Dislodgement Forces Research Articles

Quantifying gravel overlap and dislodgement forces on natural river bars: implications for particle entrainment

ABSTRACTEquations for predicting particle entrainment typically assume that clast weight is the only factor resisting motion in the force balance on individual grains. In this work, increases in the force required to dislodge surface particles due to overlapping by surrounding clasts is quantified. Field data were collected at two subaerially‐exposed bars along the Colorado River in central Texas, USA, with median gravel diameters (D50) of 37 and 64 mm. Clast size, shape, weight, the force required for vertical removal, and the fraction of clast area covered by surrounding grains were measured. Small hooks were glued to individual clasts without disrupting their positions and quasi‐static peak forces required to vertically dislodge each clast were measured using a force gauge. Clasts were also colored with dye before being dislodged, and image analysis was used to calculate the fraction of clast surface area covered by surrounding grains. The effect of overlap on the particle force balance is quantified by defining the ‘excess force ratio’ as the dislodgement force divided by the weight of the clast. Excess force ratio varies weakly but systematically with clast size: lifting larger clasts can require forces up to two times the clast's weight, while smaller clasts can require forces up to seven times their weight. The fraction of clast surface area covered by surrounding particles is also weakly correlated with excess force ratio. By assuming that critical shear stresses are proportional to the excess force ratio, the effect that overlap alone may have on particle entrainment is calculated. For a given size class, the most mobile grains should have critical shear stresses controlled only by their weight. However, clast overlap also causes broad distributions of critical stresses for partially‐exposed surface grains. The data quantify the significant fraction of bed area that should be less mobile than grain size alone would predict. Copyright © 2011 John Wiley & Sons, Ltd.

Efficacy of denture adhesives in maxillary dentures using gnathodynamometry: a comparative study

The purpose of this study was to investigate the effect of four commercially available denture adhesives on the incisal and premolar dislodgement forces of maxillary complete dentures by using an electronic and disposable gnathodynamometer and compare the measured incisal forces for differences. This study was conducted with 12 complete maxillary denture wearers. Four commercially available denture adhesives Super Corega(®), Corega Ultra(®), Super Corega Powder(®) and Fittydent Cationic(®) were investigated. Testing protocol and sequence included baseline measurements without adhesives (control) for previous and new dentures and then replications of measurements with the four adhesives. Maximum dislodgement forces were recorded in two sites between central incisors and the left 2nd premolars by using an electronic and disposable gnathodynamometer. To estimate the effect of the different adhesives on the dislodgement forces, data were analyzed by a 2- and 3-way ANOVA, while for estimating the agreement of the two devices a Bland-Altman and Mountain plots were used. ANOVAs indicated significant differences between adhesives (p < 0.05), denture types (p < 0.05) and biting sites (p < 0.05) with both devices. Bland-Altman plot and Mountain plots indicated a poor agreement of the two devices. It was concluded that denture adhesives increase the denture dislodgement forces, but with differences among them. The two devices do not highly agree with each other, but each one alone is useful in estimating dislodgement forces in clinical practice and research.

Biomechanical and histologic analysis in aortic endoprosthesis using fibrin glue

The absence of incorporation between endoprosthesis (EP) and the arterial wall may lead to device migration and endoleaks around the stent graft. Alternatives have been tested aiming to improve this incorporation. Fibrin glue is used in many operating procedures promoting adhesion and tissue regeneration; however, its use to improve EP incorporation by arteries is unknown. The objective of this study was to analyze dislodgement forces needed to extract the EPs implanted in pig aorta, compare different oversizing and fibrin glue injections, and to analyze histologic changes among groups. Straight EPs were implanted in the thoracic aorta of pigs using 10% oversizing plus fibrin glue in the interface between the EP and the artery (group 1), using 20% oversizing (group 2), and 10% oversizing (group 3). Fourteen days after the implant, the animals were killed to enable biomechanical analysis of the EP and to verify histologic changes of the aortic wall and its interface with the EP. Group 1 showed a dislodgement force of 21.9 ± 5.3 Newton (N) being higher than the other groups and statistically significant when compared to group 3 (15.6 ± 3.6N), P = .003%. Group 2 had a higher dislodgement force and statistically more significant than group 3 (19.5 ± 7.8N). Histologic analysis showed tissue reaction with inflammatory cells and fibroblasts higher in group 1 and group 2 compared to group 3. This study reports a large animal survival model of thoracic aortic stent graft placement by testing the impact of fibrin glue on EP incorporation. Compared to oversizing alone, fibrin glue placed between the stent graft and the arterial wall increases EP incorporation. Additional studies are needed to determine the potential utility of fibrin glue in the setting of human arterial endografts.

