Name: accurate determination of the equilibrium surface tension values with area perturbation tests
Uploaded: 2019-08-30T14:00:00
Description: Watch this Scientific Journal Video about Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests at JoVE.com

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...

<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. (1), 113 (1986).</li><li>Franses, E. I., Basaran, O. A., Chang, C. -. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Techniques+to+measure+dynamic+surface+tension.">Techniques to measure dynamic surface tension.</a> Current Opinion in Colloid &amp; Interface Science. 1 (2), 296-303 (1996).</li><li>Hua, X. Y., Rosen, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamic+surface+tension+of+aqueous+surfactant+solutions+1.+basic+parameters.">Dynamic surface tension of aqueous surfactant solutions 1. basic parameters.</a> Journal of Colloid and Interface Science. 124 (2), 652-659 (1988).</li><li>Rotenberg, Y., Boruvka, L., Neumann, A. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+surface+tension+and+contact+angle+from+the+shapes+of+axisymmetric+fluid+interfaces.">Determination of surface tension and contact angle from the shapes of axisymmetric fluid interfaces.</a> Journal of Colloid And Interface Science. 93 (1), 169-183 (1983).</li><li>Boyce, J. F., Schurch, S., Rotenberg, Y., Neumann, A. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Measurement+of+Surface+and+Interfacial+Tension+by+the+Axisymmetric+Drop+Technique.">The Measurement of Surface and Interfacial Tension by the Axisymmetric Drop Technique.</a> Colloids and Surfaces. 9, 307-317 (1984).</li><li>Vonnegut, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rotating+bubble+method+for+the+determination+of+surface+and+interfacial+tensions.">Rotating bubble method for the determination of surface and interfacial tensions.</a> Review of Scientific Instruments. 13 (1), 6-9 (1942).</li><li>Lin, S. -. Y., Mckeigue, K., Maldarelli, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diffusion-controlled+Surfactant+Adsorption+Studied+by+Pendant+Drop+Digitization.">Diffusion-controlled Surfactant Adsorption Studied by Pendant Drop Digitization.</a> AIChE Journal. 36 (12), 1785-1795 (1990).</li><li>Sheng, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modern+chemical+enhanced+oil+recovery:+theory+and+practice.">Modern chemical enhanced oil recovery: theory and practice.</a> Gulf Professional Publishing. , (2010).</li><li>Moody, C. A., Field, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perfluorinated+surfactants+and+the+environmental+implications+of+their+use+in+fire-fighting+foams.">Perfluorinated surfactants and the environmental implications of their use in fire-fighting foams.</a> Environmental Science and Technology. 34 (18), 3864-3870 (2000).</li></ol>

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">
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	<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 ...

