focusing is verified via the experiments and ALE method.

Anisotropy of homogenized phononic crystals with anisotropic material

The anisotropy of elastic waves propagating in two-dimension phononic crystals with cubic crystal material was theoretically investigated in the long wavelength limit. The anisotropy can be tuned efficiently by either rotating the crystalline orientation of the material or changing the filling fraction. It can even disappear at a given orientation and filling fraction. The pure vibration mode direction and the pure propagation mode direction can be efficiently tuned to a desired direction deviating from the symmetric plane of the phononic crystal lattice. These results will be useful in manipulating the anisotropy of a homogenized composite and dealing with a PC with anisotropic material.

Band gap in tubular pillar phononic crystal plate.

In this paper, a phononic crystal (PC) plate with tubular pillars is presented and investigated. The band structures and mode displacement profiles are calculated by using finite element method. The result shows that a complete band gap opens when the ratio of the pillar height to the plate thickness is about 1.6. However, for classic cylinder pillar structures, a band gap opens when the ratio is equal or greater than 3. A tubular pillar design with a void room in it enhances acoustic multiple scattering and gives rise to the opening of the band gap. In order to verify it, a PC structure with double tubular pillars different in size (one within the other) is introduced and a more than 2times band gap enlargement is observed. Furthermore, the coupling between the resonant mode and the plate mode around the band gap is characterized, as well as the effect of the geometrical parameters on the band gap. The behavior of such structure could be utilized to design a pillar PC with stronger structural stability and to enlarge band gaps.

Ultrasensitive label-free optical microfiber coupler biosensor for detection of cardiac troponin I based on interference turning point effect

Sensitive detection of cardiac biomarkers is critical for clinical diagnostics of myocardial infarction (MI) while such detection is quite challenging due to the ultra-low concentration of cardiac biomarkers. In this work, a label-free immunosensor based on optical microfiber coupler (OMC) has been developed for the ultrasensitive detection of cardiac troponin I (cTnI), a selective and highly sensitive biomarker of acute myocardial infarction (AMI). CTnI monoclonal antibodies were immobilized on the surface of the fiber through polyelectrolyte layer using layer-by-layer deposition technique. For refractive index sensing characterization, an ultra-high sensitivity of 91777.9 nm/RIU was achieved when the OMC works around the dispersion turning point, which is the highest experimental demonstration in the field of fiber-optic evanescent biosensors. For biosensing, the immunosensor with good specificity showed a linear wavelength shift in the range of 2-10 fg/mL and an ultra-low detection limit of 2 fg/mL. Such immunosensors have huge application potential for the detection of cardiac biomarkers of myocardial infarction due to simple detection scheme, quick response time, ease of handling and miniaturation.