The effect of vessel material properties and pulsatile wall motion on the fixation of a proximal stent of an endovascular graft

Migration is a serious failure mechanism associated with endovascular abdominal aortic aneurysm (AAA) repair (EVAR). The effect of vessel material properties and pulsatile wall motion on stent fixation has not been previously investigated. A proximal stent from a commercially available stent graft was implanted into the proximal neck of silicone rubber abdominal aortic aneurysm models of varying proximal neck stiffness ( β = 25.39 and 20.44). The stent was then dislodged by placing distal force on the stent struts. The peak force to completely dislodge the stent was measured using a loadcell. Dislodgment was performed at ambient pressure with no flow (NF) and during pulsatile flow (PF) at pressures of 120/80 mmHg and 140/100 mmHg to determine if pulsatile wall motions affected the dislodgement force. An imaging analysis was performed at ambient pressure and at pressures of 120 mmHg and 140 mmHg to investigate diameter changes on the model due to the radial force of the stent and internal pressurisation. Stent displacement forces were ∼50% higher in the stiffer model (7.16–8.4 N) than in the more compliant model (3.67–4.21 N). The mean displacement force was significantly reduced by 10.95–12.83% from the case of NF to the case of PF at 120/80 mmHg. A further increase in pressure to 140/120 mmHg had no significant effect on the displacement force. The imaging analysis showed that the diameter in the region of the stent was 0.37 mm greater in the less stiff model at all the pressures which could reduce the fixation of the stent. The results suggest that the fixation of passively fixated aortic stents could be comprised in more compliant walls and that pulsatile motions of the wall can reduce the maximum stent fixation.

The effect of injectable biocompatible elastomer (PDMS) on the strength of the proximal fixation of endovascular aneurysm repair grafts: An in vitro study

One of the major concerns in the long-term success of endovascular aneurysm repair (EVAR) is stent graft migration, which can cause type I endoleak and even aneurysm rupture. Fixation depends on the mechanical forces between the graft and both the aortic neck and the blood flow. Therefore, there are anatomical restrictions for EVAR, such as short and angulated necks. To improve the fixation of EVAR grafts, elastomer (PDMS) can be injected in the aneurysm sac. The support given by the elastomer might prevent dislocation and migration of the graft. The aim of this study was to measure the influence of an injectable biocompatible elastomer on the fixation strength of different EVAR grafts in an in vitro model. The proximal part of three different stent grafts was inserted in a bovine artery with an attached latex aneurysm. The graft was connected to a tensile testing machine, applying force to the proximal fixation, while the artery with the aneurysm was fixated to the setup. The force to obtain graft dislodgement (DF) from the aorta was recorded in Newtons (N). Three different proximal seal lengths (5, 10, and 15 mm) were evaluated. The experiments were repeated after the space between the graft and the latex aneurysm was filled with the elastomer. Independent sample ttests were used for the comparison between the DF before and after elastomer treatment for each seal length. The mean DF (mean +/- SD) of all grafts without elastomer sac filling for a proximal seal length of 5, 10, and 15 mm were respectively, 4.4 +/- 3.1 N, 12.2 +/- 10.6 N, and 15.1 +/- 6.9 N. After elastomer sac filling, the dislodgement forces increased significantly (P < .001) to 20.9 +/- 3.8 N, 31.8 +/- 9.8 N, and 36.0 +/- 14.1 N, respectively. The present study shows that aneurysm sac filling may have a role as an adjuvant procedure to the present EVAR technique. The strength of the proximal fixation of three different stent grafts increases significantly in this in vitro setting. Further in vivo research must be done to see if this could facilitate the treatment of aneurysms with short infrarenal necks.

