I-Kao Chiang

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves

Techniques that can dexterously manipulate single particles, cells, and organisms are invaluable for many applications in biology, chemistry, engineering, and physics. Here, we demonstrate standing surface acoustic wave based “acoustic tweezers” that can trap and manipulate single microparticles, cells, and entire organisms (i.e., Caenorhabditis elegans) in a single-layer microfluidic chip. Our acoustic tweezers utilize the wide resonance band of chirped interdigital transducers to achieve real-time control of a standing surface acoustic wave field, which enables flexible manipulation of most known microparticles. The power density required by our acoustic device is significantly lower than its optical counterparts (10,000,000 times less than optical tweezers and 100 times less than optoelectronic tweezers), which renders the technique more biocompatible and amenable to miniaturization. Cell-viability tests were conducted to verify the tweezers’ compatibility with biological objects. With its advantages in biocompatibility, miniaturization, and versatility, the acoustic tweezers presented here will become a powerful tool for many disciplines of science and engineering.

Three-dimensional continuous particle focusing in a microfluidic channel via standing surface acoustic waves (SSAW)†

Three-dimensional (3D) continuous microparticle focusing has been achieved in a single-layer polydimethylsiloxane (PDMS) microfluidic channel using a standing surface acoustic wave (SSAW). The SSAW was generated by the interference of two identical surface acoustic waves (SAWs) created by two parallel interdigital transducers (IDTs) on a piezoelectric substrate with a microchannel precisely bonded between them. To understand the working principle of the SSAW-based 3D focusing and investigate the position of the focal point, we computed longitudinal waves, generated by the SAWs and radiated into the fluid media from opposite sides of the microchannel, and the resultant pressure and velocity fields due to the interference and reflection of the longitudinal waves. Simulation results predict the existence of a focusing point which is in good agreement with our experimental observations. Compared with other 3D focusing techniques, this method is non-invasive, robust, energy-efficient, easy to implement, and applicable to nearly all types of microparticles.

Standing surface acoustic wave (SSAW) based multichannel cell sorting

We introduce a novel microfluidic device for cell sorting in continuous flow using tunable standing surface acoustic waves. This method allows individual cells to be precisely directed into five different outlet channels in a single step. It is versatile, simple, label-free, non-invasive, and highly controllable.

Surface acoustic wave driven light shutters using polymer-dispersed liquid crystals.

displays, [ 3 ] microlenses, [ 4 , 5 ] lasers, [ 6 , 7 ] and data storage, [ 8 ] due to their excellent electro-optical properties. PDLC fi lms can be prepared between two conductive, transparent substrates using methods such as encapsulation, thermally induced phase separation, solvent-induced phase separation, and polymerizationinduced phase separation. [ 9 ] Within a PDLC fi lm, liquid crystals (LCs) are generally trapped in a transparent polymer medium, thus forming micrometer-scale LC droplets. The random dispersion of LC droplets in the polymer matrix causes a strong scattering of light due to the signifi cant refractive index mismatch between the two materials; therefore, a PDLC fi lm is naturally opaque. Based on laser interference holography, various periodic structures such as gratings [ 10–12 ] and photonic crystals, [ 13 , 14 ] can be also introduced inside the fi lm, coined as holographic PDLCs (HPDLCs). [ 15 ] The application of an electric fi eld can re-orientate the LC molecules inside a droplet, thus modulating the refractive index difference between the polymer matrix and the LC. A complete refractive index match between the two materials can be achieved by tuning the LCs to a specifi c orientation. In such a way, the PDLC fi lm can be switched from opaque to transparent. The switching properties of PDLCs are infl uenced by many variables including the size and shape of the LC droplets, [ 16 , 17 ] and molecular interactions between the LCs and polymer matrix. [ 18 , 19 ]

Light-driven artificial molecular machines

Artificial molecular machines represent a growing field of nanoscience and nanotechnology. Stimulated by chemical reagents, electricity, or light, artificial molecular machines exhibit precisely controlled motion at the molecular level; with this ability molecular machines have the potential to make significant impacts in numerous engineering applications. Compared with molecular machines powered by chemical or electrical energy, light-driven molecular machines have several advantages: light can be switched much faster, work without producing chemical waste, and be used for dual purposes—inducing (writing) as well as detecting (reading) molecular motions. The following issues are significant for light-driven artificial molecular machines in the following aspects: their chemical structures, motion mechanisms, assembly and characterization on solid-state surfaces. Applications in different fields of nanotechnology such as molecular electronics, nano-electro-mechanical systems (NEMS), nanophotonics, and nanomedicine are envisaged.

Characterization of complementary patterned metallic membranes produced simultaneously by a dual fabrication process

An efficient technique is developed to fabricate optically thin metallic films with subwavelength patterns and their complements simultaneously. By comparing the spectra of the complementary films, we show that Babinet’s principle nearly holds for these structures in the optical domain. Rigorous full-wave simulations are employed to verify the experimental observations. It is further demonstrated that a discrete-dipole approximation can qualitatively describe the spectral dependence of the metallic membranes on the geometry of the constituent particles as well as the illuminating polarization.

Metallic Membranes with Subwavelength Complementary Patterns: Distinct Substrates for Surface-Enhanced Raman Scattering

We present a detailed comparison of surface-enhanced Raman spectroscopy (SERS) signals from metallic nanoparticle arrays and their complementary hole arrays. Using an analytical model for local field enhancement, we show that the SERS enhancements of the hole arrays are closely related to their transmission spectra. This trend is experimentally confirmed and characterized by a cos(4 )θ dependence of the SERS signal on the excitation polarization angle θ. The particle arrays, on the other hand, exhibit quite different behavior because of the existence of considerable evanescent modes in the near field. Their maximal local field gains appear at wavelengths generally much larger than their localized surface plasmonic resonant wavelengths.

Shifts in plasmon resonance due to charging of a nanodisk array in argon plasma

A method for generating charge-induced plasmonic shifts, using argon plasma to charge nanoparticle arrays, is presented. Particles develop a negative charge, due to enhanced collisions with high-temperature electrons, in low-temperature plasmas. The negative charge generated causes a blue shift in the localized surface plasmon resonance. The dynamics of the shift were recorded and discussed. This effect could be used as a real-time method for studying the dynamics for charging in plasma.