We would like to acknowledge the support given and facilities provided by the Department of Mechanical Science and Engineering Rapid Prototyping Lab at the University of Illinois to enable this work.

1. Rapid prototype mold design and fabrication
<ol>
	<li>Open AutoCAD on a PC. Select File on the taskbar, then select Open and browse to and click on a three-dimensional (3D) model file of the channel mold having .dxf or .dwg extension.</li>
	<li>Select the entire model by clicking and dragging a box around it. Export the design as a .stl file by selecting File |&#160;Export,&#160;then Other formats and choosing .stl from the dropdown box.</li>
	<li>Upload the file to a high precision resin stereolithographic (SLA) printer such as Formlabs FORM3. Pour the resin into the resin chamber and initiate printing and produce the mold with the smallest z-axis steps (25 micron for Formlabs CLEAR resin).</li>
	<li>Wait for the automatic part printing to be completed. 
	NOTE: Molds with features as small as 0.1 mm can be fabricated in this way.</li>
	<li>After removing the part from the resin, agitate it in isopropanol for 5 min to remove any remaining resin.</li>
	<li>Dry the mold with air or nitrogen gas for 2 min. 
	NOTE: Conventional microfluidic mold fabrications with silicon wafers and photolithography with any SU8 or KMPR photoresists can also be used to produce a mold with smaller features.</li>
	<li>Cure the dried mold at 60 &#176;C in UV light for a maximum of 1 h.</li>
</ol>
2. PDMS microchannel fabrication
<ol>
	<li>Place the mold on a sheet of aluminum foil. To ease the delamination of PDMS, spray coat the mold with silicone mold release in 1 or 2 passes.</li>
	<li>Pour PDMS resin and cross-linker into a disposable cup in the ratio of 10:1 by weight and mix with a disposable spoon.</li>
	<li>Pour the resulting mixture onto the mold to produce a film of required thickness. To prevent large channel wall deformation, maintain PDMS thickness of more than 5 mm or 3-4 times the maximum feature thickness.</li>
	<li>Place the mold with poured PDMS into the degas chamber and close the lid. Ensure that the O-ring hermetically seals the chamber.</li>
	<li>Close the exhaust valve and turn on the vacuum rough pump to initiate degassing.</li>
	<li>Degas the poured mixture in a vacuum pump for over 4-6 cycles with each cycle lasting approximately 5 min. Manually remove any remaining bubbles (in corners and trenches) using a fine wire.</li>
	<li>Set oven temperature to 80 &#176;C and allow it to preheat. Place the mixture in the oven at 80 &#176;C for 2 h to cure.</li>
	<li>Remove the cured mold from the oven and leave it at room temperature for 10 min to cool.</li>
	<li>Using a scalpel, carefully cut out the edges of the mold. For optimal delamination, use a syringe to inject isopropanol in between the mold and the cured PDMS.</li>
	<li>Peel the cured PDMS off from the mold and cut it into individual devices with a razor blade. The size of each device must range between 10 mm x 10 mm to 30 mm x 70 mm to be bonded with the glass slide.</li>
	<li>Make a hole of 1.0-3.0 mm diameter at the inlet and outlet using a biopsy punch.</li>
	<li>Turn on the handheld radio frequency (RF) plasma generator. To activate the glass slide, steadily pass the wire electrode over a clean dry glass slide multiple times for 2 min. Maintain a wire to glass gap of approximately 5 mm. Place the device side of the cured PDMS in contact with the activated glass slide and then place in an 80 &#176;C oven for 2 h.</li>
	<li>Cut polyethylene inlet and outlet tubing to the required length and insert them into the inlet and outlet holes.</li>
	<li>To prevent tube detachment during operation, apply silicone sealant on the contact surface and let cure for 2 h to secure the tubing.</li>
</ol>
3. Oscillatory driver assembly
<ol>
	<li>Clamp the alligator clip ends of a pair of alligator-to-pin wires to the terminals of a speaker. Here a 15 W speaker with an 8 cm cone was used although other speakers can also be used.</li>
	<li>Place the aux controller chip on an insulating container. Insert the pin ends into the screw sockets of the aux controller chip and firmly tighten with a screwdriver to ensure connectivity.</li>
	<li>Connect one end of an aux cable to the controller chip and the other end to an aux port on a computer or smartphone.</li>
	<li>Connect a 12 V direct current (DC) adapter to the power supply. Power the controller chip on by connecting the coaxial end of the DC adapter to the power socket.</li>
	<li>Using an internet browser, navigate to an online tone generator website (e.g., https://www.szynalski.com/tone-generator/ ).</li>
