The authors are grateful to the Pioneer Oil Company (Vincennes, IN) for financial support.

1. Minimum instrument specifications
<ol>
	<li>Prepare a tensiometer for the EBM with the following specifications: (i) a dispensing system for controlling the dispensing gas volume; (ii) a camera for capturing the bubble image; (iii) an image analysis software for solving the Laplace-Young equation (LY equation) with the axisymmetric bubble shape analysis algorithm11,12; and (iv) a temperature-controlled sample chamber. 
	NOTE: Usually, the instrument for the EBM can also be used for the pendant drop method, in which a small drop is formed and hangs vertically from the end of a syringe needle.</li>
	<li>Prepare a tensiometer for the SBM with the following specifications: (i) a sample tube holder that is capable of spinning a sample tube holder horizontally at high rotation frequencies of at least 6,000 rpm; (ii) a camera for capturing the image of the spinning bubble in the tube; and (iii) an image analysis software to solve the general LY equation and Vonnegut equation13. 
	NOTE: The protocol can be paused here.</li>
</ol>
2. Materials and sample preparation
<ol>
	<li>Obtain pure water from a water purification apparatus. The resistivity of the water at 25 &#176;C at the device output should be 18.2 M&#8486;&#183;cm or close.</li>
	<li>Clean all borosilicate vials, quartz cells, glassware, and magnetic stirring bars by soaking them in pure water for at least 8 h and repeat the soaking process at least one more time. 
	NOTE: The soaking process is aimed at removing residual ions from the glass containers, which can affect the surface tension values significantly.</li>
	<li>Prepare a surfactant solution of interest in the cleaned glassware. 
	NOTE: The surfactant concentration should be lower than its solubility limit in the water.</li>
	<li>Wash each container that will be used for the tension measurements with the sample solution that will be used for the actual measurements prior to sample loading.</li>
	<li>Measure the densities of the liquid samples prior to the tension measurement to three or four significant figures. 
	NOTE: The protocol can be paused here.</li>
</ol>
3. Surface tensiometry with the emerging bubble method (EBM)
<ol>
	<li>Calibrate the image-acquiring device of the tensiometer according to the vendor&#39;s user manual. 
	NOTE: The protocol can be paused here.</li>
	<li>Select an inverted stainless-steel needle based on the estimated maximum bubble diameter from the estimated surface tension values. 
	NOTE: the maximum bubble diameter can be estimated from the capillary length, dc ( <img alt="Equation 1" src="/files/ftp_upload/59818/59818eq1.jpg" />, where &#947; is the surface tension (N&#183;m-1), &#916;&#961; is the density difference of the liquid phase and the air (kg&#183;m-3), and g is the gravitational acceleration (m2&#183;s-1)). The maximum bubble volume (Vmax) can be estimated as &#960;dc3/6. 
	NOTE: The protocol can be paused here.</li>
	<li>Place the inverted stainless-steel needle, obtained from the same vendor of the tensiometer, at the tip of the dispensing device. 
	NOTE: An automated dispenser is recommended compared to a manual syringe, because it is easier and more accurate for the users to produce the desired volume, and then, volume and area perturbations to the surface. The smallest volume step of the dispenser is recommended to be less than 1 &#181;L, from 0.2-0.5 &#181;L, in order to produce precise area perturbations. The protocol can be paused here.</li>
	<li>Determine the volume of the liquid sample for the tension measurements such that the depth of the liquid sample is long enough to have the entire inverted part of the dispensing needle submerged, and to have an additional ~20 mm depth of liquid sample between the inverted needle tip and the liquid sample surface.</li>
	<li>Load a liquid sample in the quartz cell and place the cell on top of the sample platform. In our example, the liquid sample volume was 40 mL.</li>
	<li>Adjust the height of the inverted needle such that the tip of the needle is at least 20 mm below the surface of the liquid sample.</li>
	<li>Adjust the position of the inverted needle such that the boundary of the needle tip is parallel to the liquid-air surface.</li>
	<li>Inject ~1 mL of air through the submerged inverted needle to remove impurities that could possibly be present on the tip of the syringe. This procedure is used to improve the surface chemical purity of the air/liquid interface.</li>
	<li>Estimate the maximum bubble volume (Vmax) with a procedure described as follows. First, dispense ~2 &#181;L of air to form a bubble at the tip of the syringe and observe the bubble shape. Then, increase the bubble volume by ~0.5 &#181;L and observe the bubble shape. Repeat the two previous steps until the bubble detaches from the needle tip. This step specifies the Vmax.</li>
	<li>Determine the appropriate range of the bubble volume, based on the previous set of observations. 
	NOTE: The bubble shape should be non-spherical, substantially deformed by gravity, to allow accurate use of the axisymmetric drop shape analysis algorithm, and the bubble volume should be quite smaller than the Vmax to avoid bubble detachment from the needle tip. For the syringe tip with the inner diameter of 0.84 mm, the preferred initial bubble volume is about 4 &#181;L.</li>
	<li>Determine the initial bubble volume based on the bubble volume range determined from the previous step. The initial bubble volume should be close to the middle of the bubble volume range so that the volume, and area, perturbations produce bubbles inside the range.</li>
	<li>Dispense the predetermined initial bubble volume from the previous step to form a bubble at the tip of the inverted syringe tip. Make sure that the bubble is in hydrostatic equilibrium, which means that the surface tension forces balance the gravity (buoyancy) forces. 