These protocols can be incorporated into tension measurement protocols. For example, they can be used to measure equilibrium interfacial tension between water and oil phases. Each of the two protocols in this video can be used to obtain robust and reliable equilibrium surface tension values.These values can be established after testing their stability against area perturbations. To begin, prepare the tensiometer and sample as described in the text protocol. Then, select an inverted stainless steel needle based on the estimated surface tension values and place it at the tip of the dispensing device.Next, load 40 milliliters of liquid sample into a quartz cell. Place the cell on top of the sample platform. Adjust the height of the inverted needle such that the tip of the needle is at least 20 millimeters below the surface of the liquid sample.Inject one milliliter of air through the submerged inverted needle to remove impurities that could possibly be present on the tip of the syringe and to improve the surface chemical purity of the air/liquid interface. Then, experimentally determine the initial bubble volume. Dispense the computed initial bubble volume to form a bubble at the tip of the inverted syringe.Make sure that the bubble is in hydrostatic equilibrium such that the bubble does not move. Measure the dynamic surface tension based on the shape of the produced air bubble at the tip of the needle. Every second, calculate the surface tension based on the axisymmetric drop shape analysis method of the Laplace-Young equation.Compare the actual shape of the bubble with a calculated shape. If the two shapes overlap, then the equilibrium Laplace-Young equation is valid. This inference is completely valid when the bubble stops moving and the surface tension stops changing.Measure the surface tension as a function of time until the first steady-state surface tension is achieved. The steady state surface tension is defined as a plane of value beyond which the surface tension changes by less than one millinewton per meter or by less than 5%in several consecutive dynamic surface tension measurements. When steady state surface tension is achieved, record the bubble volume and the surface area.Then, decrease the bubble volume by removing one microliter of air and record the new bubble volume and area. Continue measuring the dynamic surface tension in the areas until the dynamic surface tension reaches the second steady state surface tension. Next, expand the bubble volume by injecting one microliter of air so that the volume and area are similar to the initial values.Continue measuring the dynamic surface tension values until a third steady state surface tension is reached. If the three steady state surface tension values differ from each other by less than one millinewton per meter, or by less than 5%then we define their average as the equilibrium surface tension. Place a filled sample holder inside of the spinning chamber of the spinning tensiometer and then spin the tube at 500 RPM.This should prevent the injected bubble from migrating upward or attaching to the tube wall. Next, load two microliters of air into the syringe. Insert the needle of the syringe through the rubber septum, which seals the spinning tube and inject a two microliter air bubble into the spinning tube.Increase the rotation frequency of the sample tube so that the bubble moves closer to the axis of rotation and deforms more because of the increased centrifugal forces. Continue to speed it up until the ratio of the horizontal bubble's length to its radius at the middle of the bubble is eight or greater. Then adjust the tilt angle of the measuring chamber to position the sample tube horizontally.This will prevent bubble movement and to help achieve gyrostatic equilibrium for an axisymmetric shape assumed in the Laplace-Young equation and algorithm used. Now, measure and record the dynamic surface tension values at one second intervals. Continue at a fixed rotation frequency until the surface tension reaches a steady state value.Also, record the bubble volume and area. Once recorded, alter the rotation frequency to a second rotation frequency to vary the surface area. Measure the dynamic surface tension at a fixed rotation frequency once it reaches a second steady state value at the new frequency.At this point, also record the new bubble volume and area. Next, change the rotation frequency so that it is close to the original value. Measure the dynamic surface tension values at this fixed rotation frequency until the third steady state value is reached.Again, record the new bubble volume and area. Using the emerging bubble method, the steady state surface tension values of a five millimolar solution of Triton X-100 were measured against air. This concentration is above the critical micelle concentration for this surfactant in water.The steady state surface tension of 31.5 millinewtons per meter was obtained approximately 20 seconds after the bubble was formed. After about 25 seconds, the volume and area of the bubble were reduced and the dynamic surface tension dropped to 31 and within one second and increased back to 31.5, marking the steady state surface tension number two. After about 50 seconds, the volume and the area of the bubble were increased abruptly and the dynamic surface tension value changed little, and hence, the steady state surface tension number three was determined to be 31.5 millinewtons per meter, as well.The three SST values were all the same, therefore, the equilibrium surface tension was determined to be 31.5 millinewtons per meter. Using the spinning bubble method, steady state surface tension one was found to be 30.9 millinewtons per meter, steady state surface tension two was found to be 30.6, and steady state surface tension three and the equilibrium surface tension was found to be 30.8. The two methods had a 2.2%difference in the equilibrium surface tension values when measuring five millimolar Triton X-100.This was probably due to certain systematic errors. The most important thing to remember for the emerging bubble method is to maintain conditions close to the hydrostatic equilibriums. For the spinning bubble method, be sure to apply the correct equation.The methods described in this video can also be applied to determine equilibrium interfacial tension values between water and oil phases. These methods provide a more reliable way of calculating how much a surfactant absorbs when in equilibrium at the air-water interface. It can also be used to determine the extent of surfactant aggregation in solution called micellization.

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

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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 ...