	<li>Type in the desired frequency (5-1200 Hz) in the online application. Scroll the volume bar to the required amount (e.g., 100%).</li>
	<li>Click on the Wave-Type Generator symbol and select the desired waveform (sine, square, triangle, sawtooth). Note the default is a sine waveform. Press Play to actuate the speaker.</li>
</ol>
4. Adapter assembly
NOTE: The complete speaker-to-tube adapter assembly is illustrated by the schematic in Figure 1.
<ol>
	<li>Fix the speaker&#160;(Figure 1(I)) onto the 3D printed speaker mount (Figure 1(II)) (see speakermount.stl in Supplementary File 1) by sticking a tape over the curved surface and either side of the mount.</li>
	<li>Orient the speaker vertically with the speaker cone surface facing up. Place the 3D printed adapter&#160; (Figure 1(III)) (see speakertubeadapter.stl in Supplementary File 2) concentrically on the speaker cone.</li>
	<li>Apply silicone sealant generously along the edges of the adapter and let cure for 2 h.</li>
	<li>Position the speaker and speaker mount on the microscope stage and tape down to prevent movement during operation.</li>
	<li>Cut a 200 &#956;L micro-pipette tip approximately 2 cm from its narrow end and dispose the wider half of the tip. The narrow conical end will serve as a wedge seal for reversible attachment.</li>
	<li>Connect the polyethylene tubing (Figure 1(V)) to the microchannel (Figure 1(VI)) outlet by first threading through the micro-pipette tip (Figure 1(IV)), and then through the adapter&#39;s coaxial end and finally out through the side.</li>
	<li>Firmly wedge the narrow end of the pipette tip into the adapter&#39;s coaxial end to create a detachable tight seal.</li>
</ol>
5. Operation of the experimental setup for oscillatory flows in microchannels
<ol>
	<li>Add tracer particles into a vial of 22% weight/weight (w/w) glycerol solution to produce a neutrally buoyant suspension with a volume fraction of 0.01%-0.1% polystyrene in liquid at 20 &#176;C. Mix vigorously by shaking to produce a homogenous suspension.</li>
	<li>Load a 1 mL inlet syringe with 1 mL of sample. Mount and fasten the loaded syringe onto an automatic syringe pump. Insert the syringe needle into the inlet tubing of the device to create a watertight seal.</li>
	<li>Ensure the outlet tube is routed through the adapter assembly and into a reservoir (see previous section on adapter assembly).</li>
	<li>Turn on the syringe pump. Using the touch screen, select the syringe type as Becton-Dickinson 1 mL. Then, select Infuse. Then select the required flow rate (0-1 mL/min) or flow volume (&#60; 1 mL).</li>
	<li>Initiate the steady flow using the syringe pump. Wait until sufficient volume of fluid has flowed and the outlet tube is filled with liquid up to the speaker. 
	NOTE: The oscillatory amplitude for a given setting will not vary with steady transport flow if the outlet tube is primed.</li>
	<li>Select a required frequency, amplitude, and waveform in the tone generator application as described in step 3.5 and press Play to generate oscillatory flow inside the microchannel.</li>
</ol>
6. Observation and amplitude measurement
<ol>
	<li>Mount the device on the microscope. Set up the optical configuration by selecting an objective lens with a magnification between 10x and 40x adjusting the focal plane and positioning the stage.</li>
	<li>To obtain measurements in a well-defined focal plane, ensure that the depth of field of the objective lens is smaller than the channel depth by a factor of 5 or more.</li>
	<li>To observe the oscillatory flow, use a high-speed camera with a frame rate of at least twice the oscillation frequency as calculated using the Nyquist sampling theorem. For a practically useful resolution of the waveform, measure at least 10 points per time period using a framerate &#62; 10 times that of the oscillation frequency.</li>
	<li>Alternatively, to observe only the rectified or long-time effects of pulsatile flows, perform stroboscopic imaging by setting the observation frequency to any perfect divisor of the oscillation frequency.</li>
	<li>For both direct and stroboscopic imaging, use a camera equipped with a global shutter to avoid the jello-effect. In either case, keep the exposure time considerably smaller than the oscillation time period (by a factor of 10 or more) to prevent streaking.</li>
	<li>To measure the oscillation amplitude without a high-speed camera, record at a framerate maintained close to but not equal to the stroboscopic frame rate (e.g., 49 frames/s for a 50 Hz signal). This results in a highly slowed-down oscillation from which the amplitude can be accurately measured.</li>
	<li>Observe and record the amplitude measurements.</li>
</ol>