Dislodgement of carbon nanotube bundles under pressure driven flow

Experimental and predicted flow rates through carbon nanotubes vary considerably butgenerally are reported to be well in excess of that predicted by the conventionalPoiseuille flow, and therefore nanotubes embedded in a matrix might providemembranes with exceptional mass transport properties. In this paper, appliedmathematical modelling is undertaken to estimate the three forces acting on a nanotubebundle, namely the molecular interaction force, the viscous force, and the staticpressure force. In deducing estimates of these forces we introduce a modification ofthe notion of the effective dead area for a carbon nanotube membrane, and wecalculate the total forces necessary to push one or more of the nanotubes out of thebundle, thus creating a channel through which further enhancement of flow maytake place. However, careful analysis shows that the nett dislodgement force isentirely independent on the useable flow area, but rather depends only on the totalcross-sectional area perpendicular to the flow. This rather surprising result is aconsequence of the flow being steady and a balance of the viscous and pressure forces.

How Much Force Is Required to Dislodge an Alternate Airway?

Introduction. Endotracheal tube (ETT) dislodgment is a potentially catastrophic adverse event. Newer alternate airway devices—esophageal-tracheal Combitube (ETC), King laryngeal tube disposable airway (King LT), and laryngeal mask airway (LMA)—are easier to insert, but their relative extubating forces remain unknown. Objective. To examine the applied forces required to dislodge an ETC, King LT, LMA, and ETT. Methods. We used five recently deceased adult unembalmed cadavers. In random order, we sequentially inserted an ETC, King LT, LMA, and standard ETT. Because commercial tube holders are not designed for all alternate airways, we secured the devices with a standard adhesive tape method. Using a precision digital force measuring device, we measured the minimum manually applied axial force (lb) that dislodged each airway device at least 4 cm. We compared required dislodgment forces between airway devices using a mixed-effects regression model, adjusting for cadaver height, weight, neck circumference, and thyromental distance. Results. Characteristics of the cadavers were as follows (median, interquartile range [IQR]): height 172 cm (167–177), weight 98 kg (84–120), neck circumference 46.5 cm (41–52), and thyromental distance 7.5 cm (7.5–8). Required axial dislodgment forces for each airway device were as follows (median, IQR): ETC 28.3 lb (19.0–28.6), King LT 12.5 lb (11.7–13.3), LMA 18.3 lb (14.0–21.9), and ETT 14.4 lb (13.5-22.1). The ETC required twice as much dislodgment force as the ETT (adjusted difference 16.7 lb, 95% confidence interval [CI]: 8.3 to 25.1). The King LT and LMA dislodgment forces were similar to that of the ETT (King LT vs. ETT adjusted difference 5.9 lb, 95% CI: –2.4 to 14.2; LMA vs. ETT 8.1 lb, 95% CI: –0.2 to 16.5). Conclusion. In a cadaver model of unintended airway dislodgment, the ETC required the most force for dislodgment. The King LT and LMA performed similarly to a standard ETT.