	NOTE: It is important to have the bubble pinned outside of the needle tip perimeter to prevent the presence of surfactant solution inside the syringe needle. If the bubble is pinned inside of the needle tip, repeat the step 3.8 to purify the needle tip.</li>
	<li>Measure the dynamic surface tension based on the shape of the produced air bubble at the tip of the needle tip every 1 s, or another predetermined time interval. The recommended numerical algorithm for calculating surface tension is one based on the axisymmetric drop shape analysis method of the LY equation11,12.</li>
	<li>Compare the actual shape of the bubble with the calculated shape. If the two shapes overlap completely, or nearly, one infers that the equilibrium LY equation is valid for each dynamic and slowly-varying shape. This inference is completely valid when the bubble stops moving, and the ST stops changing, to have hydrostatic equilibrium. 
	NOTE: The criterion that the surface tension value is uniform throughout the interface and that hydrodynamic effects are not important is that the calculated bubble interface shape based on the optimal inferred surface tension values overlaps visually with the actual bubble interface shape. More quantitative tests are possible but will not be considered in this article.</li>
	<li>Measure the surface tension as a function of time until the first steady-state surface tension (SST1) is achieved. The SST is defined as a plateau value beyond which the surface tension changes by less than 1 mN&#183;m-1 (or by less than 5%) in several (10 to 100) consecutive dynamic surface tension measurements.</li>
	<li>Record the bubble volume (V1) and the surface area (A1)</li>
	<li>Decrease the bubble volume by removing ~1 &#181;L of air, and record the new bubble volume, V2 and area, A2 (see Figure 1).</li>
	<li>Continue measuring the DST and the areas until the DST reaches the second SST (SST2) at the bubble volume of V2.</li>
	<li>Expand the bubble volume by injecting ~1 &#181;L of air so that V3 &#8776; V1 and A3 &#8776; A1. 
	NOTE: Having V3 and A3 exactly equal to V1 and A1 is not essential.</li>
	<li>Continue measuring DST values until a third SST (SST3) is reached. If the three SST values differ from each other by less than 1.0 mN&#183;m-1, or by 5%, then their average is defined as the equilibrium surface tension (EST). 
	NOTE: The protocol can be paused here.</li>
</ol>
4. Surface tensiometry with the spinning bubble method (SBM)
<ol>
	<li>Calibrate the image-acquiring device of the tensiometer according to the vendor&#8217;s user manual. 
	NOTE: The protocol can be paused here.</li>
	<li>Fill the glass tube of the sample holder, compatible with the spinning tensiometer for the measurement, with a liquid sample and close the lid. No air bubbles should be present inside of the glass tube. 
	NOTE: It is recommended that the sample holder and the glass tube, which are provided by the instrument vendor or are compatible with the tensiometer, be used.</li>
	<li>Place the filled sample holder inside of the spinning chamber of the spinning tensiometer. 
	NOTE: The protocol can be paused here.</li>
	<li>Spin the tube at a low rate of ~500 rpm to prevent the injected bubble from migrating upward and/or attaching to the tube wall. 
	NOTE: The protocol can be paused here.</li>
	<li>Load ~2.0 &#956;L of air in the syringe. 
	NOTE: The protocol can be paused here.</li>
	<li>Insert the syringe needle piercing through the polymeric septum sealing the top of the spinning tube.</li>
	<li>Inject an air bubble of ~2.0 &#956;L into the spinning tube. 
	NOTE: The bubble volume usually remains constant, unless the bubble breaks. If the bubble breaks, it is better to start the process again.</li>
	<li>Increase the rotation frequency of the sample holder to &#957;1 so the bubble inside the glass tube is deformed such that the ratio of the horizontal bubble length (L) and the radius of the middle of the bubble (R) to reach a value of 8 or greater. 
	NOTE: If, with the available instrument, the sample tube cannot be spun at a sufficiently high rotation frequency to allow substantial bubble deformation and have a L/R ratio of 8 or greater, the general LY equation can be used to calculate DST values.</li>
	<li>Adjust the tilt angle of the measuring chamber containing the tube, if necessary, to position the sample tube oriented horizontally, to prevent bubble movement, and to help achieve gyrostatic equilibrium (hydrostatic equilibrium in a rotating fluid) for an axisymmetric shape assumed in the LY equation and algorithm used. 
	NOTE: Gyrostatic equilibrium is defined for rotating bubbles, analogously to the hydrostatic equilibrium of non-rotating bubbles, when the bubble is not moving.</li>
	<li>Measure the DST values at a predetermined time interval. The typical value is 1 s.</li>
	<li>Continue to measure the DST at a fixed rotation frequency, &#957;1, until it reaches a steady-state value (SST1) and record SST1 and the rotation frequency &#957;1 (see Figure 2).</li>
	<li>Record the bubble volume, V1 and area, A1.</li>
	<li>Alter the rotation frequency to a second rotation frequency, &#957;2, to vary the surface area.</li>
	<li>Continue to measure the DST at a fixed rotation frequency, &#957;2, until it reaches a second steady-state value (SST2) and the rotation frequency &#957;2.</li>
	<li>Record the bubble volume, V2 and area, A2. 
	NOTE: V2 should be very close to V1.</li>
	<li>Change the rotation frequency to &#957;3. 
	NOTE: Having &#957;3 exactly equal to &#957;1 is not essential.</li>
	<li>Measure DST values at a fixed rotation frequency, &#957;3, until the third steady-state value, SST3, is reached.</li>
	<li>Record &#957;3 and A3.</li>
	<li>4.19. When the three SST values differ from each other by less than 1.0 mN&#183;m-1 (or by less than 5%), their average is taken to be the &#34;EST&#34;.</li>
</ol>