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The authors have nothing to disclose.

We have demonstrated the assembly (see protocol critical steps 3 and 4) and operation (see protocol critical steps 5 and 6) of an external speaker-based apparatus for the generation of oscillatory flow with frequencies in the range of 10 to 1000 Hz in microfluidic devices. Particle tracking of suspended tracer particles is required to determine the fidelity of the harmonic motion as well as for calibrating the range of oscillation amplitudes achievable over the range of operating frequencies. The amplitude-frequency curv...

The precise control of liquid flow rate in microchannels is crucial for lab-on-a-chip applications such as droplet production and encapsulation1, mixing2,3, and the sorting and manipulation of suspended particles4,5,6,7. The predominantly used method for flow control is a syringe pump that produces highly controlled steady flows dispensing either a fixed volume of liquid or a fixed volumetric flow rate, often limited to entirely unidirectional flow. Alternative strategies for producing unidirectional flow include using gravitational head8, capillary forces9, or electro-osmotic flow10. Programmable syringe pumps allow for a time-dependent bidirectional control of flow rates and dispensed volumes but are limited to response times greater than 1 s due to the mechanical inertia of the syringe pump.
Flow control at shorter time scales unlocks a plethora6,11,12,13,14,15 of otherwise inaccessible possibilities due to qualitative changes in flow physics. The most practical means of harnessing this varied flow physics is through acoustic waves or oscillatory flows with time periods ranging from 10-1- 10-9 s or 101&#160;-109 Hz. The higher end of this frequency range is accessed using bulk acoustic wave (BAW; 100 kHz-10 MHz) and surface acoustic wave (SAW; 10 MHz-1 GHz) devices. In a typical BAW device, the entire substrate and the fluid column are vibrated by applying a voltage signal across a bonded piezoelectric. This enables relatively high throughputs but also results in heating at higher amplitudes. In SAW devices, however, the solid-liquid interface is oscillated by applying voltage to a pair of interdigitated electrodes patterned on a piezoelectric substrate. Due to the very short wavelengths (1 &#181;m-100 &#181;m) particles as small as 300 nm can be precisely manipulated by the pressure wave generated in SAW devices. Despite the ability to manipulate small particles, SAW methods are limited to local particle manipulation since the wave rapidly attenuates with distance from the source.
At the 1-100 kHz frequency range, oscillatory flows are usually generated using piezo-elements that are bonded to a polydimethylsiloxane (PDMS) microchannel above a designed cavity16,17. The PDMS membrane above the patterned cavity behaves like a vibrating membrane or drum that pressurizes the fluid within the channel. At this frequency range, the wavelength is larger than the channel size, but the oscillation velocity amplitudes are small. The most useful phenomenon in this frequency regime is the generation of acoustic/viscous streaming flows, which are rectified steady flows caused due to non-linearity inherent in the flow of liquids with inertia18. The steady streaming flows typically manifest as high-speed counter-rotating vortices in the vicinity of obstacles, sharp corners, or micro-bubbles. These vortices are useful for mixing19,20 and separating 10 &#181;m sized particles from the flow stream21.
For frequencies in the range of 10-1000 Hz, both the velocity of the oscillatory component and its associated steady viscous streaming are considerable in magnitude and useful. Strong oscillatory flows in this frequency range can be used for inertial focusing22, facilitate droplet generation23, and can generate flow conditions (Womersley numbers) that mimic blood flow for in vitro studies. On the other hand, streaming flows are useful for mixing, particle trapping, and manipulation. Oscillatory flow in this range of frequencies can also be accomplished using a piezo-element bonded to the device as described above23. A significant hurdle to implementing oscillatory flows through a bonded piezo element is that it requires features to be designed beforehand. Furthermore, the bonded speaker elements are not detachable, and a new element must be bonded to each device24. However, such devices present the advantage of being compact. An alternative method is using an electromechanical relay valve20. These valves require pneumatic pressure sources and custom control software for operation and therefore increase the technical barrier to testing and implementation. Nevertheless, such devices enable the application of set pressure amplitude and frequency.