Influence of hydrodynamic forces on population structure of Pinna nobilis L., 1758 (Mollusca: Bivalvia): The critical combination of drag force, water depth, shell size and orientation

The effects of hydrodynamics on size, shape and distribution of benthic organisms are still not completely understood. Benthic organisms that inhabit wave-swept environments usually have small sizes and ethological adaptations to reduce drag and increase resistance force. Water speeds produced by waves in intertidal habitats can be more than 10 times higher than those in subtidal environments. However, comparatively small water speeds can produce high drag forces ( F d) on large subtidal organisms. Pinna nobilis is a subtidal epibenthic large bivalve-mollusc endemic to the Mediterranean Sea, a common inhabitant of Posidonia oceanica meadows. It lives partially buried in the seabed and shows a characteristic population structure. Small individuals are usually located at shallow sites whereas large individuals are only observed in deeper levels or sheltered locations. Also, some populations show a common orientation of the shell. These features are widespread throughout the Mediterranean, but their causes are unknown. The present work is a study on the relationship between population structure of P. nobilis and the habitat hydrodynamics. The main factor considered was Drag force due to the water flow produced by waves. Drag forces ( F d) supported by two populations located at different depths in the same P. oceanica meadow were estimated according to both the size and orientation of shells. Also, F d acting on the individuals during the greatest storm recorded in the zone in the previous 9 years, were calculated. Drag coefficients ( C d), necessaries to estimate F d, were estimated in the towing tank of the “Ecuela Técnica Superior de Ingenieros Navales (ETSIN)” of the Polytechnic University of Madrid. The results show significant differences in F d acting in both populations. Despite the important increment of water speed with wave shoaling, individuals of the shallow population (SP), located at 6 m depth, withstand less F d than those of the deep population (DP), at 13 m depth. The main reasons of this F d reduction in SP are both the small size of the individuals and their common orientation, having the dorso-ventral side of the shell towards the main water flow. This fact, together with previous data showing higher mortality, less density of individuals, and less maximum asymptotic length, evidence that selective pressures regulate these population parameters, producing a trade-off between hydrodynamics, shell size and orientation, for each shore type and water depth. Combining the data of F d supported by each population for different wave types, approximate values of the optimal mean F d and the maximum dislodgement force withstood by P. nobilis were estimated (< 9 Newton (N) and ≈ 45 N respectively).

Crowns cemented on crown preparations lacking geometric resistance form. Part II: effect of cement.

This study evaluated the effect of different cements on resistance to dislodgment of crowns cemented on preparations lacking geometric resistance form. A preparation that offered no geometric resistance form, with 20 degrees total occlusal convergence (TOC), 0.9 mm wide shoulder finish line, and a 2.5 mm axial wall height was created on an ivorine tooth using a milling machine. Ten metal test specimen die replicas and 10 standardized metal crowns with recipient sites for the application of external forces through a universal testing machine were fabricated. The crowns were cemented on the dies under 5 and 10 kg external loads, the marginal openings measured, loaded to dislodgment, and cleaned of cement. The process was repeated using zinc oxide and eugenol (ZOE), zinc phosphate (ZPh), resin modified glass ionomer (RMGI), and composite resin (CR) cements. Marginal openings under 5 kg cementation loads were 74.63 (+/-15.04) for ZOE, 75.98 (+/-18.20) microm for ZPh, 98.58 (+/-22.62) microm for RMGI, and 105.82 (+/-20.07) microm for CR cements respectively; under 10 kg cementation loads they were 57.62 (+/-15.86) microm, 59.55 (+/-15.41) microm, 95.00 (+/-19.52) microm, 101.30 (+/-12.52) microm respectively. Oblique dislodgment forces, measured with a Universal testing machine, were 40.18 (+/- 6.76) N for ZOE, 215.65 (+/-45.79) N for ZPh, 165.43 (+/-19.53) N for RMGI, and 181.54 (+/-30.75) N for CR respectively when crowns were cemented under 5 kg loads. The corresponding values for 10 kg loads were 38.62 (+/-4.19), 274.86 (+/-54.22), 139.70 (+/-21.71), and 160.40 (+/-21.21) respectively. Only zinc phosphate cement produced statistically enhanced resistance when crowns were cemented under 10 kg force (p value = 0.035). Under the conditions of the present study only crowns cemented with zinc phosphate displayed increased resistance to dislodgment on preparations lacking resistance form.