<ol>
	<li>Shah, D. O., Schechter, R. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Improved oil recovery by surfactant and polymer flooding. , (1977).</li><li>Hiemenz, P. C., Rajagopalan, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Principles of Colloid and Surface Chemistry. , (1997).</li><li>Adamson, S. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Physical Chemistry of Surfaces. , (1990).</li><li>Doe, P. H., El-Emary, M., Wade, W. H., Schechter, R. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surfactants+for+producing+low+interfacial+tensions:+II.+Linear+alkylbenzenesulfonates+with+additional+alkyl+substituents.">Surfactants for producing low interfacial tensions: II. Linear alkylbenzenesulfonates with additional alkyl substituents.</a> Journal of the American Oil Chemists' Society. 55 (5), 505-512 (1978).</li><li>Chung, J., Boudouris, B. W., Franses, E. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+Tension+Behavior+of+Aqueous+Solutions+of+a+Propoxylated+Surfactant+and+Interfacial+Tension+Behavior+against+a+Crude+Oil.">Surface Tension Behavior of Aqueous Solutions of a Propoxylated Surfactant and Interfacial Tension Behavior against a Crude Oil.</a> Colloids and Surfaces A: Physicochemical and Engineering Aspects. 537, 163-172 (2018).</li><li>Miller, R., Lunkenheimer, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+determination+of+equilibrium+surface+tension+values+of+surfactant+solutions.">On the determination of equilibrium surface tension values of surfactant solutions.</a> Colloid &amp; Polymer Science. 261 (7), 585-590 (1983).</li><li>Miller, R., Lunkenheimer, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adsorption+kinetics+measurements+of+some+nonionic+surfactants.">Adsorption kinetics measurements of some nonionic surfactants.</a> Colloid &amp; Polymer Science. 264 (4), 357-361 (1986).</li><li>Lunkenheimer, K., Miller, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Properties+of+homologous+series+of+surface-chemically+pure+surfactants+at+the+water-air+interface+Part+I:+Equilibrium+properties.">Properties of homologous series of surface-chemically pure surfactants at the water-air interface Part I: Equilibrium properties.</a> Abhandlungen der Akademie der Wissenschaften der DDR, abteilung Mathematik, Naturwissenschaften, Technik. 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The authors have nothing to disclose.

