A method for determining surface tension, viscosity, and elasticity of gels via ultrasonic levitation of gel drops.

A method for obtaining the elasticity, surface tension, and viscosity of ultrasonically levitated gel drops is presented. The drops examined were made of agarose, a hydrogel. In contrast to previous studies where fluid properties are obtained using ultrasonic levitation of a liquid drop, herein the material studied was a gel which has a significant elasticity. The work presented herein is significant in that gels are of growing importance in biomedical applications and exhibit behaviors partially determined by their elasticities and surface tensions. Obtaining surface tension for these substances is important but challenging since measuring this quantity using the standard Wilhelmy plate or DuNuoy ring methods is not possible due to breakage of the gel. The experiments were conducted on agarose gels having elasticities ranging from 12.2 to 200.3 Pa. A method is described for obtaining elasticity, surface tension, and viscosity, and the method is experimentally demonstrated for surface tension and viscosity. For the range of elasticities explored, the measured surface tension ranged from 0.1 to 0.3 N/m, and the viscosity ranged from 0.0084 to 0.0204 Pa s. The measurements of surface tension are, to the authors' knowledge, the first obtained of a gel using ultrasonic levitation.

Experimental observation of Faraday waves in soft gels.

We report the experimental observation of Faraday waves on soft gels. These were obtained using agarose in a mechanically vibrated cylindrical container. Low driving frequencies induce subharmonic standing waves with spatial structure that conforms to the geometry of the container. We report the experimental observation of the first 15 resonant Faraday wave modes that can be defined by the mode number (n,ℓ) pair. We also characterize the shape of the instability tongue and show the complex dependence upon material properties can be understood as an elastocapillary effect.

Elastocapillary Transition in Gel Drop Oscillations.

We report experimental observations of surface oscillations in an ultrasoft agarose gel drop. Ultrasonic levitation is used to excite shape oscillations in the gel drop and we report the natural frequency of the drop as it depends upon a nondimensional elastocapillary number, which we define as the ratio of the elastocapillary length to drop size. Our experiments span a wide range of experimental parameters and we recover the appropriate scaling laws in the elastic and capillary wave limits. The crossover between these two limits is observed and agrees well with a proposed frequency relationship.

Extracting the surface tension of soft gels from elastocapillary wave behavior.

Mechanically-excited waves appear as surface patterns on soft agarose gels. We experimentally quantify the dispersion relationship for these waves over a range of shear modulus in the transition zone where the surface energy (capillarity) is comparable to the elastic energy of the solid. Rayleigh waves and capillary-gravity waves are recovered as limiting cases. Gravitational forces appear as a pre-stress through the self-weight of the gel and are important. We show the experimental data fits well to a proposed dispersion relationship which differs from that typically used in studies of capillary to elastic wave crossover. We use this combined theoretical and experimental analysis to develop a new technique for measuring the surface tension of soft materials, which has been historically difficult to measure directly.

Demisting using an ultrasonic standing wave field.

Removing drops from an air flow can be challenging, particularly, for small drops. Herein a method for demisting is presented that employs ultrasonics to force small drops to combine. Specifically, a cylindrical ultrasonic standing wave field is established in a tube, forming pressure nodes that take the form of cylinders located within the tube and having the same axis as the tube. Droplets are driven toward these pressure nodes by the acoustic radiation force, forcing smaller drops to combine to form larger drops, which eventually fall due to gravity, thereby demisting the flow. Experiments presented herein show that, for the setup employed, this method can remove a fraction of drops that approaches 0.8 and that the improvement due to ultrasonics, compared to the case without ultrasonics, is as large as 2.8. The effect of air flow rate and power is investigated.

The directional sensitivity of the acoustic radiation force to particle diameter.