Retentive properties of five different luting cements on base and noble metal copings

Statement of Problem. The retention of indirectly fabricated restorations can be compromised by short or over-tapered tooth preparations. Purpose. The aim of this study was to evaluate the retentive properties of 5 different luting cements on base and noble metal copings to short and over-tapered preparations. Material and Methods. Eighty extracted mandibular premolars were prepared to receive full cast copings with a flat occlusal surface, 33° taper, and 3-mm axial length. Half of the standardized metal copings were cast in an AuAgPd alloy, whereas the other half were cast in an NiCr alloy. Cementation was performed with 5 different luting cements through use of 5 kg of pressure in 90% relative humidity. Specimens were stored in distilled water at 37°C for 24 hours and thermocycled between 5°C and 55°C for 5000 cycles, with a dwell time of 30 seconds. After thermocycling, vertical tensile force was applied in a Zwick universal testing machine with a constant speed of 1 mm/min until separation was noted. A 2-factor analysis of variance was used to analyze the data, with a significance level of α =.05. Results. Mean dislodgement forces for AuAgPd crowns and NiCr crowns were 120.88 N and 143.09 N, respectively, for zinc phosphate cement; 135.45 N and 150.38 N for Principle; 145.88 N and 220.71 N for Meron; 276.85 N and 225.61 N for Avanto; and 300.92 N and 381.02 N for Fuji Plus. Conclusion. Within the limitations of this study, Fuji Plus and Avanto showed significantly higher retentive strengths for AuAgPd copings in comparison to the other cements tested (P <.05). The retentive strength of Fuji Plus was significantly higher than those of the other cements tested with NiCr copings (P <.05). (J Prosthet Dent 2002;88:491-7.)

Predicting wave dislodgment of mussels: variation in attachment strength with body size, habitat, and season

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 213:157-164 (2001) - doi:10.3354/meps213157 Predicting wave dislodgment of mussels: variation in attachment strength with body size, habitat, and season Heather L. Hunt*, Robert E. Scheibling Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada *Present address: Institute of Marine and Coastal Sciences, 71 Dudley Rd. Rutgers University, New Brunswick, New Jersey 08901-8521, USA. E-mail: hunt@imcs.rutgers.edu ABSTRACT: Breaking waves impose large hydrodynamic forces which may dislodge mussels and other organisms living on exposed rocky shores. We examined the effect of variation in attachment strength with size, habitat and season on the predicted probability of wave dislodgment of mussels Mytilus trossulus and M. edulis on an exposed shore on the Atlantic coast of Nova Scotia, Canada. Mussels on this shore are exposed to maximum significant wave heights of at least 7 to 9 m each winter and to maximal water velocities of at least 12 m s-1. We used Denny¹s (1995) mechanistic approach to predict probabilities of dislodgment from measures of attachment strength of mussels. Predicted probabilities of dislodgment indicated that mussels of 10 to 25 mm shell length are vulnerable to dislodgment by water velocities of >7 m s-1. As a result of variation in dislodgment forces, probabilities of dislodgment did not differ consistently between tidepools and emergent rock, or with mussel size. Attachment strength increased from July to February as mussels were exposed to stronger wave action, reducing the probability of dislodgment by a given water velocity. This study indicates that knowledge of patterns of change in attachment strength are necessary for prediction of probabilities of dislodgment of mussels. KEY WORDS: Disturbance · Wave forces · Mussels · Tide pools · Emergent rock · Seasonal variation · Mytilus Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 213. Online publication date: April 04, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Predator-prey interactions in a benthic stream community: a field test of flow-mediated refuges.