In this article, the construction, operation, and characterization of a user-friendly method to generate oscillatory flows in the frequency range of 10-1000 Hz in microchannels is described. The method offers numerous advantages such as cost-effective assembly, ease of operation, and ready to interface with standard microfluidic channels and accessories such as syringe pumps and tubing. Additionally, compared to previous similar approaches25, the method offers the user selective and independent control of oscillation frequencies and amplitudes, including the modulation between sinusoidal and non-sinusoidal waveforms. These features allow users to easily deploy oscillatory flows and, therefore, facilitate widespread adoption into a broad range of currently existing microfluidic technologies and applications in the fields of biology and chemistry.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Oscillatory Driver Assembly</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Alligator-to-pin wire</td>
			<td>Adafruit</td>
			<td>3255</td>
			<td>Small alligator clip to male jumper wire (12)</td>
		</tr>
		<tr>
			<td>Aux cable</td>
			<td>Adafruit</td>
			<td>2698</td>
			<td>3.5 mm Male/Male stereo cable 1 m</td>
		</tr>
		<tr>
			<td>Controller chip</td>
			<td>Damgoo</td>
			<td>TPA3116</td>
			<td>50w+50w 2 channel audio amplifier (bluetooth and AUX)</td>
		</tr>
		<tr>
			<td>DC adapter</td>
			<td>Adafruit</td>
			<td>798</td>
			<td>12 V DC 1A regulated switching power adapter</td>
		</tr>
		<tr>
			<td>Micro-pipette tip</td>
			<td>VWR Signature</td>
			<td>37001-532</td>
			<td>200 ul micropipette tip</td>
		</tr>
		<tr>
			<td>Silicone sealant</td>
			<td>Loctite</td>
			<td>908570</td>
			<td>Clear silicone waterproof sealant (80 ml)</td>
		</tr>
		<tr>
			<td>Speaker</td>
			<td>Drok</td>
			<td>6843996</td>
			<td>4.5 inch 4 Ohm 40 W speaker</td>
		</tr>
		<tr>
			<td>Speaker mount</td>
			<td></td>
			<td></td>
			<td>3D printed from &#39;speakermount.stl&#39; in supplementary files</td>
		</tr>
		<tr>
			<td>Speaker-to-tube adapter</td>
			<td></td>
			<td></td>
			<td>3D printed from &#39;speaketubeadapter.stl&#39; in supplementary files</td>
		</tr>
		<tr>
			<td>Microchannel Manufacture</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Biopsy punch</td>
			<td>Miltex</td>
			<td>15110</td>
			<td>Biopsy punch with plunger (1 - 4 mm)</td>
		</tr>
		<tr>
			<td>Degasser</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Disposable cup</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Disposable spoon</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Slides</td>
			<td>VWR Signature</td>
			<td>16004-430</td>
			<td>3&#34; x 1&#34; pre clean 1 mm thick</td>
		</tr>
		<tr>
			<td>Mold</td>
			<td></td>
			<td></td>
			<td>Si - SU-8 or 3D printed</td>
		</tr>
		<tr>
			<td>Oven</td>
			<td>Fischer Scientific</td>
			<td></td>
			<td>Isotemp</td>
		</tr>
		<tr>
			<td>PDMS resin and cross-linker</td>
			<td>Dow Chemical</td>
			<td>4019862</td>
			<td>Sylgard 184 PDMS resin and crosslinker (500 g)</td>
		</tr>
		<tr>
			<td>Polyethylene tubing</td>
			<td>Becton Dickinson Intramedic</td>
			<td>427440</td>
			<td>Polyethylene tubing (PE 60 - PE 200)</td>
		</tr>
		<tr>
			<td>Razor blades</td>
			<td>VWR</td>
			<td>55411-050</td>
			<td>Single edge industrial razor blades</td>
		</tr>
		<tr>
			<td>RF plasma generator</td>
			<td>Electro-Technic Products</td>
			<td>BD - 20</td>
			<td>High frequency generator</td>
		</tr>
		<tr>
			<td>Silicone Mold Release</td>
			<td>CRC</td>
			<td>03301</td>
			<td>Food Grade Silicon Mold release (16 oz)</td>
		</tr>
		<tr>
			<td>Observation and Characterization</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Camera</td>
			<td>Edgertronic</td>
			<td></td>
			<td>SC2+</td>
		</tr>
		<tr>
			<td>Lens</td>
			<td>Nikon</td>
			<td></td>
			<td>Plan Fluor 10x</td>
		</tr>
		<tr>
			<td>Microscope</td>
			<td>Nikon</td>
			<td></td>
			<td>Ti Eclipse manual stage</td>
		</tr>
		<tr>
			<td>Needles</td>
			<td>Becton Dickinson</td>
			<td>305175</td>
			<td>&#160;PrecisionGlide 20G</td>
		</tr>
		<tr>
			<td>Syringe</td>
			<td>Becton Dickinson</td>
			<td>1180100555</td>
			<td>Monoject 1 ml</td>
		</tr>
		<tr>
			<td>Syringe pump</td>
			<td>Harvard Apparatus</td>
			<td></td>
			<td>Dual syringe programmable syringe pump</td>
		</tr>
		<tr>
			<td>Tracer Particles</td>
			<td>Spherotech</td>
			<td>PP-10-10</td>
			<td>Polystyrene tracer particles 1 um</td>
		</tr>
	</tbody>
</table>

