Siew-Wan Ohl

Sonochemistry and sonoluminescence in microfluidics

One way to focus the diffuse energy of a sound field in a liquid is by acoustically driving bubbles into nonlinear oscillation. A rapid and nearly adiabatic bubble collapse heats up the bubble interior and produces intense concentration of energy that is able to emit light (sonoluminescence) and to trigger chemical reactions (sonochemistry). Such phenomena have been extensively studied in bulk liquid. We present here a realization of sonoluminescence and sonochemistry created from bubbles confined within a narrow channel of polydimethylsiloxane-based microfluidic devices. In the microfluidics channels, the bubbles form a planar/pancake shape. During bubble collapse we find the formation of OH radicals and the emission of light. The chemical reactions are closely confined to gas–liquid interfaces that allow for spatial control of sonochemical reactions in lab-on-a-chip devices. The decay time of the light emitted from the sonochemical reaction is several orders faster than that in the bulk liquid. Multibubble sonoluminescence emission in contrast vanishes immediately as the sound field is stopped.

Creation of cavitation activity in a microfluidic device through acoustically driven capillary waves

We present a study on achieving intense acoustic cavitation generated by ultrasonic vibrations in polydimethylsiloxane (PDMS) based microfluidic devices. The substrate to which the PDMS is bonded was forced into oscillation with a simple piezoelectric transducer attached at 5 mm from the device to a microscopic glass slide. The transducer was operated at 100 kHz with driving voltages ranging between 20 V and 230 V. Close to the glass surface, pressure and vibration amplitudes of up to 20 bar and 400 nm were measured respectively. It is found that this strong forcing leads to the excitation of nonlinear surface waves when gas-liquid interfaces are present in the microfluidic channels. Also, it is observed that nuclei leading to intense inertial cavitation are generated by the entrapment of gas pockets at those interfaces. Subsequently, cavitation bubble clusters with void fractions of more than 50% are recorded with high-speed photography at up to 250,000 frames/s. The cavitation clusters can be sustained through the continuous injection of gas using a T-junction in the microfluidic device.

Removal of particles from holes in submerged plates with oscillating bubbles

This study is motivated by a common problem in submerged tubes and structures, which is the blockage of the tubes by pollutant particles or debris from the surrounding fluid. To clear the obstruction from the tube, an expanding bubble is used to propel the obstruction away from the tube (the tube is represented as a submerged transparent plate with a hole in our experiments). In some cases the obstruction removal effect is reinforced by the impacting jet of such a collapsing bubble. The bubble is generated via a simple low voltage electric spark discharge circuit. The pressure generated by the oscillating bubble effectively pushes the particle away from the tube, thereby successfully clearing the obstruction. High-speed photography is used to record and analyze the phenomenon. The speed of the particle is found to be around 1 m/s shortly after the collapse of the bubble. Interestingly, there is a clear difference between air-backed plates and water-backed plates in terms of bubble and particle dynamics. T...

Jets in quiescent bubbles caused by a nearby oscillating bubble

An oscillating bubble near another (stationary) bubble can give rise to interesting interactions. Such a nonequilibrium (oscillating) bubble can create a jet in a smaller nearby (initially stationary) bubble as demonstrated in this study both experimentally and numerically. In the experimental study, a spark-generated bubble (through a short circuit with two electrodes) was generated near a stationary smaller bubble. In order to keep the millimeter-sized bubble stationary, it was trapped in a droplet of silicone oil attached to one of the electrodes. The jet in the initially stationary bubble can reach velocities up to 250 m/s, but the velocity becomes lower for bubbles that are larger or situated further away. The current article also describes some experiments with the appearance of a crown-like secondary jet on the free surface (regarded as a large stationary bubble) relatively long after the bubble has collapsed. Some other interesting interactions of a spark-generated bubble with more than one statio...

Jet orientation of a collapsing bubble near a solid wall with an attached air bubble

The interaction between a cavitation bubble and a non-oscillating air bubble attached to a horizontal polyvinyl chloride plate submerged in de-ionized water is investigated using a low-voltage spark-discharge setup. The attached air bubble is approximately hemi-spherical in shape, and its proximity to a spark-induced oscillating bubble (represented by the dimensionless stand-off distance H′) determines whether or not a jet is formed in the oscillating bubble during its collapse. When the oscillating bubble is created close to the plate, it jets towards or away from the plate. The ratio of oscillating bubble oscillation time and the wall-attached bubble oscillation time (T ′) is found to be an important parameter for determining the jet direction. This is validated with numerical simulations using an axial-symmetrical boundary element model. Our study highlights prospects in reducing cavitation damage with a stationary bubble, and in utilizing a cavitation collapse jet by controlling the jets direction.