When viscous corrections to the inviscid acoustic radiation force theory are implemented and applied to a standing wave field, the direction of the acoustic radiation force on particles varies from theory to theory. Specifically, some theories predict that the direction of the force depends on the particle diameter, while others reveal that the direction of the force is independent of particle diameter. The present study is an experimental investigation of the direction of the acoustic radiation force which suggests that particle diameter does affect the direction. Experiments were conducted in air using an ultrasonic standing wave field with a nominal frequency of 30 kHz. Smoke particles and fine water droplets having a range of diameters were flowed into the region of a standing wave field. The direction of the acoustic radiation force was determined by observing whether the particles accumulated in the nodes or the anti-nodes of the standing wave. Results show a change in the direction of the acoustic radiation force at a particle diameter of 0.3±0.1 µm, which corresponds to a particle diameter to acoustic-boundary-layer thickness ratio of 0.023±0.008.

A method for the temperature calibration of an infrared camera using water as a radiative source.

Presented here is an effective low-cost method for the temperature calibration of infrared cameras, for applications in the 0-100 degrees C range. The calibration of image gray level intensity to temperature is achieved by imaging an upwelling flow of water, the temperature of which is measured with a thermistor probe. The upwelling flow is created by a diffuser located below the water surface of a constant temperature water bath. The thermistor probe is kept immediately below the surface, and the distance from the diffuser outlet to the surface is adjusted so that the deformation of the water surface on account of the flow is small, yet the difference between the surface temperature seen by the camera and the bulk temperature measured by the thermistor is also small. The benefit of this method compared to typical calibration procedures is that, without sacrificing the quality of the calibration, relatively expensive commercial blackbodies are replaced by water as the radiative source (epsilon approximately 0.98 for the wavelengths considered here). A heat transfer analysis is provided, which improves the accuracy of the calibration method and also provides the user with guidance to further increases in accuracy of the method.

A method for removing surfactants from an air/water interface.

The study of heat, mass, and momentum transport across an air/water interface is an aspect of fluid mechanics where the presence of surfactant monolayers can play a significant role. Experimental studies of air/water transport typically require a method for cleaning the air/water interface so that it is free from any contaminating surfactant monolayer. This may be for the sake of running an experiment under clean surface conditions, or to clean the surface prior to deposition of a known surfactant. Herein a method is described for maintaining a clean air/water interface during conditions of finite air flow over the water surface. The unique aspect of this method is its ability to maintain clean surfaces while experiments are conducted.

Edge detection methods applied to the analysis of spherical raindrop images.

Optical imaging of raindrops provides important information on the statistical distribution of raindrop size and raindrop shape. These distributions are critical for extracting rainfall rates from both dual- and single-polarization radar signals. A large number of raindrop images are required to obtain these statistics, necessitating automatic processing of the imagery. The accuracy of the measured drop size depends critically on the characteristics of the digital image processing algorithm used to identify and size the drop. Additionally, the algorithm partially determines the effective depth of field of the camera/image processing system. Because a large number of drop images are required to obtain accurate statistics, a large depth of field is needed, which tends to increase errors in drop size measurement. This trade-off between accuracy and depth of field (dof) is also affected by the algorithm used to identify the drop outline. In this paper, eight edge detection algorithms are investigated and compared to determine which is best suited for accurately extracting the drop outline and measuring the diameter of an imaged raindrop while maintaining a relatively large depth of field. The algorithm which overall gave the largest dof along with the most accurate estimate of the size of the drop was the Hueckel algorithm [J. Assoc. Comput. Mach. 20, 634 (1973)].

Infrared imaging of a solid phase surfactant monolayer.

A new method for visualizing solid phase surfactant monolayers is presented. This method utilizes infrared (IR) imaging of the surface of a warm subphase covered by the monolayer. When the subphase is deep, natural convection occurs, resulting in a complex surface temperature field that is easily visualized using an IR camera. The presence of a surfactant monolayer changes the hydrodynamic boundary condition at the interface, dramatically altering the surface temperature field, and permitting the differentiation of surfactant-covered and surfactant-free regions. In this work, solid phase monolayers are imaged using this IR method. Fractures in the monolayer are dramatically visualized because of the sudden elimination of surfactant in the region opened up by the crack. The method is demonstrated in a wind/water tunnel, where a stearic acid monolayer is deposited and a crack is created through shear on the surfactant surface, created by suddenly increasing the velocity of the air over the water.