The EBM and the SBM are simple and robust methods for determining tension values for air/water or oil/water interfaces at atmospheric pressure. Prerequisite information for these methods is the density of each phase, and no contact angle information is required for determining tension values9. A major limitation of the techniques is that the samples should have a low viscosity, and be single-phase or below the surfactant solubility. The two protocols, the EBM and the SBM, are used for measuring DS...

The determination of the equilibrium surface tension (EST), or the equilibrium interfacial tension (EIFT), of a given air/water or oil/water interface is a critical step for applications in a wide range of industrial areas such as detergency, enhanced oil recovery, consumer products, and pharmaceutics1,2,3,4. Such tension values should be determined indirectly from the dynamic surface tension (DST) or the dynamic interfacial tension (DIFT), because only dynamic tension values are directly measurable. Dynamic surface tension values (i.e., measuring tension values as a function of time) are determined at regular time intervals. Equilibrium tension values are deemed to be determined when the DST values are at steady state. True equilibrium surface tension values are better established when they are stable against perturbations5. Several observations of the tension relaxation after surface area compression have been previously reported by Miller and Lunkenheimer, who used &#160;two classical tensiometry methods, the Du No&#252;y ring and the &#160;Wilhelmy plate methods6,7,8. Those methods are less accurate than the ones used in this study, and those DSTs were measured every few minutes. Numerous techniques have been developed for measuring the surface tension (ST) or interfacial tension (IFT) values of interfaces, but there are only a handful of techniques that can be used to measure DST or DIFT values and allow one to apply perturbations to test the stability of the acquired steady-state tension values9. If the aqueous solution contains surfactant mixtures, and when one of the components adsorbs much faster than the others, then there may be a temporary plateau in the DST curves10. Then the presented methods may not work well in the short time-scales as for one component surfactants, but they still may work if the procedures are extended slightly to cover longer time-scales.
The protocols described here show representative data only for surface tension values of an air/aqueous solution. However, these protocols also apply for the IFT of an aqueous solution against a second liquid, such as an oil, which is immiscible with the aqueous solution and has a smaller density than that of the aqueous solution. Here, we present two robust methods that satisfy these criteria, the emerging bubble method (EBM) and the spinning bubble method (SBM). In both methods, one determines ST values that are based on bubble shapes and do not require contact angle information, which can introduce significant uncertainties and errors to the measurements. For the EBM, area perturbations are introduced by abruptly changing the volume of the bubble emerging from a syringe needle tip. For the SBM, changes in the rotation frequency of the samples are used for area perturbations. The detailed protocols are aimed to guide researchers in the field, such that they can avoid common mistakes or errors in dynamic and equilibrium tensiometry and help prevent inaccurate interpretations of the acquired data.

<table border="0" cellpadding="0" cellspacing="0" style="width:1303px;" width="1303">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:623px;">10 &#181;L, Model 1701 SN SYR, Cemented NDL, Custom gauge, length, point style</td>
			<td style="width:227px;">Hamilton</td>
			<td style="width:119px;">80008</td>
			<td style="width:335px;">gauge: 26s, needle length: 2.5 inch, point style: 2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:623px;">Anton Paar Density Meter</td>
			<td style="width:227px;">Anton Paar</td>
			<td style="width:119px;">DMA 5000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:623px;">Barnstead MicroPure Water Purification System</td>
			<td style="width:227px;">Thermo Fisher Scientific</td>
			<td style="width:119px;">50132374</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:623px;">Emerging bubble tensiometer</td>
			<td style="width:227px;">Ram&#233;-Hart Instrument Company</td>
			<td style="width:119px;">Model 790</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:623px;">Spinning bubble tensiometer</td>
			<td style="width:227px;">DataPhysics Instruments</td>
			<td style="width:119px;">SVT 20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:623px;">Triton X-100</td>
			<td style="width:227px;">Sigma-Aldrich</td>
			<td style="width:119px;">X100</td>
			<td></td>
		</tr>
	</tbody>
</table>

accurate determination of the equilibrium surface tension values with area perturbation tests