Microbubble-mediated sonoporation for highly efficient transfection of recalcitrant human B- cell lines.

Sonoporation has not been widely explored as a strategy for the transfection of heterologous genes into notoriously difficult‐to‐transfect mammalian cell lines such as B cells. This technology utilizes ultrasound to create transient pores in the cell membrane, thus allowing the uptake of extraneous DNA into eukaryotic and prokaryotic cells, which is further enhanced by cationic microbubbles. This study investigates the use of sonoporation to deliver a plasmid encoding green fluorescent protein (GFP) into three human B‐cell lines (Ramos, Raji, Daudi). A higher transfection efficiency (TE) of >42% was achieved using sonoporation compared with <3% TE using the conventional lipofectamine method for Ramos cells. Upon further antibiotic selection of the transfected population for two weeks, we successfully enriched a stable population of GFP‐positive Ramos cells (>70%). Using the same strategy, Raji and Daudi B cells were also successfully transfected and enriched to 67 and 99% GFP‐positive cells, respectively. Here, we present sonoporation as a feasible non‐viral strategy for stable and highly efficient heterologous transfection of recalcitrant B‐cell lines. This is the first demonstration of a non‐viral method yielding transfection efficiencies significantly higher (42%) than the best reported values of electroporation (30%) for Ramos B‐cell lines.

THE DYNAMICS OF AN OSCILLATING BUBBLE NEAR BIO-MATERIALS

The simulation of an oscillating bubble near various bio-materials (brain and cornea) using the Boundary Element Method (BEM) is presented. The bio-materials are modeled as an elastic fluid with different Youngs modulus and density. It is found that the bubble tends to split into two or more bubbles when it is oscillating near soft bio-materials (such as brain). The bubble will collapse with a high speed jet (about 100 m/s) near hard bio-materials (such as cornea). It is found from simulation that under certain conditions (light and soft elastic material), the oscillating bubble will jet away from the elastic boundary similar to a bubble near a free surface. An experiment using a laser generated bubble is carried out to confirm this phenomenon.

Intense cavitation in microfluidics for bio-technology applications

This study reports the use of intense ultrasonic cavitation in the confinement of a microfluidics channel [1], and the applications that has been developed for the past 4 years [2]–[5]. The cavitation bubbles are created at the gas-water interface due to strong capillary waves which are generated when the system is driven at its natural frequency (around 100 kHz) [1]. These bubbles oscillate and collapse within the channel. The bubbles are useful for sonochemistry and the generation of sonoluminescence [2]. When we add bacteria (Escherichia coli), and yeasts (Pichia pastoris) into the microfluidics channels, the oscillating and collapsing bubbles stretch and lyse these cells [3]. In another application, human red blood cells are added to a microchamber. Cell stretching and rapture are observed when a laser generated cavitation bubble expands and collapses next to the cell [4]. A numerical model of a liquid pocket surrounded by a membrane with surface tension which was placed next to an oscillating bubble ...

Surfactant-free emulsification in microfluidics using strongly oscillating bubbles

In this study, two immiscible liquids in a microfluidics channel has been successfully emulsified by acoustic cavitation bubbles. These bubbles are generated by the attached piezo transducers which are driven to oscillate at resonant frequency of the system (about 100 kHz) [1, 2]. The bubbles oscillate and induce strong mixing in the microchamber. They induce the rupture of the liquid thin layer along the bubble surface due to the high shear stress and fast liquid jetting at the interface. Also, they cause the big droplets to fragment into small droplets. Both water-in-oil and oil-in-water emulsions with viscosity ratio up to 1000 have been produced using this method without the application of surfactant. The system is highly efficient as submicron monodisperse emulsions (especially for water-in-oil emulsion) could be created within milliseconds. It is found that with a longer ultrasound exposure, the size of the droplets in the emulsions decreases, and the uniformity of the emulsion increases. Reference:...

Shock Wave Interaction with Single Bubbles and Bubble Clouds

Research on the interaction of shock waves with bubbles is highlighted by describing historic studies and recent experiments. We distinguish between the interaction of stable gas bubbles and cavitation bubbles. Gas bubbles and stabilized liquid menisci demonstrate a rapid jetting mechanism if exposed to shock waves. Cavitation bubbles can by themselves interact through the emission of acoustic transients and shock waves. We summarize some of the work on the interaction of stable bubbles and cavitation bubbles in clouds with shock waves. Most of the experimental findings are compared to simulation results using Boundary Element Method, Free Lagrange methods, and various techniques to solve the Euler equations with Finite Differences and Finite Volume techniques. We conclude this chapter by presenting recent advances from molecular dynamics simulations to predict nanobubble shock wave interaction.