Higher Gel Concentrations Research Articles

Experimental simulation of proppant permeability in hydraulic fracturing at extended time under bottom-hole conditions

During hydraulic fracturing treatment, huge quantities of gel are pumped into the formation to initiate the fracture, maintain it open and transport the proppant. The fracture dimensionless conductivity (Fcd) is a key parameter to optimize the fracturing design, to estimate the productivity Index (PI) and the folds of increase (FOI). However, these parameters are affected by the gel residues which decrease the fracture conductivity; thus, the proppant cleanup is a very important step to avoid additional damage caused by fracturing fluid due to high gel concentration and the extended time of gel staying in the fracture before cleanup. Throughout the life of Hassi Messaoud, Algeria field, hydraulic fracturing technique has been aggressively used mainly in four producing formations in the Cambrian, with hard formation characteristics, an average permeability range and low reservoir pressure (0.15–0.45 psi/ft) and high stress value between 6000 and 9000 psi. In this paper, an experimental simulation is applied using a self-made cell to determine the effect of different parameters on the fracture conductivity under various bottom-hole conditions where different variables were used: effect of Proppant type, guar gel concentration, temperature, breaker concentration and closure pressure at extended time. An important drop in fracture conductivity was observed varied between 10 and 80% under stresses at interval of 2000 psi and 8000 psi, gel concentration up to 200 lb/1000 gal at extended time and temperature.

Printability and bio-functionality of a shear thinning methacrylated xanthan–gelatin composite bioink

3D bioprinting is a recent technique that can create complex cell seeded scaffolds and therefore holds great promise to revolutionize the biomedical sector by combining materials and structures that more closely mimic the 3D cell environment in tissues. The most commonly used biomaterials for printing are hydrogels, however, many of the hydrogels used still present issues of printability, stability, or poor cell-material interactions. We propose that bioinks with intrinsic self-assembling and shear thinning properties, such as xanthan gum, can be methacrylated (XGMA) and combined with a bio-functional material such as gelatin methacryloyl (GelMa) to create a stable, cell-interactive bioink with improved properties for 3D bioprinting. These biomaterials have reduced viscosity under high shear and recover their viscosity rapidly after the shear is removed, retaining their shape, which translates to easier extrusion whilst maintaining accurate fidelity after printing. This was confirmed in printing studies, with measured normalized strand widths of 1.2 obtained for high gel concentrations (5+5 % XGMA-GelMA). Furthermore, the introduction of a secondary photo-cross-linking method allowed tuning of the mechanical properties of the hydrogel with stiffness between 15 and 30 kPa, as well as improving the stability of the hydrogel with retention of 75 % of its mass after 90 d. The hydrogel was shown to be biocompatible and bio-active with 97 % cell viability, and cell spreading after 7 d of culture for low gel concentrations (3+3 % XGMA-GelMA). Shear stresses were relatively low while printing (1 kPa) as a result of the shear thinning property of the material, which supported cell viability during extrusion. Finally, printed hydrogels retained high cell viability for lower gel concentrations, and showed improved cell viability for more concentrated hydrogels when compared to cells cultured in bulk hydrogels, presumably due to improved nutrient/oxygen diffusion and cell migration. In conclusion, stability and formulation of a XGMA-GelMA shear thinning composite hydrogel has been optimized to create a bio-functional bioink, with improved printability, and in vitro culture stability via secondary photo-induced cross-linking, making this composite a promising bioink for 3D bioprinting.

Gel-Modulated Growth of High-Quality Zeolite Membranes

Trade-off between thickness (and thus gas permeance) and quality (and thus selectivity) of zeolite membranes significantly restricts their wide application. It is challenging to maintain the membrane thickness while minimizing nonselective defects in the selective membrane layer. Currently, continuous change of the synthesis gel concentration during membrane synthesis, instead of high gel concentration for membrane "skeleton" growth and low gel concentration for slow crystal growth to merge "skeleton" crystals, usually leads to thick membranes to compensate for the low selectivity. In this work, we report a gel-modulated synthesis approach to engineer the zeolite membrane synthesis. The gel concentration was suddenly reduced by a quenching process in the middle of the membrane synthesis, leading to reduced nuclei formation and crystal growth rate in the following synthesis process. Membrane quality was significantly improved without increasing membrane thickness, leading to a great increase in membrane selectivity but without sacrifice of gas permeance. Moreover, as an example, highly reproducible SAPO-34 membranes were successfully prepared by the gel-modulated growth method. We expect this novel synthesis strategy might be a viable and economic way of growing thin and high-quality zeolite membranes.