We demonstrate two robust protocols for determining the equilibrium surface tension (EST) values with area perturbation tests. The EST values should be indirectly determined from the dynamic surface tension (DST) values when surface tension (ST) values are at steady-state and stable against perturbations. The emerging bubble method (EBM) and the spinning bubble method (SBM) were chosen, because, with these methods, it is simple to introduce area perturbations while continuing dynamic tension measurements. Abrupt expansion or compression of an air bubble was used as a source of area perturbation for the EBM. For the SBM, changes in the rotation frequency of the sample solution were used to produce area perturbations. A Triton X-100 aqueous solution of a fixed concentration above its critical micelle concentration (CMC) was used as a model surfactant solution. The determined EST value of the model air/water interface from the EBM was 31.5 &#177; 0.1 mN&#183;m-1 and that from the SBM was 30.8 &#177; 0.2 mN&#183;m-1. The two protocols described in the article provide robust criteria for establishing the EST values.

Dynamic surface tension and equilibrium surface tension of an aqueous Triton X-100 solution with the EBM 
The SST values of the Triton X-100 solutions against air were measured, and their stability was tested for 5 mM aqueous solution; the CMC for this surfactant in water is 0.23 mM14. The SST1, 31.5 &#177; 0.1 mN&#183;m-1, was obtained approximately 20 s after the bubble was formed (Figure 3). After about 25 s, the surface ...

Watch this Scientific Journal Video about Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests at JoVE.com

Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests

Two protocols for determining the equilibrium surface tension (EST) values using the emerging bubble method (EBM) and the spinning bubble method (SBM) are presented for a surfactant-containing aqueous phase against air.

We demonstrate two robust protocols for determining the equilibrium surface tension (EST) values with area perturbation tests. The EST values should be ...

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7.4K Views.  Purdue University. Two protocols for determining the equilibrium surface tension (EST) values using the emerging bubble method (EBM) and the spinning bubble method (SBM) are presented for a surfactant-containing aqueous phase against air.

Video: Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests

Taking Advantage of Reduced Droplet-surface Interaction to Optimize Transport of Bioanalytes in Digital Microfluidics

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; platforms. Often, droplet motion relies on the wetting of a surface, directly associated with the application of an electric field; surface interactions, however, make motion dependent on droplet contents, limiting the breadth of applications of the technique.
Some alternatives have been presented to minimize this dependence. However, they rely on the addition of extra chemical species to the droplet or its surroundings, which could potentially interact with droplet moieties. Addressing this challenge, our group recently developed Field-DW devices to allow the transport of cells and proteins in DMF, without extra additives.
Here, the protocol for device fabrication and operation is provided, including the electronic interface for motion control. We also continue the studies with the devices, showing that multicellular, relatively large, model organisms can also be transported, arguably unaffected by the electric fields required for device operation.

The protocol for fabrication and operation of field dewetting devices (Field-DW) is described, as well as the preliminary studies of the effects of electric fields on droplet contents.

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of &#8220;lab-on-a-chip&#8221; ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 &#956;m from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,&#945;&#947;)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

We illustrate the application of 1H(15N,&#945;&#947;)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and ...

The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various characteristic length scales can result in pressure fluctuations over a wide frequency range. This property of turbulence requires sensing devices to have sufficient sensitivity over a wide range of frequencies. Furthermore, the small characteristic length scales of turbulent structures require small sensing areas and the ability to place the sensors in very close proximity to each other. The complex geometries of the solid bodies, often including large surface curvatures or discontinuities, require the probe to have the ability to be set up in very limited spaces. The development of a remote microphone probe, which is inexpensive, consistent, and repeatable, is described in the present communication. It allows for the measurement of pressure fluctuations with high spatial resolution and dynamic response over a wide range of frequencies. The probe is small enough to be placed within the interior of typical wind tunnel models. The remote microphone probe includes a small, rigid, and hollow tube that penetrates the model surface to form the sensing area. This tube is connected to a standard microphone, at some distance away from the surface, using a "T" junction. An experimental method is introduced to determine the dynamic response of the remote microphone probe. In addition, an analytical method for determining the dynamic response is described. The analytical method can be applied in the design stage to determine the dimensions and properties of the RMP components.

Here, we present a protocol to measure, with high spatial resolution, the unsteady surface pressure in turbulent flows. This method demonstrates the construction of a remote microphone probe (RMP) and the determination of its frequency-dependent, complex transfer function. An analytical determination of the dynamic response is presented and validated.