assembly and characterization of an external driver for the generation of sub-kilohertz oscillatory flow in microchannels

Microfluidic technology has become a standard tool in chemical and biological laboratories for both analysis and synthesis. The injection of liquid samples, such as chemical reagents and cell cultures, is predominantly accomplished through steady flows that are typically driven by syringe pumps, gravity, or capillary forces. The use of complementary oscillatory flows is seldom considered in applications despite its numerous advantages as recently demonstrated in the literature. The significant technical barrier to the implementation of oscillatory flows in microchannels is likely responsible for the lack of its widespread adoption. Advanced commercial syringe pumps that can produce oscillatory flow, are often more expensive and only work for frequencies less than 1 Hz. Here, the assembly and operation of a low-cost, plug-and-play type speaker-based apparatus that generates oscillatory flow in microchannels is demonstrated. High-fidelity harmonic oscillatory flows with frequencies ranging from 10-1000 Hz can be achieved along with independent amplitude control. Amplitudes ranging from 10-600 &#181;m can be achieved throughout the entire range of operation, including amplitudes &#62; 1 mm at the resonant frequency, in a typical microchannel. Although the oscillation frequency is determined by the speaker, we illustrate that the oscillation amplitude is sensitive to fluid properties and channel geometry. Specifically, the oscillation amplitude decreases with increasing channel circuit length and liquid viscosity, and in contrast, the amplitude increases with increasing speaker tube thickness and length. Additionally, the apparatus requires no prior features to be designed on the microchannel and is easily detachable. It can be used simultaneously with a steady flow created by a syringe pump to generate pulsatile flows.