Rotational Diffusion of Macromolecules and Nanoparticles Modeled as Non-Overlapping Bead Arrays in an Effective Medium

In this work, the retarding influence of a gel on the rotational motion of a macromolecule is investigated within the framework of the Effective Medium (EM) model. This is an extension of an earlier study that considered the effect of a gel on the translational motion of a macromolecule [Allison, S. et al. J. Phys. Chem. B 2008, 112, 5858-5866]. The macromolecule is modeled as an array of non-overlapping spherical beads with no restriction placed on their size or configuration. Specific applications include the rotational motion of right circular cylinders and wormlike chains modeled as strings of identical touching beads. The procedure is then used to examine the electric birefringence decay of a 622 base pair DNA fragment in an agarose gel. At low gel concentration (M ≤ 0.010 gm/mL), good agreement between theory and experiment is achieved if the persistence length of DNA is taken to be 65 nm and the gel fiber radius of agarose is taken to be 2.5 nm. At higher gel concentrations, the EM model substantially underestimates the rotational relaxation time of DNA and this can be attributed to the onset of direct interactions that become significant when the effective particle size becomes comparable to the mean gel fiber spacing.

Rheological Properties of Starch and Whey Protein Isolate Gels

The paste viscosity of starches Amioca (~0.5% amylose), native corn (~27% amylose), Hylon VII (~70% amylose), tapioca (~19% amylose), and their whey protein isolate (WPI; 50/50) mixtures at varying solid concentration of 2.5, 5, 10, 15 and 20% were determined. At higher gel concentration (20%) during the cooling cycle, WPI paste measured by rapid visco-analyser (RVA) showed a tremendous increase in viscosity over that of the micro visco-amylograph (MVA), 5784 and 184 cP, respectively. RVA gels had a more uniform network of coarse particles (~250—500 nm) than the fine structures (~50 nm) in those of the MVA. Inclusion of WPI reduced the paste viscosity of all starches by at least 50%, except for Hylon VII at 20% solid concentration. The strength of Hylon VII and corn starch gels was reduced by WPI. In contrast, gels of WPI/tapioca ( G' = 45.4 Pa) and WPI/amioca ( G' = 18.3 Pa) had similar rheological properties as their pure starch control ( G' = 47.4 Pa and G' = 15.3 Pa for Tapioca and Amioca, respectively).

Dynamics of Cellulose Whiskers Spatially Trapped in Agarose Hydrogels

The motion of cellulose whiskers trapped in agarose hydrogels is measured by depolarized dynamic light scattering (DDLS) under conditions of optical heterodyning, in which the rigid polymer network provides a static depolarized reference signal. At low concentration in free suspension, the rotational relaxation rate ϑ of the whiskers is decoupled from the osmotic properties of the suspension. When incorporated inside a gel, however, as the agarose concentration increases, ϑ first increases beyond that in the free suspension, reflecting a reduced amplitude of rotational fluctuations of the whiskers due to the repulsive interactions with their surroundings. At the same time, the dynamic light scattering Rayleigh ratio RVH obtained from DDLS decreases with increasing gel concentration. At high gel concentrations, however, ϑ tends to decrease due to the steric hindrance of the network. Except at the lowest whisker concentrations, ϑ is governed by the same fluctuations as those associated with RVH.

Effects of purified poloxamer 407 gel on vascular occlusion and the coronary endothelium☆