Ecological theory suggests that the impact of predation can be strongly modified by the existence of regions of the environment in which prey are less accessible to predators, which underscores the need for empirical studies examining the factors influencing the availability and importance of such prey refuges. Our study tested whether benthic microhabitats with high flows provide suspension-feeding larval black flies (Simulium␣vittatum) with a spatial refuge in which the negative impact of predatory flatworms (Dugesia dorotocephala) is reduced. We conducted a short-term field experiment in Chester Creek (southeastern Pennsylvania, United States) to examine how the number of black fly larvae inhabiting tile substrates responded to manipulated variations in flatworm abundance and current speed. The abundance of flatworms declined with increasing current speed, thereby creating the potential for sites with high flows to provide larvae with a refuge from these predators. Multiple regression analysis revealed that the final abundance of larvae exhibited a significant negative relationship to flatworm abundance and a significant positive relationship to current speed. After adjusting for variations in elapsed time and initial larval abundance, flow and predators explained 38% of the variation in the rate of change in larval abundance. The positive correlation between larval abundance and flow had two components: a positive, direct effect of flow on larvae, which arises because these food-limited consumers prefer to reside within sites with faster flows where they can feed at higher rates; and a negative effect of flow on predators, and of predators on larvae, which combine to yield a positive indirect effect of flow on larvae. This indirect effect demonstrates the existence of flow-mediated refuges (i.e., microhabitats in which the impact of predation is reduced due to high flows), although the effect accounts for a small proportion of total variation in larval abundance. A consideration of biomechanical relationships suggests that microhabitats with high flows are likely to create prey refuges in a wide range of freshwater and marine benthic environments. In particular, predators will often experience greater dislodgement forces than prey because of their larger size and because they project farther above the bed where current speeds are faster. Moreover, the ability to resist a given dislodgement force may be greater for many prey, especially those that are sessile or semi- sessile.

鋳造支台築造とレジン支台築造の保持力に関する研究

Recently, adhesive dentistry has made a lot of progress, and now its effects should be shared by post and core techniques for restoration of endodontically treated teeth. The purpose of this study was to compare the prefabricated metal postand composite resin cores with the conventional cast post and cores with respectto retention. Three kinds of luting cement for the cast post and cores, that is, zinc phosphate cement (EC), Panavia 21 (P 21), and Super-bond C&B (SB) were examined. In addition, three kinds of core build-up technique for the prefabricated post and composite resin cores were prepared for dislodgment tests. They were the prefabricated post cemented with P 21 and light polymerized Clearfil photo core (P 21+PC), the same post and dual polymerized Clearfil DC core (P 21+DC), and the prefabricated post cemented with DC and DC core (DC + DC).The results were as follows:1. The dislodgment forces of the cast post and cores cemented with EC were the smallest, although the forces did not decrease after thermocycling.2. Three kinds of the prefabricated post and composite resin core were equal in retention to the cast metal post and cores cemented with resin cements, P 21 and SB.3. The cast post and cores cemented with SB, and two types of the prefabricated post and core, P 21+PC and DC+ DC, showed significantly lower values in retention after thermocycling.4. SEM photographs showed different images in each post and core technique. Fractured dentin surfaces in EC were covered with a smear layer. P 21 and SB were covered using a resin cement layer, and P 21+ PC, P 21 + DC, and DC + DC were covered using a bonding layer.