To illustrate the capability and performance of the above setup, representative results of oscillatory flow in a simple linear microchannel with a square cross-section are presented. The width and height of the channel are 110 &#181;m and its length is 5 cm. First, we describe the motion of spherical polystyrene tracer particles and how these can be used to check the fidelity of the oscillatory signal as well as the range of oscillation amplitudes achievable. We then discuss the effect of specific fluid properties or mic...

Watch this Scientific Journal Video about Assembly and Characterization of an External Driver for the Generation of Sub-Kilohertz Oscillatory Flow in Microchannels at JoVE.com

Assembly and Characterization of an External Driver for the Generation of Sub-Kilohertz Oscillatory Flow in Microchannels

The protocol demonstrates a convenient method to produce harmonic oscillatory flow from 10-1000 Hz in microchannels. This is performed by interfacing a computer-controlled speaker diaphragm to the microchannel in a modular manner.

Microfluidic technology has become a standard tool in chemical and biological laboratories for both analysis and synthesis. The injection of liquid ...

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Research

JoVE Journal

Engineering

2.3K Views.  University of Illinois at Urbana-Champaign. The protocol demonstrates a convenient method to produce harmonic oscillatory flow from 10-1000 Hz in microchannels. This is performed by interfacing a computer-controlled speaker diaphragm to the microchannel in a modular manner.

Video: Assembly and Characterization of an External Driver for the Generation of Sub-Kilohertz Oscillatory Flow in Microchannels

The Generation of Higher-order Laguerre-Gauss Optical Beams for High-precision Interferometry

Thermal noise in high-reflectivity mirrors is a major impediment for several types of high-precision interferometric experiments that aim to reach the standard quantum limit or to cool mechanical systems to their quantum ground state. This is for example the case of future gravitational wave observatories, whose sensitivity to gravitational wave signals is expected to be limited in the most sensitive frequency band, by atomic vibration of their mirror masses. One promising approach being pursued to overcome this limitation is to employ higher-order Laguerre-Gauss (LG) optical beams in place of the conventionally used fundamental mode. Owing to their more homogeneous light intensity distribution these beams average more effectively over the thermally driven fluctuations of the mirror surface, which in turn reduces the uncertainty in the mirror position sensed by the laser light.
We demonstrate a promising method to generate higher-order LG beams by shaping a fundamental Gaussian beam with the help of diffractive optical elements. We show that with conventional sensing and control techniques that are known for stabilizing fundamental laser beams, higher-order LG modes can be purified and stabilized just as well at a comparably high level. A set of diagnostic tools allows us to control and tailor the properties of generated LG beams. This enabled us to produce an LG beam with the highest purity reported to date. The demonstrated compatibility of higher-order LG modes with standard interferometry techniques and with the use of standard spherical optics makes them an ideal candidate for application in a future generation of high-precision interferometry.

Large laser-interferometers are being constructed to create a new type of astronomy based on gravitational waves. Their sensitivities, as for many other high-precision experiments, are approaching fundamental noise limits such as the atomic vibration of their components. We are pioneering technologies to overcome these limits using novel laser beam shapes.

Thermal noise in high-reflectivity mirrors is a major impediment for several  types of high-precision interferometric experiments that aim to reach the  ...