Vascular occlusion during off-pump coronary surgery often results in sub-optimal visualization and endothelial damage of the target vessel. We have previously reported the safety of purified poloxamer 407, a gel with reverse thermosensitive properties, in a model of off-pump coronary occlusion. The aim of this study was to evaluate different gel concentrations and their effects on coronary occlusion time, myocardial contractility, endothelial function, and markers of myocardial injury and apoptosis. Yorkshire swine (30-35 kg) underwent sternotomy and mid-LAD occlusion using either microvascular clamps (n=6) or varying quantities of three different concentrations (20%; n=6, 22.5%; n=3, and 25%; n=3) of purified poloxamer 407 gel. Distal LAD flow, left ventricular pressure, and in vitro microvascular reactivity were assessed. Molecular markers of myocardial injury and apoptosis were assessed using Western blotting. Duration of ischemia correlated significantly with the amount of injected gel for the 20% and 22.5% formulations (Spearman r=0.64, p=0.02 and r=0.85, p=0.03, respectively) but not for the 25% gel (r=0.22, p=0.66). There were no significant differences in LV contractility (maximum+dp/dt) (p=0.86) as well as LAD flow (p=0.25) during reperfusion in the four groups. Microvessel relaxation to adenosine diphosphate in the ischemic territory was impaired with higher concentrations of the gel, 22.5% (-14.8+/-7.5% vs control at 10(-5)mol/L, p<0.05) and 25% gel (-24.0+/-7.6% vs control at 10(-5)mol/L, p<0.001) but not with the 20% gel. Endothelium-independent relaxation to sodium nitroprusside was preserved in all groups. Cleaved poly(ADP-ribose) polymerase, a marker of myocardial injury and apoptosis, was elevated in the ischemic myocardium in all groups (p=0.02 vs non-ischemic myocardium) with no significant differences between the groups (p=0.70). Phosphorylation of Bad, an anti-apoptotic protein, was significantly reduced in the ischemic territory (p=0.04). No changes were observed in other markers of myocardial apoptosis. Purified poloxamer 407 gel is effective for temporary coronary vascular occlusion. Coronary artery occlusion time is related to gel concentration and the quantity of gel injected. The higher gel concentrations may cause endothelial dysfunction of the distal vasculature and therefore should be avoided.

Moving boundaries in sodium dodecyl sulfate‐polyacrylamide gel electrophoresis

In sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), moving boundaries I-IV (Table 1) form. They migrate isotachophoretically at displacement rates that increase in the order of I to II to III. Moving boundaries IV and V comprising Pyronin-SDS as a leading constituent are retarded at high gel concentrations in comparison with the isotachophoretically migrating species. Since analytes depending on their net mobilities stack within any of those moving boundaries, previous R(f) values and Ferguson plots may have to be revised.

Experimental pattern transitions in a Liesegang system

We experimentally found various patterns of precipitation in a lead iodide Liesegang system: periodic precipitation, continuous tree-like crystal aggregates and periodic tree-like crystal aggregates. Gel and reactant concentrations are two major controlling factors. At high gel concentrations, Liesegang band forms, whereas at low gel concentrations, tree-like crystal aggregates similar to diffusion-limited aggregation (DLA) develop in the entire system. At an intermediate gel concentration, Liesegang band and tree-like crystal aggregates coexist. At lower reactant concentration, the periodic tree-like crystal aggregates tends to form. A simple scaling argument that the kinetics of bulk and surface nucleation plays an important role, is presented to understand the effect of gel concentration on pattern transition.

Preparation of hydroxyapatite-gelatin nanocomposite

A nanocomposite of gelatin[GEL]-hydroxyapatite[HAp] was prepared using the biomimetic process. The hydroxyapatite nanocrystals were precipitated in aqueous solution of gelatin at pH 8 and 38°C. The chemical bonding between calcium ions of HAp and carboxyl ions of GEL molecules induced a red-shift of the 1339 cm −1 band of GEL in FT-IR analysis. TEM images and electron diffraction patterns for the nanocomposite strongly indicate the self-organization of HAp nanocrystals along the GEL fibrils. Electron diffraction for the nanocomposites showed a strong preferred orientation of the (0 0 2) plane in HAp nanocrystals. The development of HAp nanocrystals in an aqueous GEL solution was highly influenced by the concentration ratio of GEL to HAp. A higher concentration of GEL induced the formation of tiny crystallites (4 nm×9 nm size), while a lower concentration of GEL contributed to the development of bigger crystallites (30 nm×70 nm size). From DT/TGA data, the HAp-GEL nanocomposite showed typically three exothermic temperatures. The increase in decomposition temperatures indicates the formation of a primary chemical bond between HAp and GEL. The higher concentration of GEL supplies abundant reaction sites containing groups such as carboxyl, which can bind with calcium ions. The abundant supply of reaction sites leads to a very large number of HAp nuclei. However, the formation of a large number of nuclei depletes the concentration of calcium ions that available for growth to the extent that the nuclei cannot grow very large. This in turn will lead to the creation of a large number of tiny nanocrystals at this higher GEL concentration.

DNA electrophoresis in agarose gels: effects of field and gel concentration on the exponential dependence of reciprocal mobility on DNA length.