Use of thresholding algorithms in the processing of raindrop imagery.

Several thresholding algorithms are applied to the analysis of drop images, and their performance is compared. Images were obtained by use of a digital camera setup in which drops were illuminated from behind, resulting in an image of the drop silhouette. Each algorithm was evaluated based on the accuracy of the drop diameter obtained from the thresholded image and on the size of the depth of field. Because of the difficulty associated with creating drops that have a known diameter, solid spheres composed of a glass with an index of refraction close to that of water were used in computing the depth of field and in determining the accuracy of measured diameter. The application of this study is to the automatic measurement of raindrops and images were obtained during several storms. With each thresholding algorithm this raindrop imagery was used to compute the probability density function of drop diameter, and the rain rate. The performance of each thresholding algorithm was quantified by comparison of these measurements with simultaneous measurements obtained by use of a Joss-Waldvogel disdrometer.

Determination of tar, nicotine, and carbon monoxide yields in the mainstream smoke of selected international cigarettes.

OBJECTIVE: Survey of nicotine, tar, and carbon monoxide (CO) smoke deliveries from 77 cigarette brands purchased in 35 countries was conducted using a standardised machine smoking method. The goal of this study was to determine regional variations and differences in the tar, nicotine, and CO smoke yields of a cigarette brand manufactured by a leading transnational corporation and of non-US locally popular cigarette brands. DESIGN: The majority of the cigarettes were purchased in each of the participating countries by delegate members of the World Health Organization and forwarded to the Centers for Disease Control and Prevention for analysis. Smoke deliveries were determined using a standardised smoking machine method and subsequent gravimetric and gas chromatography analysis. RESULTS: The smoke deliveries varied widely. Mainstream smoke deliveries varied from 6.8 to 21.6 mg tar/cigarette, 0.5 to 1.6 mg nicotine/cigarette, and 5.9 to 17.4 mg CO/cigarette. In addition to the smoke deliveries, the cigarettes were examined to determine physical parameters such as filter composition, length, and ventilation levels. CONCLUSION: Analysis of the smoke deliveries suggested that cigarettes from the Eastern Mediterranean, Southeast Asia, and Western Pacific WHO regions tended to have higher tar, nicotine, and CO smoke deliveries than did brands from the European, American, or African WHO regions surveyed.

Single-camera method to determine the optical axis position of ellipsoidal drops.

The sizing of droplets by optical imaging typically requires a small depth of field so that variations in the magnification ratio are minimized. However, if the location of the drop along the optical axis can be determined, a variable magnification ratio can be imposed on each imaged drop, and the depth of field can be increased. Previous research suggested that droplet location can be determined with a characteristic of droplet images that is obtained when the droplet is illuminated from behind. In this prior research, the method was demonstrated with spherical glass objects to simulate raindrops. Raindrop are known to deviate significantly from a spherical shape, especially when the drop size is large. We demonstrate the ability to locate the position of objects that deviate from sphericity. Deformed water drops and glass ellipsoids are tested, along with glass spheres. The role of refractive index is also discussed.

Internal reflection beneath capillary water waves: a method for measuring wave slope.

Ray-tracing simulations were performed to explore total internal reflection of light rays beneath capillary water waves. A vertically oriented light ray, scanned laterally below the wave surface, is mapped to a position that oscillates at a frequency f. It was found that f varies over 2 orders of magnitude as the dimensionless wave height a/lambda varies from 0.34 to 0.73. This presents a possible frequency domain method for wave slope measurement in wave tank experiments. A linear relationship between the maximum displacement of the mapped ray and a/lambda is also demonstrated for a/lambda between 0.54 and 0.73, presenting a second wave slope measurement approach. The consequences of partial internal reflection are considered.