Construction and Characterization of External Cavity Diode Lasers for Atomic Physics

Since their development in the late 1980s, cheap, reliable external cavity diode lasers (ECDLs) have replaced complex and expensive traditional dye and Titanium Sapphire lasers as the workhorse laser of atomic physics labs1,2. Their versatility and prolific use throughout atomic physics in applications such as absorption spectroscopy and laser cooling1,2 makes it imperative for incoming students to gain a firm practical understanding of these lasers. This publication builds upon the seminal work by Wieman3, updating components, and providing a video tutorial. The setup, frequency locking and performance characterization of an ECDL will be described. Discussion of component selection and proper mounting of both diodes and gratings, the factors affecting mode selection within the cavity, proper alignment for optimal external feedback, optics setup for coarse and fine frequency sensitive measurements, a brief overview of laser locking techniques, and laser linewidth measurements are included.

This is an instructional paper to guide the construction and diagnostics of external cavity diode lasers (ECDLs), including component selection and optical alignment, as well as the basics of frequency reference spectroscopy and laser linewidth measurements for applications in the field of atomic physics.

Since their development in the late 1980s, cheap, reliable external cavity diode lasers (ECDLs) have replaced complex and expensive traditional dye and ...

Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.

We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities &#8211; high spatial resolution and richness of microstructural data &#8211; with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

The use of electron channeling contrast imaging in a scanning electron microscope to characterize defects in III-V/Si heteroexpitaxial thin films is described. This method yields similar results to plan-view transmission electron microscopy, but in significantly less time due to lack of required sample preparation.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging ...

Characterization of Full Set Material Constants and Their Temperature Dependence for Piezoelectric Materials Using Resonant Ultrasound Spectroscopy

During the operation of high power electromechanical devices, a temperature rise is unavoidable due to mechanical and electrical losses, causing the degradation of device performance. In order to evaluate such degradations using computer simulations, full matrix material properties at elevated temperatures are needed as inputs. It is extremely difficult to measure such data for ferroelectric materials due to their strong anisotropic nature and property variation among samples of different geometries. Because the degree of depolarization is boundary condition dependent, data obtained by the IEEE (Institute of Electrical and Electronics Engineers) impedance resonance technique, which requires several samples with drastically different geometries, usually lack self-consistency. The resonant ultrasound spectroscopy (RUS) technique allows the full set material constants to be measured using only one sample, which can eliminate errors caused by sample to sample variation. A detailed RUS procedure is demonstrated here using a lead zirconate titanate (PZT-4) piezoceramic sample. In the example, the complete set of material constants was measured from room temperature to 120 &#176;C. Measured free dielectric constants <img alt="Equation 1" src="/files/ftp_upload/53461/image001.jpg" /> and&#160;<img alt="Equation 2" src="/files/ftp_upload/53461/image002.jpg" /> were compared with calculated ones based on the measured full set data, and piezoelectric constants d15 and d33 were also calculated using different formulas. Excellent agreement was found in the entire range of temperatures, which confirmed the self-consistency of the data set obtained by the RUS.

This protocol describes the procedure of measuring the temperature dependence of the full set material constants of piezoelectric materials using resonant ultrasound spectroscopy (RUS).

During the operation of high power electromechanical devices, a temperature rise is unavoidable due to mechanical and electrical losses, causing the ...

Characterization of Ultra-fine Grained and Nanocrystalline Materials Using Transmission Kikuchi Diffraction

One of the challenges in microstructure analysis nowadays resides in the reliable and accurate characterization of ultra-fine grained (UFG) and nanocrystalline materials. The traditional techniques associated with scanning electron microscopy (SEM), such as electron backscatter diffraction (EBSD), do not possess the required spatial resolution due to the large interaction volume between the electrons from the beam and the atoms of the material. Transmission electron microscopy (TEM) has the required spatial resolution. However, due to a lack of automation in the analysis system, the rate of data acquisition is slow which limits the area of the specimen that can be characterized. This paper presents a new characterization technique, Transmission Kikuchi Diffraction (TKD), which enables the analysis of the microstructure of UFG and nanocrystalline materials using an SEM equipped with a standard EBSD system. The spatial resolution of this technique can reach 2 nm. This technique can be applied to a large range of materials that would be difficult to analyze using traditional EBSD. After presenting the experimental set up and describing the different steps necessary to realize a TKD analysis, examples of its use on metal alloys and minerals are shown to illustrate the resolution of the technique and its flexibility in term of material to be characterized.

