Saliva Film Research Articles

Determinants of Microscale pH in In Situ-Grown Dental Biofilms.

Dental biofilm pH is the most important determinant of virulence for the development of caries lesions. Confocal microscopy-based pH ratiometry allows monitoring biofilm pH with high spatial resolution. Experiments performed on simplified biofilm models under static conditions identified steep pH gradients as well as localized acidogenic foci that promote enamel demineralization. The present work used pH ratiometry to perform a comprehensive analysis of the effect of whole saliva flow on the microscale pH in complex, in situ-grown 48-h and 96-h biofilms (n = 54) from 9 healthy participants. pH was monitored in 12 areas at the biofilm bottom and top, and saliva flow with film thicknesses corresponding to those in the oral cavity was provided by an additively manufactured microfluidic flow cell. Biofilm pH was correlated to the bacterial composition, as determined by 16S rRNA gene sequencing. Biofilm acidogenicity varied considerably between participants and individual biofilms but also between different areas inside one biofilm, with pH gradients of up to 2 units. pH drops were more pronounced in 96-h than in 48-h biofilms (P = 0.0121) and virtually unaffected by unstimulated saliva flow (0.8 mm/min). Stimulated flow (8 mm/min) raised average biofilm pH to near-neutral values but it did not equilibrate vertical and horizontal pH gradients in the biofilms. pH was significantly lower at the biofilm base than at the top (P < 0.0001) and lower downstream than upstream (P = 0.0046), due to an accumulation of acids along the flow path. pH drops were positively correlated with biofilm thickness and negatively with the thickness of the saliva film covering the biofilm. Bacterial community composition was significantly different between biofilms with strong and weak pH responses but not their species richness. The present experimental study demonstrates that stimulated saliva flow, saliva film thickness, biofilm age, biofilm thickness, and bacterial composition are important modulators of microscale pH in dental biofilms.

Protein-induced delubrication: How plant-based and dairy proteins affect mouthfeel

Understanding how certain proteins cause astringency is necessary in order to improve the mouthfeel and popularity of plant-based foods. To this end, we studied protein interactions during oral processes using a PDMS-PDMS interface lubricated by ex-vivo human saliva. Friction measurements and in-contact imaging were implemented, while food consumption was simulated by introducing model plant and animal-based proteins. All but one of the protein samples caused an increase in measured friction and this correlated with astringency ratings from a human taste panel. This is attributed to delubrication as the salivary pellicle is removed, since food proteins interact with salivary proteins thus disrupting their adhesion. This interaction is shown to occur both on the surface and in the bulk of the fluid. However, the debonding of the pellicle requires frictional shear stress (i.e., rubbing) . Food proteins in isolation are themselves shown to be surface-active and form boundary films, which can adhere following removal of the pellicle. The mechanical action of protein particles in the delubrication process was isolated by filtering and shown to account for a moderate (<33%) increase in friction magnitude accompanied by a significant (>90%) increase in frictional noise. The flow and deformation of these particles was also visualised thus demonstrating how the microscale breakdown of food can be studied. • Tribological measurements combined with in-contact fluorescence imaging were used to study the breakdown of the salivary pellicle by food proteins within a simulated tongue-palate interface. • During salivary pellicle formation, salivary proteins agglomerations are trapped at the inlet and act to feed the contact. • Measured friction increase resulting from the introduction of food protein onto a saliva film on PDMS agreed with that from a biological tongue and correlated with astringency ratings from a human taste panel. • Both plant-based and dairy proteins can cause astringency by interacting with saliva proteins thus reducing their adhesion to the surface. This interaction is shown to occur both on the surface and in the bulk of the fluid. • Shear stress is required to remove the weakly bound saliva proteins to cause delubrication. • The role of insoluble food protein particles entrainment in astringency is to cause abrasive wear that increases the average friction coefficient and contributes to significant frictional noise.

Lubrication behavior of ex-vivo salivary pellicle influenced by tannins, gallic acid and mannoproteins

The objective of this study was to investigate the influence of tannins and gallic acid on the salivary lubrication behavior. Furthermore, the effects of pH and mannoproteins in combination with gallic acid on the lubrication of saliva were studied. The addition of gallic acid and tannins were found to increase friction caused by the removal of the saliva film. Tannins resulted in higher friction compared to gallic acid. Lowering pH increased friction of gallic acid mixtures with saliva, due to stronger interactions between gallic acid and saliva. The increased friction caused by gallic acid was inhibited by the addition of mannoproteins due to the hydrogen bond interactions between gallic acid and mannoproteins, thereby decreasing the complex formation between gallic acid and salivary proteins. A correlation of 0.96 was found between the hydrodynamic diameter of the aggregate and the delta friction suggesting that the formation of aggregates determined the lubrication behavior.

In situ oral lubrication and smoothness sensory perception influenced by tongue surface roughness.

The human tongue is important in the oral processing of food and in sensory perception. Tongue topography could influence delicate differences in sensory perception. It is hypothesized that tongue surface roughness could alter oral lubrication status and affect perception of smoothness. Fifteen participants with varying levels of tongue surface roughness were recruited and tested. Participants' in situ oral lubrication status without and after consumption of fluid food (milk with varying fat content and maltodextrin solutions with different shear viscosities) was measured. Participants' smoothness sensory scores were also recorded. The in situ friction coefficient (0.299-1.505) was significantly positively correlated with tongue-surface roughness (54.6-140.0μm) in all types of test fluid samples across participants. Oral lubrication was significantly decreased when participants consumed the test fluid samples compared with no liquid food consumption, for all test fluid sample types (P < 0.05). No significant differences in in situ friction coefficient were found after participants consumed different test fluid samples, and this was mainly attributed to the limited quantities of fluid residuals in the oral cavity after expectoration. Participants whose tongue surface roughness differed did not exhibit significant differences in smoothness perception with different test fluid samples. Tongue surface roughness has a strong impact on in situ oral lubrication, and fluid food intake reduces in situ oral lubrication significantly. Saliva film and tongue surface roughness might play greater roles in oral lubrication and smoothness sensory perception if fluid is expectorated after consumption. The association between oral physiology and texture perception still needs further elucidation. © 2021 Society of Chemical Industry.

Ratiometric imaging of extracellular pH in Streptococcus mutans biofilms exposed to different flow velocities and saliva film thicknesses

ABSTRACT Introduction: Fluid flow has a prominent influence on the metabolism of surface-attached biofilms. Dental biofilms are covered by a thin saliva film that flows at different rates in different locations under stimulated and unstimulated conditions. Methods:The present study employed pH ratiometry to study the impact of different flow velocities, saliva film thicknesses and saliva concentrations on microscale pH developments in Streptococcus mutans biofilms of different age. Results:While saliva flow at a velocity of 0.8 mm/min (unstimulated flow) had little impact on biofilm pH, stimulated flow (8 mm/min; 80 mm/min) affected vertical pH gradients in the biofilms and raised the average pH in 48-h biofilms, but not in 72-h and 168-h biofilms. The saliva film thickness had a strong impact on biofilm pH under both static and dynamic conditions. pH drops were significantly higher in biofilms exposed to a thin saliva film (≤ 50 µm) than a thick saliva film (> 50 µm). pH drops in the biofilms were also strongly dependent on the saliva concentration and thus the buffer capacity of the salivary medium. For 48-h and 72-h biofilms, but not for 168-h biofilms, pH drops in distinct microenvironments were more pronounced when the local biofilm thickness was high.

Lepidopteran mouthpart architecture suggests a new mechanism of fluid uptake by insects with long proboscises

Proboscises of many fluid-feeding insects share a common architecture: they have a partially open food canal along their length. This feature has never been discussed in relation to the feeding mechanism. We formulated and solved a fluid mechanics model of fluid uptake and estimated the time required to completely fill the food canal of the entire proboscis through the openings along its length. Butterflies and moths are taken as illustrative and representative of fluid-feeding insects. We demonstrated that the proposed mechanism of filling the proboscis with fluid through permeable lengthwise bands, in association with a thin film of saliva in the food canal, offers a competitive pathway for fluid uptake. Compared with the conventional mechanism of fluid uptake through apically restricted openings, the new mechanism provides a faster rate of fluid uptake, especially for long-tongued insects. Accordingly, long-tongued insects with permeable lengthwise bands would be able to more rapidly exploit a broader range of liquids in the form of films, pools, and discontinuous columns, thereby conserving energy and minimizing exposure to predators, particularly for hovering insects.