Electrophoretic mobilities of DNA molecules ranging in length from 200 to 48 502 base pairs (bp) were measured in agarose gels with concentrations T = 0.5% to 1.3% at electric fields from E = 0.71 to 5.0 V/cm. This broad data set determines a range of conditions over which the new interpolation equation nu(L) = (beta+alpha(1+exp(-L/gamma))(-1) can be used to relate mobility to length with high accuracy. Mobility data were fit with chi(2) > 0.999 for all gel concentrations and fields ranging from 2.5 to 5 V/cm, and for lower fields at low gel concentrations. Analyses using so-called reptation plots (Rousseau, J., Drouin, G., Slater, G. W., Phys. Rev. Lett. 1997, 79, 1945-1948) indicate that this simple exponential relation is obeyed well when there is a smooth transition from the Ogston sieving regime to the reptation regime with increasing DNA length. Deviations from this equation occur when DNA migration is hindered, apparently by entropic-trapping, which is favored at low fields and high gel concentrations in the ranges examined.

Gel-Shift Assays: Migrative Dissociation of a RecA−Oligonucleotide Complex during Electrophoresis in Hydroxyethylated Agarose Gels

We investigate the dynamics of a protein-DNA complex under gel-shift assay conditions, in particular how the degree of gel confinement influences the complex stability during the electrophoretic analysis. The fiber-like complex between RecA proteins and a single-stranded oligonucleotide has been studied in hydroxyethylated agarose gels with the average pore radius being between 1 and 5 times larger than the radius of the rodshaped complex. The confining effect of the gel matrix slows down the dissociation of the complex, but in addition, migrative interactions (collisions) with the gel network during the electrophoresis perturbs the complex in two ways. At low gel concentrations, the protein-DNA complex is dismantled into free RecA and oligonucleotide with a half-time of the complex that decreases with increasing field strength in accordance with a migrative mechanism. At a given field strength, the half-time increases with increasing gel concentration despite more frequent interactions, probably because of a counteracting stabilizing cage effect from the gel. At high gel concentrations, a second type of perturbation is reflected in an increasing complex velocity over time, probably caused by trains of end-to-end attached fibers being broken up by interactions with the tight gels.

Reevaluation of the Electrophoretic Migration Behavior of Soluble Globular Proteins in the Native and Detergent-Denatured States in Polyacrylamide Gels

Although sodium dodecyl sulfate (SDS)–polyacrylamide gel electrophoresis is widely used for estimating molecular masses of proteins, considerable uncertainty still exists both about the structure of SDS–protein complexes and about their mechanism of electrophoretic migration. In this study, soluble globular proteins, with masses of 14–200 kDa, were heat-denatured in the presence of SDS and their relative total molecular volume and net charge were estimated from Ferguson plots of electrophoretic mobility vs acrylamide concentration. Native globular protein served as standards for overall molecular size and effective radii. Results revealed at least two independent electrophoretic migration mechanisms for the SDS–protein complexes: (i) for proteins in the 14–65 kDa range at <15% acrylamide, linear Ferguson plots suggested that they migrated ideally and that their effective radii could be estimated in this manner: (ii) concave plots at higher gel concentrations, and for complexes derived from larger proteins, indicated that migration in these cases could be described by reptation theory. Migration of the large proteins at lower gel concentrations and small proteins at higher gel concentrations was not well described by either theory, representing intermediate behavior not described by these mechanisms. These data support models in which all but the smallest SDS–protein complexes adopt a necklace-like structure in which spherical micelles are distributed along the unfolded polypeptide chain. Possible relations to recent alternative models of gel electrophoresis are also discussed.

Liesegang Ring Type Structures and Bifurcation in Solid–Vapor and Liquid Phase Reactions between Cobalt Nitrate and Ammonium Hydroxide

