Richard G. Stearns

Effect of electronic strain on photoacoustic generation in silicon

The photogeneration of excess free carriers gives rise to a mechanical strain in semiconductors. An experiment has been performed to investigate the contribution of this electronic strain to photoacoustic generation, and it is found that electronic strain is an important mechanism in the photogeneration of acoustic waves in silicon. A theory is developed to predict the contribution of the electronic strain to the photoacoustic generation and is found to agree well with experiment.

Thermal acoustic probe

A noncontacting method of measuring periodic surface heating is described. The perturbation of an externally generated acoustic wave is measured. The acoustic wave is generated in the air above a sample to be studied. The acoustic wave is directed onto the sample surface, coincident with a modulated light beam. Absorption of the light beam results in the periodic heating of the sample, at and near the sample surface. The air in contact with the sample surface is in turn heated, and produces a periodic phase shift in the reflected acoustic wave. This phase shift is detected and gives a direct measure of the periodic heating of the sample surface. An acoustic microscope may generate the acoustic wave. The sample is placed in water. An acoustic microscope lens produces an acoustic wave in the water, which focuses onto the sample surface, coincident with a modulated laser beam. The light beam is guided onto the sample using an optical fiber. Heating in the water directly above the illuminated sample produces a phase perturbation in the acoustic wave reflecting off the sample surface. This embodiment of the present invention allows surface heating to be measured with very high spatial resolution.

Acoustic wave generation by thermal excitation of small regions

Reciprocity theory is used to derive a general formalism for generation of acoustic waves by thermal excitation. It is shown that the most important parameters contributing to the generation include the thermal expansion of the material and its interaction with the volume dilation associated with the acoustic field of the receiving transducer. The excitation of a longitudinal acoustic wave, as a function of angle to the surface normal, is derived directly.

Moving Liquids with Sound: The Physics of Acoustic Droplet Ejection for Robust Laboratory Automation in Life Sciences.

Liquid handling instruments for life science applications based on droplet formation with focused acoustic energy or acoustic droplet ejection (ADE) were introduced commercially more than a decade ago. While the idea of “moving liquids with sound” was known in the 20th century, the development of precise methods for acoustic dispensing to aliquot life science materials in the laboratory began in earnest in the 21st century with the adaptation of the controlled “drop on demand” acoustic transfer of droplets from high-density microplates for high-throughput screening (HTS) applications. Robust ADE implementations for life science applications achieve excellent accuracy and precision by using acoustics first to sense the liquid characteristics relevant for its transfer, and then to actuate transfer of the liquid with customized application of sound energy to the given well and well fluid in the microplate. This article provides an overview of the physics behind ADE and its central role in both acoustical and rheological aspects of robust implementation of ADE in the life science laboratory and its broad range of ejectable materials.

Low Nanoliter Acoustic Transfer of Aqueous Fluids with High Precision and Accuracy of Volume Transfer and Positional Placement

The utility of acoustic droplet ejection (ADE), originally used to transfer dimethyl sulfoxide (DMSO) solutions, is expanded beyond the transfer of DMSO to a wide variety of aqueous solutions common to biochemical experiments and assays. Aqueous-based liquids are transferred with high precision (coefficient of variation <5% for volume transfers of 5–50 nL) and accuracy (within 5% of expected volume), similar to that seen with DMSO transfers. The precision and accuracy of the technique are measured via fluorescence. ADE transfers of aqueous solutions may facilitate the miniaturization of assays leading to increased throughput and reduced reagent usage.

Technologies That Enable Accurate and Precise Nano- to Milliliter-Scale Liquid Dispensing of Aqueous Reagents Using Acoustic Droplet Ejection.

Acoustic liquid handling uses high-frequency acoustic signals that are focused on the surface of a fluid to eject droplets with high accuracy and precision for various life science applications. Here we present a multiwell source plate, the Echo Qualified Reservoir (ER), which can acoustically transfer over 2.5 mL of fluid per well in 25-nL increments using an Echo 525 liquid handler. We demonstrate two Labcyte technologies—Dynamic Fluid Analysis (DFA) methods and a high-voltage (HV) grid—that are required to maintain accurate and precise fluid transfers from the ER at this volume scale. DFA methods were employed to dynamically assess the energy requirements of the fluid and adjust the acoustic ejection parameters to maintain a constant velocity droplet. Furthermore, we demonstrate that the HV grid enhances droplet velocity and coalescence at the destination plate. These technologies enabled 5-µL per destination well transfers to a 384-well plate, with accuracy and precision values better than 4%. Last, we used the ER and Echo 525 liquid handler to perform a quantitative polymerase chain reaction (qPCR) assay to demonstrate an application that benefits from the flexibility and larger volume capabilities of the ER.

Measurement of periodic surface heating using surface acoustic waves

A new technique is described to measure periodic heating at and near the surface of a material. The technique involves the phase perturbation of a surface acoustic wave propagating through the heated region. Temperature fluctuations can be measured at modulation frequencies of several hertz to hundreds of kilohertz with high sensitivity. A theory is given which predicts the acoustic phase perturbation as a function of thermal modulation frequency, and is shown to agree well with experiment.

Acoustic Methods to Monitor Protein Crystallization and to Detect Protein Crystals in Suspensions of Agarose and Lipidic Cubic Phase

Improvements needed for automated crystallography include crystal detection and crystal harvesting. A technique that uses acoustic droplet ejection to harvest crystals was previously reported. Here a method is described for using the same acoustic instrument to detect protein crystals and to monitor crystal growth. Acoustic pulses were used to monitor the progress of crystallization trials and to detect the presence and location of protein crystals. Crystals were detected, and crystallization was monitored in aqueous solutions and in lipidic cubic phase. Using a commercially available acoustic instrument, crystals measuring ~150 µm or larger were readily detected. Simple laboratory techniques were used to increase the sensitivity to 50 µm by suspending the crystals away from the plastic surface of the crystallization plate. This increased the sensitivity by separating the strong signal generated by the plate bottom that can mask the signal from small protein crystals. It is possible to further boost the acoustic reflection from small crystals by reducing the wavelength of the incident sound pulse, but our current instrumentation does not allow this option. In the future, commercially available sound-emitting transducers with a characteristic frequency near 300 MHz should detect and monitor the growth of individual 3 µm crystals.

High-Frequency Photoacoustics in Air

Absfruct-A photoacoustic measurement in air at 2 MHz is described that uses as a detector a focused acoustic transducer designed to operate in air. A detailed analysis of the photoacoustic measurement is performed, and the experimental sensitivity of the measurement is found to agree well with theoretical prediction. Several applications of the photoacoustic measurement are discussed. These include the determination of the step height of an oxide layer on silicon, the imaging of surface topography with high spatial resolution, and the imaging of variations in the surface recombination velocity along a silicon wafer.

Effect of photocarriers on acoustic wave propagation for measuring excess carrier density and lifetimes in silicon

A new technique is described to measure temporal periodic fluctuations in the free‐carrier density of silicon. It is based on the electronic dependence of the elastic constants, and involves the measurement of the perturbation in phase of an acoustic wave propagating through a region of modulated carrier density. An experiment is described in which the bulk lifetime of photogenerated carriers is determined using this technique. A theory is developed to predict the acoustic phase perturbation, and is found to agree well with experiment.