PHENOTYPIC PLASTICITY IN THE FOOT SIZE OF AN INTERTIDAL SNAIL

The risk of dislodgment due to hydrodynamic forces on rocky intertidal shores is greater on wave-exposed than on protected shores, and this is believed to represent an important selective force in intertidal communities. For intertidal snails, the probability of dislodgment by a given flow is determined, in part, by shell size and shape, and by the attachment strength of the foot. This study addressed two questions. First, do habitat-specific differences in traits that reduce the risk of dislodgment of an intertidal snail (Littorina obtusata) parallel differences in wave energies? To address this question, I measured variation in (1) shell size (defined as the maximum projected surface area, MPSA); (2) foot size; (3) maximum shear dislodgment force; and (4) tenacity (dislodgment force per unit foot size) of two wave-exposed and three protected snail populations. Second, are habitat-specific differences in foot size, and hence attachment strength, the product of selection or of phenotypic plasticity in response to increased hydrodynamic stress? I conducted field transplant experiments and a laboratory flume experiment that manipulated water velocity to test for plasticity in foot size. Wave-exposed snails exhibited traits that reduce the risk of dislodgment. Their shells were smaller (MPSA) and more squat (shell height relative to MPSA and shell length) than were shells of protected conspecifics. Wave-exposed snails also had larger foot sizes and were thus able to resist greater shear dislodgment forces than protected conspecifics of similar MPSA. Neither wave exposure nor collection site influenced tenacity, indicating that the greater dislodgment forces of wave-exposed snails were due to their larger foot sizes. Assuming that MPSA is proportional to speed-specific drag, which is reasonable for bluff bodies, I found that dislodgment force was proportional to drag. Foot size scaled isometrically with MPSA, and dislodgment force scaled isometrically with foot size, indicating that the risk of dislodgment does not change as snails grow. The field and laboratory flume experiments provide the first clear demonstration that increased hydrodynamic stress induces plastic increases in foot size. Protected snails raised on a wave-exposed shore produced a larger foot than controls raised on their native shore. Selection on the foot size of protected snails raised on both shores was not detected in either habitat. Protected snails also produced larger foot sizes when raised in high-velocity flumes relative to snails raised under low-velocity flow. In contrast, wave-exposed snails exhibited no change in foot size when raised under both high- and low-velocity flow. Hence, variation in the foot size of wave-exposed snails is less flexible than that of protected conspecifics. If natural selection favors plasticity in heterogeneous environments, an asymmetry may be favored as well, especially if there are risks associated with improper adjustments to unpredictable environmental cues.

Rational Equipment Selection Method Based on Soil Conditions

This paper presents a rational approach for evaluating a tractor dozer's capability to excavate a given soil type. A number of empirical charts exist for equipment selection, and a complex formulation based on soil mechanics theory can be applied to predict dislodgement forces. However, there is no current method to compute dislodgement forces based on soil properties and uses the results as a basis for equipment selection. This paper proposes a simplified model based on Coulomb lateral earth pressure theory to predict dislodgement forces for a dozer blade geometry. The results from the simplified model analyses are verified using the more complex existing theory. Based on the results, a set of practical decision charts are developed that can be used by the practitioner to estimate the tractor dozer's ability to excavate various types of soils. I provide an example of how the charts are used to select equipment given a set of natural soil conditions. The methodology proposed, if verified in field tests, could be used to develop a comprehensive series of charts for various equipment models and soil types.

Effect of ultrasonic vibration on post removal in extracted human premolar teeth

The purpose of this study was to determine the effectiveness of ultrasonic vibration in removing Paraposts from extracted teeth. Paraposts were cemented in mandibular premolars to a depth of 9 mm with zinc phosphate cement and the teeth placed in four groups. Group 1 received no vibration. Group 2 received vibration for 4 min, group 3 received vibration for 12 min, and group 4 received vibration for 16 min. Tensile forces were applied to the posts and mean dislodgment forces compared. The mean force (kg) required to dislodge the Parapost in group 1 was 24.92 +/- 1.64 SEM; in group 2, 25.01 +/- 1.80; in group 3, 24.08 +/- 2.29; and in group 4, 12.41 +/- 2.60. There was a significant difference between group 4 and groups 1 to 3 (p = 0.0003). Results of this study indicate that 16 min ultrasonic vibration is an effective method for removing Paraposts from human premolar teeth.

Wettability of denture soft-lining materials

Data on the wettability of long-term resilient denture base lining materials have previously been obtained by static methods. This study determined the dynamic contact angle of a range of long-term resilient lining materials and the surface free energy properties of these materials. This method was a rapid and convenient method for determining wettability parameters. The equilibrium contact angle revealed the Flexor and Novus materials to be the least wetted with the Molloplast-B material, and CoeSuper-Soft material was wetted similarly to Trevalon material. The contact angle hysteresis indicated that all the soft-lining materials tested would improve denture stability under dislodgment forces. The surface-free energies appeared to be similar for all the soft-lining materials but were significantly lower than that of Trevalon.

Resistance to dislodgement: habitat and size-specific differences in morphology and tenacity in an intertidal snail

An organism's ability to persist on wave-swept rocky shores is determined, in part, by its ability to resist dislodgement by waves. Resistance to dislodgement depends on hydrodynamic forces and on the ability of an organism to resist those forces by adhering to the substratum. In intertidal snails the structure upon which waves act is the shell and the structure that adheres is the muscular foot. We quantified: (1) shell size (defined as the maximum projected surface area, MPSA); (2) shell shape; (3) foot area; (4) maximum force to dislodge a snail in shear; and (5) tenacity (force per foot area required to dislodge) of the herbivorous intertidal snail Littorina obtusata. We compared individuals that were collected from habitats characterized by differences in exposure to waves. Wave-exposed snails were smaller (lower average MPSA). In addition, snails at the wave-exposed site were shorter, and had larger foot area and greater dislodgement force than did protected snails of similar MPSA. The greater dislodgement force at the exposed site was due to larger foot area, not to greater tenacity. In fact, sizespecific tenacity of snails from the protected habitat was greater Within each site, shape changed with size. Although shell width, length and MPSA scaled isometrically, shell height did not; longer shells were relatively short. Nor did foot area scale isometrically with MPSA; larger snails had relatively smaller foot areas. Assuming that MPSA is directly proportional to speed-specific drag, which is reasonable for bluff bodies, we found that dislodgement force was proportional to drag. This result suggests that increasing tenacity compensated for underscaling of foot area. In fact tenacity did increase with foot area, although only significantly so at the wave-exposed site.

Quantitation of intercellular binding strength by disruptive centrifugation: Application to the analysis of adhesive interactions between P815 tumour cells and activated macrophages

The cell binding assay described in the present paper provides a quantitative measurement of intercellular binding strengths between P815 mastocytoma cells and macrophages activated for cytotoxicity. Conjugates were submitted to a dislodgement force generated by centrifugation tending to remove radiolabelled probe cells from macrophages adhering to a glass support. Non-specific binding occurred between P815 cells and the glass support, but was negligible on siliconed coverslips. The adhesive interactions macrophage/glass were strong enough to resist up to about 1 X 10(-9) N, the maximal dislodgement force tested. Completion of conjugate formation was reached at 37 degrees C after 45 to 60 min and was inhibited at 4 degrees C or in Ca++-free medium. The strengths of intercellular bonds between P815 cells and macrophages varied from 7 X 10(-11) N (the minimal force applied) to more than 9 X 10(-10) N. No clear-cut separation between weak and strong interactions was observed when the centrifugation step was performed at 4 degrees C. In contrast, when centrifugation was carried out at 37 degrees C, a subpopulation of loosely bound tumour targets could be distinguished from more strongly bound P815 cells.

Adherence of Human Polymorphonuclear Leukocytes to Endothelial Monolayers: Effects of Temperature, Divalent Cations, and Chemotactic Factors on the Strength of Adherence Measured With a New Centrifugation Assay

Polymorphonuclear leukocytes (PMNs) adhere to endothelial cells at sites of acute inflammation. To examine this phenomenon in vitro, we have developed a new assay to measure adherence of PMNs to cultured endothelial cells. Human PMNs were labeled with 111indium-oxine and incubated in microtiter wells with monolayers of either human umbilical vein or bovine aortic endothelial cells. Following incubation, the wells were sealed, inverted, and centrifuged at varying speeds. Results are expressed as the percentage of PMNs added initially that remained attached to the monolayers after being subjected to dislodgment forces (ie, relative centrifugal forces) ranging from 1 to 1,200 g. Adherence of PMNs to endothelial monolayers was temperature dependent, dependent on the concentration of extracellular Mg2+ (but not Ca2+), and enhanced significantly by the chemotactic peptides, N-formyl-methionyl-leucyl-phenylalanine (fMLP) and human C5a. It was found that fMLP and C5a not only increased the number of PMNs that adhered to endothelial cells, but also increased the strength of adherence.