Microphones are widely applied to measure pressure fluctuations at the walls of solid bodies immersed in turbulent flows. Turbulent motions with various ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

The integration of conductive nanoparticles, such as graphene nanoplatelets, into glass fiber composite materials creates an intrinsic electrical network susceptible to strain. Here, different methods to obtain strain sensors based on the addition of graphene nanoplatelets into the epoxy matrix or as a coating on glass fabrics are proposed.

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...

Ultrasonic Fatigue Testing in the Tension-Compression Mode

Ultrasonic fatigue testing is one of a few methods which allow investigating fatigue properties in the ultra-high cycle region. The method is based on exposing the specimen to longitudinal vibrations on its resonance frequency close to 20 kHz. With use of this method, it is possible to significantly decrease the time required for the test, when compared to conventional testing devices usually working at frequencies under 200 Hz. It is also used to simulate loading of material during operation in high speed conditions, such as those experienced by components of jet engines or car turbo pumps. It is necessary to operate only in the high and ultra-high cycle region, due to the possibility of extremely high deformation rates, which can have a significant influence on the test results. Specimen shape and dimensions have to be carefully selected and calculated to fulfill the resonance condition of the ultrasonic system; thus, it is not possible to test the full components or specimens of arbitrary shape. Before each test, it is necessary to harmonize the specimen with the frequency of the ultrasonic system to compensate for deviations of the real shape from the ideal one. It is not possible to run a test until a total fracture of the specimen, since the test is automatically terminated after initiation and propagation of the crack to a certain length, when the stiffness of the system changes enough to shift the system out of the resonance frequency. This manuscript describes the process of evaluation of materials' fatigue properties at high-frequency ultrasonic fatigue loading with use of mechanical resonance at a frequency close to 20 kHz. The protocol includes a detailed description of all steps required for a correct test, including specimen design, stress calculation, harmonizing with the resonance frequency, performing the test, and final static fracture.

A protocol for ultrasonic fatigue testing in the high and ultra-high cycle region in axial tension-compression loading mode.

Ultrasonic fatigue testing is one of a few methods which allow investigating fatigue properties in the ultra-high cycle region. The method is based on ...

Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip (LOC) system. The simultaneous excitation of the fundamental frequency and its third harmonic, which is termed as dual-frequency excitation, to a pair of interdigital transducers (IDTs) could generate a new type of standing acoustic waves in a microfluidic channel. Varying the power and the phase in the dual-frequency excitation signals results in a reconfigurable field of the acoustic radiation force applied to the microparticles across the microchannel (e.g., the number and location of the pressure nodes and the microparticle concentrations at the corresponding pressure nodes). This article demonstrates that the motion time of the microparticle to only one pressure node can be reduced ~2-fold at the power ratio of the fundamental frequency greater than ~90%. In contrast, there are three pressure nodes in the microchannel if less than this threshold. Furthermore, adjusting the initial phase between the fundamental frequency and the third harmonic results in different motion rates of the three SSAW pressure nodes, as well as in the percentage of microparticles at each pressure node in the microchannel. There is a good agreement between the experimental observation and the numerical predictions. This novel excitation method can easily and non-invasively integrate into the LOC system, with a wide tenability and only a few changes to the experimental set-up.

A protocol for manipulating the microparticles in a microfluidic channel with a dual-frequency excitation is presented.

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip ...

Label-Free Imaging of Single Proteins Secreted from Living Cells via iSCAT Microscopy

We demonstrate interferometric scattering (iSCAT) microscopy, a method capable of detecting single unlabeled proteins secreted from individual living cells in real time. In this protocol, we cover the fundamental steps to realize an iSCAT microscope and complement it with additional imaging channels to monitor the viability of a cell under study. Following this, we use the method for real-time detection of single proteins as they are secreted from a living cell which we demonstrate with an immortalized B-cell line (Laz388). Necessary steps concerning the preparation of microscope and sample as well as the analysis of the recorded data are discussed. The video protocol demonstrates that iSCAT microscopy offers a straightforward method to study secretion at the single-molecule level.

We present a protocol for the real-time optical detection of single unlabeled proteins as they are secreted from living cells. This is based on interferometric scattering (iSCAT) microscopy, which can be applied to a variety of different biological systems and configurations.

We demonstrate interferometric scattering (iSCAT) microscopy, a method capable of detecting single unlabeled proteins secreted from individual living ...