New results on the reaction between (i) cobalt nitrate (A) and ammonium hydroxide (B) in gelatin and (ii) cobalt nitrate (solid) and ammonia vapor are reported. Spatial bifurcation of the blue wave into a number of normal Liesegang type bands is observed in a one-dimensional tube in gel media depending on the concentration of electrolytes and gel. Similar bifurcation is also observed when the reaction was carried out in gel media between two microslides. Results show thatKsp(solubility product), as envisaged in recent theories, plays a significant role in initiating bifurcation depending onK, the magnitude of the product of concentration ofAandB, which determines the degree of supersaturation. The value ofKat the instant of bifurcation, when (i) concentration of Co(NO3)2is alone changed or when (ii) concentration of NH4OH alone is changed, is similar when uncertainties are taken into account. The bifurcation behavior when gel concentration is changed suggests that in case of high gel concentration pore size is affected, thereby influencing the diffusion of ions. The results satisfy the spacing law. The results further satisfy the relationd2= m′t + C′ at any timet, wheredis the distance between the lower front of the precipitate and the junction andm′ andC′ are constants, thereby suggesting that the role of diffusion is vital, as envisaged in various theories of the phenomenon. A colored banded structure separated by air gaps (vacant spaces) is formed when the solid cobalt nitrate reacts with ammonia vapor. This type of behavior has not been reported earlier. Initially, a single band is formed in a one-dimensional experiment, but in the course of time (∼1 month) several bands are formed simultaneously. The diffusion kinetics in the initial stages of the solid-vapor reaction has been studied in a one-dimensional tube. The data obey the relation ξ2=kt, where ξ is the thickness of the product layer at any timetandkis some constant. Similar studies were made on microslides which showed that during the reaction, a colored product is formed which undergoes change in the sequence pink → blue → brown. Results in the one-dimensional case support recent theories which postulate nucleation followed by aggregation.

Use of Poly electrolyte-Enhanced Ultra filtration to Remove Chromate from Water

Abstract Poly electrolyte-enhanced ultra filtration (PEUF) is a process which can be used to remove multivalent ions from water. In PEUF a poly electrolyte of opposite charge to the target ion is added to the water to bind the ion to be removed. The solution is then treated using ultra filtration with membrane pore sizes small enough to reject the polymer and bound ion. In this study, chromate (CrO2 4 −) is removed from water using poly(diallyldimethyl ammonium chloride) with an average molecular weight of 240K as the poly electrolyte. In the absence of other added electrolytes, chromate rejections of up to 99.8% were observed. The presence of added NaCl reduces the chromate rejection substantially. A study of the flux of the system yielded a gel concentration of 0.55 M cationic poly electrolyte. This high gel concentration and high rejection mean that the ultra filtration can produce a concentrated, low-volume waste stream, and a purified stream containing chromate at low concentration.

A spin-spin relaxation rate investigation of the gelatin ferrous sulphate NMR dosimeter

Spin-spin NMR relaxation rate in the ferrous sulphate gelatin dosimeter has been studied in terms of pH, gelatin concentration, the addition of benzoic acid, and sample size. It is demonstrated that R2 is more sensitive to changes in Fe3+ ion concentration than R1 when measuring at frequencies of 64 and 100 MHz. pH has an important effect on dose response curves, and oxygen depletion occurs significantly more rapidly in FeSO4 gelatin than in the liquid FeSO4, resulting in a saturation dose of approximately 80 Gy at depths greater than approximately 3 mm in phantom. The concentration of gelatin can be increased to 12% by weight, and the dosimeter will continue to exhibit a linear dose response. Sensitivity is maintained at higher gel concentrations by pH compensation. Addition of low-concentration benzoic acid to the system does not alter the dose response of the gelatin FeSO4 system. Finally, spontaneous oxidation of Fe2+ ions does not significantly alter the shape of dose response curves but does result in increases in R2 by up to 4% per day.

Information on DNA conformation derived from the Ferguson plot of DNA fragments of up to 9 kb in size, using polyacrylamide gel electrophoresis in a discontinuous buffer system

The Ferguson plot in polyacrylamide gel electrophoresis (PAGE)(15%CDATD, moving boundary electrophoresis buffer system operative at pH 8.9, 4 degrees C, 8 mA/cm2 of gel) of DNA fragments up to 9.4 kb in size was found to exhibit a linear segment at polyacrylamide concentrations starting at 3% T and undergoing a gradual transition into a concave segment at higher gel concentrations, confirming previous findings by Stellwagen. The larger the DNA, and the higher the gel concentration, the less extended the linear and the more extended the concave segment of the plot. The lowest % T of the linear range for DNA in polyacrylamide remains unknown since mobilities at nongelling concentrations below 3% T have not as yet been measured. As previously suggested, the transition from the linear to the concave segment corresponds to that from the randomly oriented DNA to the anisotropically stretched, "reptating" DNA. For a DNA of 9.4 kb in size, the end of the linear range of the Ferguson plot can be extended from 3.5 to 5% T when 15% DATD rather than 2.5% Bis is used to crosslink the polyacrylamide. Increasing the temperature of PAGE from 4 degrees C to 25 and 50 degrees C widens the linear segment progressively, indicating an increasingly random orientation with rising temperature. When current density is increased from 8 to 40 mA/cm2, the concave curvature of the Ferguson plot of DNA 1 to 9.4 kb in size decreases, suggesting a transition from a "reptating" to a randomly distributed molecule, due to increased Joule heat.(ABSTRACT TRUNCATED AT 250 WORDS)

Gel electrophoretic analysis of DNA branched junctions

Gel electrophoresis has provided much of the detailed information we have about the properties of DNA junctions, stable branched molecules formed from oligonucleotide or polynucleotide strands. Here we review these applications, and present the results of an electrophoretic investigation of conformationally restricted junctions formed by covalently connecting two different pairs of strands in a junction with four arms. Native gel electrophoresis is employed to establish the formation and stoichiometry of the multistrand complexes. Ferguson analysis of native gel mobility shows that junctions have retardation coefficients that are distinct from those of linear DNA duplexes. Denaturing gel electrophoresis is the primary tool for characterizing junctions that have been covalently linked together to form both linear and macrocyclic oligomers of junctions (oligojunctions). Radioactively labelled strands enable one to monitor the progress of the ligation reaction: both linear and closed cyclic molecules result, and these can be distinguished by applying Ferguson analysis to denaturing gels. Combinations of exonuclease III, restriction enzymes and sequencing reactions have been applied to oligojunction molecules, and the results are all analyzed on denaturing gels. Junctions containing intramolecular "tethers" that restrict the conformation freedom of the complex comprise a new system for analyzing the conformations of branched molecules. In these tethered junctions, the ability of arms to move relative to each other is restricted substantially by covalently connecting pairs of arms in the original complex with short, flexible loops. The two tethers used here constrain the helical domains of the structure to be roughly parallel or anti-parallel. In this article, we use Ferguson analysis to compare two tethered junctions with an untethered junction. At high gel concentrations, the mobility of the untethered complex is found to be closer to that of the molecule tethered anti-parallel than to the one tethered parallel. Curvature in the Ferguson plots for all three of these junctions is detected over a range of compositions. At low gel concentrations, differences in electrophoretic mobility persist, suggesting that the untethered junction differs in charge as well as conformational freedom from the tethered analogs. We expect that studies of this kind will be able to define the conformational repertoire of junctions of different kinds, and to explore the effects of electrophoresis on these states.

Electrophoretic mobility of λ phage HIND III and HAE III DNA fragments in agarose gels: A detailed study

AbstractThe electrophoretic mobility (μ) of DNA fragments from λ phage and ΦX 174, split by restriction enzyme to molecular lengths from 3 × 102 to 2.36 × 104 base pairs, has been investigated in 0.6–4% agarose gels at various field strengths, ionic strengths, and temperatures. As already observed, μ is seen to be very sensitive to the field, increasing with field strength. The sensitivity increases with the molecular length of the DNA and decreases at high gel concentration. Our data are in qualitative agreement with recent theoretical predictions that concern the influence of the electric field on electrophoretic mobility. Mobility data have been extrapolated to zero field. This enables a comparison of our experimental results with theoretical predictions on the dependence of μ on the molecular weight of the DNA fragments. Our data fit, quite closely, a reptation model, where the tube path is described as a semiflexible entity with a persistence length equal to the pore diameter. The influence of the agarose concentration and the ionic strength of the buffer on the two parameters of the model—intrinsic electrophoretic mobility (μ0) and the number of base pairs per element of the tube (g)—are well described by the model. The temperature dependence of the electrophoretic mobility, together with the influence of the agarose concentration on μ0, indicate that the hydrodynamic drag is the leading frictional force on the DNA molecules in the gel.

Laser light scattering study of polyacrylamide gels

AbstractLaser light scattering experiments with polyacrylamide gels containing diffusing bovine plasma albumin are described. Complete heterodyne beating has not been assumed, and an account is given of a novel method for finding the degree of spectral broadening by scanning angles of scatter while measuring the temporal autocorrelation function. Results obtained by this method for polyacrylamide gels and un‐cross‐linked solutions in the absence of diffusing protein, are compared with those of other authors. In the presence of the protein there is a complex interaction between the diffusion coefficient of the protein and the coefficient associated with the diffusing fluctuations in segment density of polyacrylamide. The diffusing protein dominates at low gel concentrations, and the gel density fluctuations dominates at high gel concentrations. It is proposed that the protein preferentially occupies the less dense regions of the gel, so that at low gel concentrations, where it diffuses faster than the gel fluctuations, it tends to cancel them out, whereas at high gel concentrations where the protein is highly impeded by the gel structure, it tends to be carried along by the gel density fluctuations.