Droplet Technique Research Articles

Studies on nucleation from solution of some analytically important metal chelates

The combination of precipitation from homogeneous solution and Vonnegut's droplet technique is employed in the study of nucleation from solution of bis(1,2-cyclohexanedionedioxunato)Ni(II), bis(1,2-cycloheptanedionedioximato)Ni(II), bis(1,2-cyclohexanedionedioximato)Pd(II) and bis(1,2-cycloheptanedionedioximato)-Pd(II). Values of the kinetic constant, surface energy, critical radius and the number of molecules per critical nucleus are calculated and discussed in terms of the limitations of the classical theory of nucleation when applied to crystal nucleation from aqueous solutions of electrolytes.

Nucleation in undercooled Bi-Te alloys

Thermal analysis of bulk samples of Bi-Te alloys as a function of compositional change from hypo-eutectic to hyper-eutectic composition has proved that the intermediate phase Bi7Te3 with a rhombohedral crystal structure is a very efficient nucleant for bismuth. This technique is a modification of an earlier one applied by the authors as a critical experiment to test Jackson’s objection to Sundquist and Mondolfo’s droplet technique for determination of nucleation catalysis. The modified method described in this paper is unique and eliminates another possible source of error in the determination of nucleation capacity in eutectic systems.

Nucleation experiments on some alloys with low eutectic temperatures

The effect of an addition of a small amount of solute on the nucleation of three low melting point elements, lead, tin and bismuth has been investigated by the conventional droplet technique on droplets of 350 μ dia. It was discovered that alloying with small amounts of Te caused a marked reduction in the super-cooling for the nucleation of all three elements. This was especially true in the Bi-Te system where the behavior of these alloys may be explained by the formation of the bismuth telluride phase followed by the subsequent nucleation of bismuth catalyzed by the telluride.

The supercooling dependence for nucleation of some metals on the liquid drop size

The supercooling dependence for the nucleation of pure lead, tin and bismuth on the liquid drop size was studied using the conventional droplet technique. It was found, for each element, that there exists a good linear relationship between the ratio of the mean supercooling for nucleation and the mean freezing temperature ( ΔT/T s ) and the logarithmic diameter of the liquid drop (In d). Classical theory cannot explain these results, and a new model was developed which satisfactorily accounts for all of the data.

Morphology of Relaxed and Contractile Forms of Purified Pyocin Particles

Pyocin particles produced by Pseudomonas aeruginosa have been described by Ishii et al and by Higerd et al as double hollow cylinders similar to the contractile tail protein of the T-even phages of E. coli. Induction and a 1000-fold purification of the bacteriocin was accomplished as described by Higerd et al. Specimens for electron microscopy were prepared by pseudoreplication on copper grids, uranyl-magnesium acetate negative contrast, and observed in an RCA EMU-3H at instrumental magnifications of 24,000 to 50,000 diameters. Active pyocin particles in 0.02 M magnesium chloride-tris-HCl buffer pH 7.5 , are uncontracted forms (Fig. 1), 18 by 100 mμ, containing a 5 mμ diameter core which becomes evident when the particles contract (Fig. 2). Contraction can be promoted by re-suspending the particles in different selected salts or by increasing the molarity of magnesium chloride from 0.02 M to 0.5 M. The pseudoreplication technique was preferred for this study since phosphotungstate, silicotungstate, manganous chloride, and calcium chloride caused contraction, thus excluding them in specimen droplet techniques.

Microphotometry utilizing a shrinking droplet technique

A method for spectrophotometric determinations in the submicrogram range is described. By means of a shrinking droplet technique the optical density of small volumes was increased up to 300 times. The samples were shrunk in a Teflon cup by exposure to heated air. Measurements of the diameters of the small spheres formed, when oil was added, made it possible to calculate the volume reduction. The optical density of the sample was measured in a microscope photometer—either directly with the spherical droplets still remaining in the transparent Teflon cup or, with higher precision, after transfer of the sample to a hemocytometer cuvet sealed with oil.