This paper provides a detailed method to characterize the microstructure of ultra-fine grained and nanocrystalline materials using a scanning electron microscope equipped with a standard electron backscatter diffraction system. Metal alloys and minerals presenting refined microstructures are analyzed using this technique, showing the diversity of its possible applications.

One of the challenges in microstructure analysis nowadays resides in the reliable and accurate characterization of ultra-fine grained (UFG) and ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous light irradiation. The photo-excitation of dopants that can quickly dissipate light into heat, coupled with anisotropic thermal expansion and self-shadowing of the film, gives rise to the self-sustained deformation. The oscillations observed are influenced by the dimensions and the modulus of the film, and by the directionality and intensity of the light. The system developed offers applications in energy conversion and harvesting for soft-robotics and automated systems.
The general method described here consists of creating free-standing liquid crystalline films and characterizing the mechanical and thermal effects observed. The molecular alignment is achieved using alignment layers (rubbed polyimide), commonly used in the display manufacturing industry. To obtain actuators with large deformation, the mesogens are aligned and polymerized in a splay/bend configuration, i.e., with the director of the liquid crystals (LCs) going gradually from planar to homeotropic through the film thickness. Upon irradiation, the mechanical and thermal oscillations obtained are monitored with a high-speed camera. The results are further quantified by image analysis using an image processing program.

The goal of the protocol is to create liquid crystalline polymer films that can mechanically oscillate under continuous light irradiation. We describe in great detail the conception of free-standing films, from the liquid crystal alignment method to the photo-actuation. The experimental protocol applied to prepare this material is broadly applicable.

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous ...

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. This paper proposes a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation. The polydimethylsiloxane (PDMS) chip was fabricated using the multi-layer soft lithography method. Hexadecane containing surfactant was used as the continuous phase, and PEGDA with the ultraviolet (UV) photo-initiator was the dispersed phase. Surfactants allowed the local surface tension to drop and formed a more cusped tip which promoted breaking into tiny micro-droplets. As the pressure of dispersed phase was constant, the size of droplets became smaller with increasing continuous phase pressure before dispersed phase flow was broken off. As a result, droplets with size variation from 1 &#181;m to 80 &#181;m in diameter could be selectively achieved by changing the pressure ratio in two inlet channels, and the average coefficient of variation was estimated to be below 7%. Furthermore, droplets could turn into micro-beads by UV exposure for photo-polymerization. Conjugating biomolecules on such micro-beads surface have many potential applications in the fields of biology and chemistry.

Generation of Size-controlled Poly ethylene Glycol Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. ...

Studying Large Amplitude Oscillatory Shear Response of Soft Materials

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl sulfoxide (DMSO) and xanthan gum in water &#8212; two concentrated polymer solutions used as viscosifiers in foods, enhanced oil recovery, and soil remediation. Understanding the nonlinear rheological behavior of soft materials is important in the design and controlled manufacturing of many consumer products. It is shown how the response to LAOS of these polymer solutions can be interpreted in terms of a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. The LAOS results are analyzed via the fully quantitative Sequence of Physical Processes (SPP) technique, using free MATLAB-based software. A detailed protocol of performing a LAOS measurement with a commercial rheometer, analyzing nonlinear stress responses with the freeware, and interpreting physical processes under LAOS is presented. It is further shown that, within the SPP framework, a LAOS response contains information regarding the linear viscoelasticity, the transient flow curves, and the critical strain responsible for the onset of nonlinearity.

We present a detailed protocol outlining how to perform nonlinear oscillatory shear rheology on soft materials, and how to run the SPP-LAOS analysis to understand the responses as a sequence of physical processes.

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl ...