Compressional Forces Research Articles

GRANITE FORMING EVENTS AND THEIR ROLE IN CRUST FORMATION OF THE INDIAN SHIELD

Continental crustal evolution in the Indian shield appears to have started around the middle Archaean and completed by the early Proterozoic. In this time span of around 700 - 800 Ma, the primordial gneissic crust has evolved, recycled and stabilized. The earliest phase of the granitoid activity, from the middle-to late Archaean consists predominantly of a tonalite-trondhjemite-granodiorite (TTG) suite, while the second phase of the Archaean - Proterozoic transition (APT) period consists predominantly of granodiorite-adamellite suite. Minor granite forming events have occurred in the middle-to late Proterozoic. The compositional changes in the granites from the Archaean to the Proterozoic appear to have been mainly controlled by the tectonics and the source compositions at each stage and contribution from the mantle appears to be negligible. Increasing of some large ion lithophile (LIL) elements and decreasing of compatible trace elements in the APT granites compared to the earlier gneissic phase and the overall compositions of the latter (the gneisses) suggest that the middle- to late Archaean polyphase gneisses are the partial melts of the mafic rocks from greenstones and the various phases of the tonalite-trondhjemite-granodiorite (TTG), viz., the hornblende-biotite- and feldspathic gneisses (younging in the same order) are the successive fractionate of similar or same source. The tectonic setting at the time of formation of the TTG gneisses is more of a compressive regime and crustal thickening appears to have played a role in the initiation of partial melting of the source. The appearance of extensive K-rich batholithic granites throughout the shield in a limited time span of 200-300 Ma during the early Proterozoic, increase of LIL elements and high Th, Rb and Sr variations in these batholithic granites suggest that these granites are anatectic in nature and have formed due to the release of compressional forces and initiation of an extensional regime in the shield area. Though sporadic important alkalic granite bodies have formed in the middle to late Proterozoic, they are mostly restricted to the orogenic belts like the Aravalli, the Delhi, the Satpura and the Eastern Ghats. Many of these post-tectonic granites (mostly A-type) give evidence for crust-mantle interaction in their formation and their emplacement in a stable continental crust setting.

GRANITE FORMING EVENTS AND THEIR ROLE IN CRUST FORMATION OF THE INDIAN SHIELD

Continental crustal evolution in the Indian shield appears to have started around the middle Archaean and completed by the early Proterozoic. In this time span of around 700 - 800 Ma, the primordial gneissic crust has evolved, recycled and stabilized. The earliest phase of the granitoid activity, from the middle-to late Archaean consists predominantly of a tonalite-trondhjemite-granodiorite (TTG) suite, while the second phase of the Archaean - Proterozoic transition (APT) period consists predominantly of granodiorite-adamellite suite. Minor granite forming events have occurred in the middle-to late Proterozoic. The compositional changes in the granites from the Archaean to the Proterozoic appear to have been mainly controlled by the tectonics and the source compositions at each stage and contribution from the mantle appears to be negligible. Increasing of some large ion lithophile (LIL) elements and decreasing of compatible trace elements in the APT granites compared to the earlier gneissic phase and the overall compositions of the latter (the gneisses) suggest that the middle- to late Archaean polyphase gneisses are the partial melts of the mafic rocks from greenstones and the various phases of the tonalite-trondhjemite-granodiorite (TTG), viz., the hornblende-biotite- and feldspathic gneisses (younging in the same order) are the successive fractionate of similar or same source. The tectonic setting at the time of formation of the TTG gneisses is more of a compressive regime and crustal thickening appears to have played a role in the initiation of partial melting of the source. The appearance of extensive K-rich batholithic granites throughout the shield in a limited time span of 200-300 Ma during the early Proterozoic, increase of LIL elements and high Th, Rb and Sr variations in these batholithic granites suggest that these granites are anatectic in nature and have formed due to the release of compressional forces and initiation of an extensional regime in the shield area. Though sporadic important alkalic granite bodies have formed in the middle to late Proterozoic, they are mostly restricted to the orogenic belts like the Aravalli, the Delhi, the Satpura and the Eastern Ghats. Many of these post-tectonic granites (mostly A-type) give evidence for crust-mantle interaction in their formation and their emplacement in a stable continental crust setting.

Compressional Characterization of Two Dextrose-Based Directly Compressible Excipients Using an Instrumented Tablet Press

The main objective of this work was to evaluate the compaction behavior and record the work and the force involved in the compaction of blends and granules of two dextrose-based directly compressed excipients using a single-punch instrumented tablet press. The second objective was to identify the predominant form of deformation for the two different directly compressible excipients. Anhydrous theophylline (10% w/w) was used as a drug model, Emdex and/or Maltrin M 510 (89.5% w/w) were used as diluent, and magnesium stearate (0.5% w/w) was used as lubricant. All formulations were compressed at four different compressional forces and at a target tablet weight of 450 mg ± 5%. Results show that compacts prepared from Emdex using the direct compression method produced the lowest elastic work and die wall friction, and the best degree of lubrication. Wet granulation for Maltrin M 510 decreased elastic work, frictional work, and ejection force, and enhanced both net work and degree of lubrication. In general, wet granulation for both Emdex and Maltrin M 510 decreased the crushing strength of the tablets and enhanced the degree of lubrication, compared to direct compression formulations. All formulations showed similar shape pattern for plastic deformation, suggesting that the predominant mechanism of deformation is plastic deformation type a Heckel plots.

Tableting of controlled release multiparticulates, the effect of millisphere size and protective overcoating

Controlled release enalapril millispheres of different diameters were prepared and tableted with excipients in a 1:1 ratio on a Carver press. Drug release rate profiles showed that the microporous coating on the millispheres was severely damaged during the compression process. Overcoating of the microporous coating with hydroxypropyl methylcellulose E5 (HPMC E5) protects the microporous coating during compression by absorbing and dissipating the compressional forces. This protection becomes more pronounced as the size of the millispheres is reduced. A novel investigation of individual millisphere strength was conducted to identify the most favorable coating and size combination which would preserve the controlled release properties of the millispheres.

Periodic deformation of oceanic crust in the central Indian Ocean

New seismic reflection profiles of ≈5370 km, running through the Ocean Drilling Program Leg 116 sites and Deep Sea Drilling Project Sites 215 and 218, were obtained to investigate the spatial extent, timing, and nature of the Tertiary deformation of the equatorial central Indian Ocean. Analyses of the data revealed basement and sedimentary structures and structural unconformities that resulted from the release of compressional forces. Long‐wavelength (150–300 km) anticlinal basement structures with 1–2 km relief and tight folding and high‐angle faulting (5–20 km long) of oceanic basement and overlying sediments exist between the Afanasy Nikitin seamount and the Ninety east Ridge. The deformation zone extends from 10°S to the north up to ≈7°N latitude. The changes in the regional basement trend along 87°E longitude between latitudes 11°S and 15°N coincide with the significant tectonic events of the first major plate reorganization occurred in the eastern Indian Ocean at 90±5 Ma and changes in the Indian plate motion at 65±5 Ma. The basement deformation at selected places, a widespread unconformity of the upper Miocene, and subsequent younger unconformities (lower Pliocene and upper Pleistocene) indicate that the deformation activity might have begun earlier than the generally believed age of 7.5 Ma and appears to be periodic. The upper Miocene and upper Pleistocene deformational unconformities are, in general, observed south of 1°S, while the basement deformation and lower Pliocene deformational unconformity are mostly present in the area north of 1°S. It is surmised that the compressional stresses built up since the hard collision of India with Eurasia may have released for a short period prior to the early Miocene time and deformed the oceanic basement in the northeastern Indian Ocean. Thereafter the stress regime seems to have transferred to the north of the Indian shield and caused deformation of the continental lithosphere. Later, the activity reoccurred during the late Miocene (≈7.5 Ma), early Pliocene (≈4 Ma) and late Pleistocene (≈0.8 Ma) with a cyclicity of ≈3.5 m.y. and deformed the oceanic crust and sedimentary strata in the central Indian Ocean.

Patterns And Structural Positions of Productive Fractures In the Asmari Reservoirs, Southwest Iran

Abstract In wrench faulted areas, similar to the one introduced here, identifying structural styles and their histories could be one of the most important tasks in studying tectonic related fractures and faults. The main purpose of this paper is to examine the patterns and structural positions of productive fractures in different reservoir structures of the Asmari formation (Oligo-Miocene) in the Khuzistan oil fields belt of southwest Iran. Details of productive fracture positions have been studied on four major Asmari reservoirs, using:production data (e.g., production rates and productivity indices) which are the most reliable indirect fracture identifiersmud losses, to identify open fracture portionfaulted well assemblages to find tectonically active Portionssecond derivative map to find the portions of the maximum rate-of-change-of-dip on top of the reservoir structures, where it is expected to have maximum fracture density. The results were then compared with the surface fracture patterns and densities in the mountain front boundary in the area. In general, productive fractures were found to be associated mainly with the plunges and bends of the Asmari reservoir structures. To assist in determining optimal horizontal well trajectories in the anticlinal traps of the studied area, idealized fracture pattern models have been presented in this paper. Structural Styles and History of the Area Identifying structural styles and their histories could be one of the most important tasks in studying tectonic related fractures. These styles are generally differentiated on the basis of the extent of the basement involvement and/or the detachment of sedimentary cover. In the Khuzistan oil fields belt of southwest Iran, reservoir structures have become further complicated by the superimposition of fundamentally different tectonic regimes, which are still active based on a high concentration of earthquake foci in the area (Figure 1). Some evidence of recent activity of the regional basement controlled faults is indicated by a high concentration of the earthquake epi-centres (foci), the existence of sinuosity (bending) in the reservoir structures and high length to width ratio in most of the anticlinal structures of the Plio-Pleistocene age. The major deformation took place in the Miocene, following the collision of the Arabian plate with the central Iranian block. This convergence, with its accompanied subduction, caused the folding of sedimentary cover into the present styles; huge anticlines, with general trends of northwest-southeast perpendicular to orogenic (folding) compressional forces which are southwestnortheast trending. These styles are mostly formed during three different stages:Early orogenic shocks, which caused the early folding phase in the Miocene.Main orogenic shocks, which caused the late folding phase in the Pliocene.Recent orogenic shocks, which caused the recent folding phase. Each folding phase created its own related fracture pattern, which are discussed in this paper.

Evaluation of Two Dextrose-Based Directly Compressible Excipients

The objectives of this research were to evaluate the physical properties and compaction behavior of two dextrose-based directly compressed excipients. Anhydrous theophylline (10% w/w) was used as a drug model, Emdex and or Maltrin M510 (89.5% w/w) were used as diluent, and magnesium stearate (0.5% w/w) was used as lubricant. Direct compression and wet granulation methods were used for preparing the compacts. In general, the wet granulation method reduced the density of the mixture and consequently its flow rate compared to the mixture prepared only by solid-solid mixing. All formulations were compressed at four different compres-sional forces and at a target weight of 450 mg ± 5%. Tablets obtained were different in physical properties and mechanical strength based on type of excipient used and methods of tablet preparation (direct compression versus wet granulation). Compacts prepared from Maltrin M510 had a longer disintegration time and slower drug release than compacts of the same composition but prepared with Emdex. Disintegration time and drug dissolution from tablets containing Maltrin M510 as diluent and prepared by wet granulation appeared to be controlled by a “gel” layer formation around the tablets and not by the tablets porosity. This study demonstrates that full characterization of excipients is needed because a different manufacturing process for the same excipients may produce differences in the pharmaceutical products.

Structure and origin of the 85°E ridge

The submerged 85°E Ridge in the Bay of Bengal trends approximately N‐S between 19°N and 6°N latitudes. Off the southeast coast of Sri Lanka it takes an arcuate shape and seems to terminate with the northward extension of the Afanasy Nikitin seamounts situated around 2°S latitude. The ridge is characterized by positive magnetic (100–400 nT) and negative free‐air gravity (<−60 mGal) anomalies with variable widths of 100–180 km. Magnetic model studies revealed that the rocks of the 85°E Ridge are magnetized with reversed polarity. The well‐defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin. Two alternative processes for the ridge emplacement have been suggested. Ridge emplacement may be (1) due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early Cretaceous crust in the Bay of Bengal, more precisely at M0 isochron or during the middle Albian reversals within the Cretaceous long normal polarity (K‐T superchron) epoch when the Earth's magnetic polarity changed from normal to reversed polarity, and/or (2) due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin. However, more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model. Further, the Rajmahal traps (normally polarized) and the 85°E Ridge (reversely polarized) appear to be associated with two different episodes of eruption that might have been triggered by the Kerguelen mantle plume. The 85°E Ridge seems to extend into the onshore West Bengal Basin as a subsurface ridge and merges with the reported NNE‐SSW trending zone of strong geophysical anomalies east of Rajmahal traps up to 25°N latitude.

Error analysis in acoustic elastography. II. Strain estimation and SNR analysis.

Accurate displacement estimates are required to obtain high-quality strain estimates in elastography. In this paper the strain variance is derived from the statistical properties of the displacement field to define a point signal-to-noise ratio for elastography (SNR0). Displacements caused by compressional forces applied along the axis of the transducer beam are modeled by scaling and shifting the axial reflectivity profile of the tissue. The strain variance is given as a function of essential experimental parameters, such as the amount of tissue compression, echo waveform window length, and the amount of window overlap. SNR0 is defined in terms of applied compression and strain variance and normalized by the input signal-to-noise ratio (SNRi) for echo signals, to formulate the performance metric SNR0/SNRi. This quantity characterizes the noise properties, dynamic range, and sensitivity of strain images based on the spatial resolution requirements. The results indicate that low noise, high sensitivity, and limited dynamic range strain images are obtained for high-frequency bandpass signals when the applied strain is small. For large strains, however, one strategy for low-noise strain imaging employs base-band signals to obtain images with large dynamic range but limited peak sensitivity and noise figure. A better strategy includes companding, which eliminates the average strain in the echo signal before cross-correlation to reduce the dynamic range requirement and increase peak sensitivity for strain estimates.

Function of proteoglycans in the extracellular matrix

Proteoglycans are glycosylated proteins which have covalently attached highly anionic glycosaminoglycans. Many forms of proteoglycans are present in virtually all extracellular matrices of connective tissues. The major biological function of proteoglycans derives from the physicochemical characteristics of the glycosaminoglycan component of the molecule, which provides hydration and swelling pressure to the tissue enabling it to withstand compressional forces. This function is best illustrated by the most abundant proteoglycan in cartilage tissues, aggrecan. During the past decade, diverse species of proteoglycans have been identified in many connective tissues, on cell surfaces and in intracellular compartments. These proteoglycans have distinct biological functions apart from their hydrodynamic functions, and their involvement in many aspects of cell and tissue activities has been demonstrated. For example, decorin, which is widely distributed in many connective tissues, may have functions in regulating collagen fibril formation and in modifying the activity of transforming growth factor-beta; perlecan, the major heparan sulfate proteoglycan in the glomerular basement membrane, may play an important role as the major anionic site responsible for the charge selectivity in glomerular filtration. Specific interactions between proteoglycans (through both their glycosaminoglycan and core protein components) and macromolecules in the extracellular matrix are the key factors in the functions of proteoglycans. Exciting biological functions of proteoglycans are now gradually emerging.

Tablet compression force measurement using strain gauges

An instrumented tableting machine yields some of the most significant information about the compressional and ejectional forces involved in the tableting operation. A number of versions of instrumented tablet presses are commercially available but tend to be very expensive to be used widely in academic institutions. The present project involving the instrumentation of a single punch tableting machine (Manesty, Model F3) was undertaken to propose a low cost alternative to researchers in the area of product development. This paper deals with the measurement of the compression force using strain gauges mounted in two different configurations: upper punch and lower pin. Good linearity between applied force and measured strain was observed.

Stress of Corrugated Fiberboard Made by Compression under Uniform Bending of Moment Axis in Machine Direction (On Single Wall Fiberboard with Symmetrical Thickness Strength)

We investigated stress generation in a semichemical corrugated medium (SM) by compressional forces under uniform bending with the bending moment axis of the machine direction (MD) in single-wall corrugated fiberboard (SWCF), and attempted to derive an elastic expression for its generation stress σ sp. Using this expression, the analysis of σ sp was carried out. Values of the lateral force W sx and fixed moment M s0 per unit length at the joint between kraftliner (KL) and SM are determined, and relationships between those values and the shape of SWCF are considered. The following results are obtained from the expression. The maximum value σ pmax of σ sp is found at the joint on the outer surface of SM. W sx, M s0 and σ pmax increase markedly with the increase of wavelength L and decrease markedly with the increase of wave height h for SM.

Stabilized polyglycolic acid fibre-based tubes for tissue engineering

Polyglycolic acid (PGA) fibre meshes are attractive candidates to transplant cells, but they are incapable of resisting significant compressional forces. To stabilize PGA meshes, atomized solutions of poly( l-lactic acid) (PLLA) and a 50/50 copolymer of poly( d,l-lactic-co-glycolic acid) (PLGA) dissolved in chloroform were sprayed over meshes formed into hollow tubes. The PLLA and PLGA coated the PGA fibres and physically bonded adjacent fibres. The pattern and extent of bonding was controlled by the concentration of polymer in the atomized solution and the total mass of polymer sprayed on the device. The compression resistance of devices increased with the extent of bonding, and PLLA bonded tubes resisted larger compressive forces than PLGA bonded tubes. Tubes bonded with PLLA degraded more slowly than devices bonded with PLGA. Implantation of PLLA bonded tubes into rats revealed that the devices maintained their structure during fibrovascular tissue ingrowth, resulting in the formation of a tubular structure with a central lumen. The potential of these devices to engineer specific tissues was exhibited by the finding that smooth muscle cells and endothelial cells seeded onto devices in vitro formed a tubular tissue with appropriate cell distribution.

Very broadband seismic analysis of the 1992 Flores, Indonesia, earthquake (Mw = 7.9)

The December 12, 1992, Flores Island earthquake (Mw = 7.9), which caused a destructive tsunami, is located in the back arc of the transition zone between the Sunda and Banda arcs, a region in active collision with the Australian continental margin. We study the earthquake using four seismological techniques spanning a period range of 1–1000 s and infer that the Flores event may be characterized as a shallow back arc thrust with fault plane strike 70°±30°, dip 28°±10° and rake 80° to 140° (linear trade‐off with strike), a rupture consisting of three discrete pulses with a total duration of 70 s (centroid time 26.3 s) and an estimated moment release between 7.5×1020 and 8.0×1020 N m. From the slip vector we infer that compressional forces play an important role in the deformation mechanism of the back arc north of Flores Island. Body waves constrain the hypocentral depth to be 16 km with rupture mainly propagating updip to shallow depth and bilaterally along strike with most subevents located toward the northeast. Evidence for rupture directivity from surface wave deconvolutions is inconclusive, but the overall source function is consistent with body wave results. Our body wave finite fault model is in rough agreement with two models proposed to explain tsunami run‐up observations. Normal mode modeling of W phase observations at periods longer than 70 s shows that slow slip is negligible. From this and the slip distribution determined from the body wave analysis we infer that the seismic source is an unlikely candidate to explain anomalously high tsunami run‐ups measured on northeastern Flores Island.

A GEOMORFOLOGIA DA SERRA DO ESPINHAÇO EM MINAS GERAIS E DE SUAS MARGENS

The geomorphological evolution of the Serra do Espinhaço is discussed based on: a) a critical analysisof previous papers about the geomorphology of the range, b) new interpretation based on own field data,c) regional interpretation, considering informations about the evolution of adjacent areas west and east ofthe range (São Francisco River, Doce River and Jeqitinhonha-Araçuaí Rivers Basins). The global contextis presented as a synthetical characterization of the main geological conditions, regional physiographyand drainage system organization and anomalies.The regional geomorphological organization is rigid controlled by the geology: a) the shape of therange reproduce the original collisional arc developed during the proterozoic orogeny, b) the existence oftwo strongly diferenciated plateaux (Planalto Meridional and Planalto Setentrional) reflect the litho-structuralcontrast between the southern and northern parts of the range, c) the morphology expresses the associationbetween lithological resistance to erosion and the tectonic organization in all scales.The morphology of the higher portions of the plateaux is characterized by relicts of two tertiaryplanation surfaces. They are probably peneplains resulting from erosion under tropical humid climaticconditions. During the Pleistocene, the valleys recorded the formation of several stepped terraces. thespacial variation on its dislevelments reflects the intensity of crustal uplifts and the position of the streamsin the drainage system hierarchy. The hillsides were, during the Upper Pleistocene, covered by two finecolluvial layers. In both cases, the cenozoic sediments register evidences of humid and hot paleoclimaticconditions.The Serra do Espinhaço acquired its present main physiography after the Miocene, which wascharacterized by tectonic stability. During the Pliocene, a strong compressional tectonic event reactivatedthe main precambrian thrust faults and detachment planes. As a consequence, the high scarp related to thewest thrust front was enhanced together with a eastward tilting of the plateaux. At the same time, in thePlanalto Setentrional, several valley heads of the São Francisco drainage basin were deviated and capturedby the Jequitinhonha Basin drainage system. In the Planalto Meridional, the geological conditions (porouslithology and anticline structure) favoured the capture of streams by the São Francisco Basin drainagesystem.As a consequence of the compressional forces that uplifted the range, an extensional stress fielddeveloped in the eastern border, generating grabens structures where the main rivers installed its valleys.The neotectonic activity is also recorded in several parts of the range and adjacent areas by normal, reverseand strike-slip faults in pleistocenic alluvial and colluvial deposits. Fault planes data analysis indicates ageneral NW-SE compressional stress field that remains up to the present time, as confirmed by results ofseismological data.(*)

Fault tectonics of the Shillong plateau and adjoining regions, north-east India using remote sensing data

Abstract The Shillong plateau in north-east India shows innumerable fracture lineaments on satellite images. In fact the whole plateau is criss-crossed by fractures. The plateau has been subjected to extensive compressional forces mainly in N-S and E-W directions resulting from the collision of the Indian plate with the Tibetan and Burmese plates, respectively. This type of plate tectonism has been responsible for the formation of many significant faults, folds and other tectonic features in the Shillong plateau and adjoining regions. The well known E-W trending Dauki fault, N-S trending faults, NE-SW trending shear zone and other prominent lineaments and their associated structural features could be studied on the basis of remote sensing techniques. Folding in soft sediments along the Dauki fault reveals the compression direction. Directional compressional tectonism has been expressed through lateral movements along the NE-SW trending shear zone and N-S trending faults and folds in the area just south of...

Fabricating Tubular Devices from Polymers of Lactic and Glycolic Acid for Tissue Engineering

Polymers of lactic and glycolic acid are attractive candidates to fabricate devides to transplant cells and engineer new tissues. These polymers are biocompatible, and exhibit a wide range of erosion times and mechanical properties. This manuscript describes the fabrication and characterization, in vitro and in vivo, of hollow, tubular devices from porous films of various polymers of this family. Porous films of these polymers were formed using a particulate leaching technique, and sealed around Teflon cylinders to form hollow tubular devices. The erosion rate of devices was controlled by the specific polymer utilized for fabrication, and ranged from months to years. Devices fabricated from a 50/50 copolymer of D,L-lactic acid and glycolic acid were completely eroded by 2 months, while devices fabricated from a homopolymer of L-lactic acid showed little mass loss after 1 year. Erosion times for devices fabricated from the other polymers [poly-(D,L-lactic acid) and a 85/15 copolymer] were between these two extremes. Devices were capable of resisting significant compressional forces (150 raN) in vitro, and the compression resistance was controlled by the polymer utilized to fabricate the devices. The ability of the devices to maintain their structure after implantation into the mesentery or omentum of laboratory rats was also dependent of the specific polymer utilized to fabricate the device. These results indicate that it is possible to fabricate tubular devices for tissue engineering applications that exhibit a wide range of erosion rates and mechanical properties.

The release of 5-fluorouracil from a swellable matrix of a triblock copolymer of epsilon-caprolactone and ethylene oxide.

The purpose of this study was to investigate the influence of hydration characteristics on the in vitro release of 5-fluorouracil from a swellable matrix prepared using a novel triblock copolymer of poly(epsilon-caprolactone) and poly(oxyethylene). Matrices were prepared by dry compression of mixtures of the drug and copolymer using low compressional forces. Release studies were performed using a custom made rotating basket dissolution apparatus. The positions of the eroding and swelling fronts within the matrices during hydration were monitored using freeze fracture scanning electron microscopy. Analysis of the release data revealed a predominantly diffusion controlled mechanism. Observations of the swelling characteristics of the copolymer matrices on immersion in Sørensen's buffer at pH 7.4 revealed gel formation and preferential swelling in the radial direction with visible erosion of the matrix after 4h. During hydration, a gradual increase in gel layer thickness was noted prior to the erosion and eventual dissolution of the matrix. This study demonstrates a means of differentiating the relative importance of the swelling characteristics in determining the release mechanism and subsequent release rate from swellable matrices.

Three‐dimensional crustal structure of the Mendocino Triple Junction region from local earthquake travel times

The large‐scale, three‐dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19,000 P wave arrival times from over 1400 local earthquakes to estimate the three‐dimensional velocity structure and hypocentral parameters. A velocity grid 175 km (N‐S) by 125 km (E‐W) centered near Garberville, California, was constructed with 25 km horizontal and 5 fan vertical node spacing. The model was well resolved near Cape Mendocino, where the earthquakes and stations are concentrated. At about 40.6° N latitude a high‐velocity gradient between 6.5 and 7.5 km/s dips gently to the south and east from about 15 km depth near the coast. Relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted Gorda Plate. Therefore the depth to the top of the Gorda Plate near Cape Mendocino is interpreted to be ∼15 km. The Gorda Plate appears intact and dipping ∼8° eastward due to subduction and flexing downward 6°–12° to the south. Both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40.3° N latitude and maintains a linear trend 15° south of east to at least 123° W longitude. The top of a large low‐velocity region at 20–30 km depth extends about 50 km N‐S and 75 km E‐W (roughly between Garberville and Covelo) and is located above and south of the southern edge of the Gorda Plate. We interpret this low velocity area to be locally thickened crust (8–10 km) due to either local compressional forces associated with north‐south compression caused by the northward impingement of the rigid Pacific Plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the Cascadia Subduction Zone. South of Cape Mendocino and southwest of the Garberville fault, high velocities indicative of oceanic crust are detected at 15 km depth. We interpret this evidence as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California (Page and Brocher, 1994). This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep‐seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake (Benz et al, 1992; Zandt et al., 1992).

Influence of Processing on the Rheology of Titanium Dioxide Pigment Suspensions

A knowledge of the rheology of titanium dioxide pigment suspensions and the variation with processing conditions is essential for optimum plant operation and in ultimate paint production. The influence of pH, solids concentration, and particle size on the rheological properties of four commercial TiO 2 pigment suspensions is reported. The suspensions exhibit pseudoplastic behavior, conforming to the power-law model in the shear rate range investigated. Extreme sensitivity with pH was observed, whereby the suspension viscosity can be changed by as much as an order of magnitude with pH variation at constant volume fraction. Also, the pH at maximum viscosity coincides with the isoelectric point, independent of solids concentration. The maximum viscosity (viscosity at the isoelectric point) increases exponentially with solids concentration and increases with decreasing particle size. The important outcome from the investigation is the dependence of rheology on the compressional history used for dewatering the suspensions. Two identical pigments from two different filtration devices exhibited a large difference in their viscosity, as a consequence of being subjected to different compressional forces. Finally, the presence of a monolayer coating of organic on the pigment surface illustrates how to lower the observed suspension viscosity with steric stabilization