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The principle of the conventional ultrasound test states that the detectable voids cannot be smaller than the acoustic wavelength. However, by using effective medium approximation, the fraction of small voids can be estimated by the variation of the effective density. In this study, a non-contacting ultrasound-based porosity fraction mapping methodology is developed for estimated small voids in coal with long operating wavelength in air. This novel ultrasonic technique based on the mechanical properties of coal offers a rapid scan of the effective density mapping and distribution of void fraction over a large sample area, which overcame the limitation of small voids detection in the conventional ultrasound testing. 

We studied laser-induced liquid indentations generated by the Marangoni effect. We showed experimental results along with the simulation model based on the lubrication theory. 

Liquid suspensions of carbon nanotubes, graphene and transition metal dichalcogenides have exhibited excellent performance in optical limiting. However, the underlying mechanism has remained elusive and is generally ascribed to their superior nonlinear optical properties such as nonlinear absorption or nonlinear scattering. Using graphene as an example, we show that photo-thermal microbubbles are responsible for optical limiting as strong light scattering centers: graphene sheets absorb incident light and become heated up above the boiling point of water, resulting in vapor and microbubble generation. This conclusion is based on the direct observation of bubbles above the laser beam as well as a strong correlation between laser-induced ultrasound and optical limiting. In situ Raman scattering of graphene further confirms that the temperature of graphene under laser pulses rises above the boiling point of water but still remains too low to vaporize graphene and create graphene plasma bubbles. Photo-thermal bubble scattering is not a nonlinear optical process and requires very low laser intensity. This understanding helps us to design more efficient optical limiting materials and understand the intrinsic nonlinear optical properties of nanomaterials. 

Enabled initially by the development of microelectromechanical systems, current microfluidic pumps still require advanced microfabrication techniques to create a variety of fluid-driving mechanisms. Here we report a generation of micropumps that involve no moving parts and microstructures. This micropump is based on a principle of photoacoustic laser streaming and is simply made of an Au-implanted plasmonic quartz plate. Under a pulsed laser excitation, any point on the plate can generate a directional long-lasting ultrasound wave which drives the fluid via acoustic streaming. Manipulating and programming laser beams can easily create a single pump, a moving pump, and multiple pumps. The underlying pumping mechanism of photoacoustic streaming is verified by high-speed imaging of the fluid motion after a single laser pulse. As many light-absorbing materials have been identified for efficient photoacoustic generation, photoacoustic micropumps can have diversity in their implementation. These laser-driven fabrication-free micropumps open up a generation of pumping technology and opportunities for easy integration and versatile microfluidic applications.

 

The discovery of photoacoustic laser streaming has opened up a new avenue to manipulate and drive fluids with light, but the necessity of an in situ “launch pad” has limited its utility in real‐world microfluidic applications due to both the size constraint and the complexity of fabrication. Here, it is demonstrated that 1) a versatile microfluidic pump can be materialized by using laser streaming from an optical fiber, and 2) laser streaming can be generated from a flat metal surface without any fabrication process. In the latter case, by focusing laser on the tip of a sewing needle tip, the needle can be turned into a micropump with controllable flow direction. Additionally, high‐speed imaging of the fluid motion and computational fluid dynamics simulations to confirm the photoacoustic principle of laser streaming are employed, and it is revealed that the streaming direction is determined by the direction of strongest intensity in the divergent ultrasound wavefront. Finally, the potential of laser streaming for microfluidic and optofluidic applications is demonstrated by successfully driving fluid inside a capillary tube.