Crystal nucleation in polyethylene: The droplet experiment

AbstractThe droplet technique was utilized to obtain estimates of the isothermal rate of homogeneous crystal nucleation in highly supercooled melts of unfractionated linear polyethylene. In a typical experiment a cell containing a suspension of several hundred micron‐size spherical droplets of the polymer was placed on a microscope hot stage and quenched from a temperature well above the melting point of polyethylene to the desired crystallization temperature. When viewed between crossed polarizers, the droplets were initially invisible but, upon freezing, they appeared as visible birefringent spheres. The time dependence of the process was followed by time‐lapse photography. Half times (T1/2) for the freezing of those droplets remaining unfrozen by the time the crystallization temperature was attained were thus obtained at a series of undercoolings and related to the desired nucleation rate constant I (nuclei cm.−1 sec.−1) by the equation I = In 2/T1/2/v, where v is the droplet volume. The temperature dependence of I, as obtained by the above procedure, was analyzed in accord with current theories of homogeneous nucleation of chain‐folded polymer crystals. From this analysis, an estimate, 14,960 ergs3 cm.−6 was made of the quantity (%sG2%sGe), where %sG and %sGe are, respectively, the lateral and end‐surface free energies of the crystal.

Virus Particle Counts with the Negative Staining-Spray Droplet Technique.

Virus Particle Counts with the Negative Staining-Spray Droplet Technique.

The lowered drop method for the preparation of specimens of partially purified virus lysates for quantitative electron micrographic analysis

A method for estimating concentrations of physical particles in virus suspensions is presented. In this method, macrodrops of the virus suspension are dialyzed against a volatile buffer through a Formvar film which floats on the buffer. The drops are lowered onto gold grids and allowed to evaporate under controlled conditions. The number of particles per unit area of the grid is determined by direct counting in the electron microscope. Estimates of latex particle concentrations obtained in this way have been determined and compared to estimates obtained using the spray droplet technique, and using dry weight measurements. The error of estimate of the method when applied to latex suspensions and killed poliovirus vaccine preparations has been investigated. The method has been used to estimate particle concentrations as low as 5 × 10 7 particles per milliliter in formaldehyde-killed poliovirus vaccines.

A method for staining virus particles and identifying their nucleic acid type in the electron microscope

A method is described for staining virus particles from fluid suspensions with uranyl acetate and phosphotungstic acid so as to enhance contrast for electron microscopy and yet maintain good structural detail. The method requires 100–1000 times less virus concentration than the spray droplet technique. Staining can be substituted for metal shadowing in the agar pseudoreplication technique and can be used for counting virus particles. Treatment with uranyl acetate reduces structural distortion of herpes virus particles on drying. Substantial differences have been noted between the appearance of RNA- and DNA-containing animal viruses after staining. Under the same conditions the cores of DNA-type viruses stain intensely with uranyl acetate, whereas the RNA-type viruses do not. The combined use of specific nuclease, negative staining (phosphotungstate), and positive staining (uranyl acetate) is suggested as a means of differentiating RNA from DNA viruses by microscopy.

Kinetics of Crystal Nucleation in Some Normal Alkane Liquids

The crystal nucleation behavior of some normal alkane liquids—C16H34, C17H36, C18H38, C24H50, and C32H66—was investigated by the droplet technique. The relative undercoolings, 1—λ, (i.e., the actual undercooling in units of the absolute melting temperature) at which the crystal nucleation frequency in these liquids becomes 107 cm—3 sec—1 are 0.037 to 0.043, and almost an order of magnitude smaller than those for simple molecular substances. Crystal nucleation frequencies in droplets of C17H36 and C18H38 samples of very high purity were determined quantitatively. The frequencies were proportional to droplet volume and increased by a factor of 5000 to 8000 per 1° decrease in temperature. The results are satisfactorily described by the classical nucleation theory with values of the crystal nucleus - liquid interfacial tension σ, 7.2 erg-cm—2 for C17H36 and 9.6 erg-cm—2 for C18H38. The experimental pre-exponential factors in the nucleation expression agree with the theoretical values to well within the experimental uncertainty. The small relative undercooling and the weak glass-forming tendency in the high-molecular weight n-alkane liquids is satisfactorily explained by the hypothesis of nucleation of small segments of molecules.

An improved spray droplet technique for quantitative electron microscopy

Improvements are described to the spray droplet method of Backus and Williams for direct particle counting by electron microscopy. Decreasing the droplet size makes it possible to use much higher magnifications without sacrificing the ability to photograph each droplet trace entire on one frame, and droplets of the desired size are sorted and collected in a reproducible manner in a cascade impactor. The technique is also useful for studying the state of aggregation of a suspension. Some factors affecting the accuracy of measurement of the polystyrene latex standard particles are discussed. Different methods yield different values, and there is at present no valid basis for selecting the best estimate.