Tribological Behaviour of Ti or Ti Alloy vs. Zirconia in Presence of Artificial Saliva

Abutment is the transmucosal component in a dental implant system and its eventual appearance has a major impact on aesthetics: use of zirconia abutments can be greatly advantageous in avoiding this problem. Both in the case of one and two-piece zirconia abutments, a critical issue is severe wear between the zirconia and titanium components. High friction at this interface can induce loosening of the abutment connection, production of titanium wear debris, and finally, peri-implant gingivitis, gingival discoloration, or marginal bone adsorption can occur. As in vivo wear measurements are highly complex and time-consuming, wear analysis is usually performed in simulators in the presence of artificial saliva. Different commercial products and recipes for artificial saliva are available and the effects of the different mixtures on the tribological behaviour is not widely explored. The specific purpose of this research was to compare two types of artificial saliva as a lubricant in titanium–zirconia contact by using the ball on disc test as a standard tribological test for materials characterisation. Moreover, a new methodology is suggested by using electrokinetic zeta potential titration and contact angle measurements to investigate the chemical stability at the titanium–lubricant interface. This investigation is of relevance both in the case of using zirconia abutments and artificial saliva against chronic dry mouth. Results suggest that an artificial saliva containing organic corrosion inhibitors is able to be firmly mechanically and chemically adsorb on the surface of the Ti c.p. or Ti6Al4V alloy and form a protective film with high wettability. This type of artificial saliva can significantly reduce the friction coefficient and wear of both the titanium and zirconia surfaces. The use of this type of artificial saliva in standard wear tests has to be carefully considered because the wear resistance of the materials can be overestimated while it can be useful in some specific clinical applications. When saliva is free from organic corrosion inhibitors, wear occurs with a galling mechanism. The occurrence of a super-hydrophilic saliva film that is not firmly adsorbed on the surface is not efficient in order to reduce wear. The results give both suggestions about the experimental conditions for lab testing and in vivo performance of components of dental implants when artificial saliva is used.

An experimental model to investigate the biomechanical determinants of pharyngeal mucosa coating during swallowing

The development of innovative experimental approaches is necessary to gain insights in the complex biomechanics of swallowing. In particular, unraveling the mechanisms of formation of the thin film of bolus coating the pharyngeal mucosa after the ingestion of liquid or semi-liquid food products is an important challenge, with implication in dysphagia treatment and sensory perceptions.The aim here is to propose an original experimental model of swallowing (i) to simulate the peristaltic motions driving the bolus from the oral cavity to the esophagus, (ii) to mimic and vary complex physiological variables of the pharyngeal mucosa (lubrication, deformability and velocity) and (iii) to measure the thickness and the composition of the coatings resulting from bolus flow. Three Newtonian glucose solutions were considered as model food boli, through sets of experiments covering different ranges of each physiological parameter mimicked.The properties of the coatings (thickness and dilution in saliva film) were shown to depend significantly on the physical properties of food products considered (viscosity and density), but also on physiological variables such as lubrication by saliva, velocity of the peristaltic wave, and to a lesser extent, the deformability of the pharyngeal mucosa.The biomechanical peristalsis simulator developed here can contribute to unravel the determinants of bolus adhesion on pharyngeal mucosa, necessary both for the design of alternative food products for people affected by swallowing disorders, and for a better understanding of the dynamic mechanisms of aroma perception.

Prosthodontic Management of Xerostomic Patient: A Technical Modification.

Xerostomia is often a contributing factor in both minor and serious health problems. It can affect nutrition and dental as well as psychological health. Common problems faced by such patients are glossitis, mucositis, angular cheilitis, dysgeusia, and difficulty in chewing and swallowing. One of the major problems associated with xerostomic patients is the poor tolerance and retention of removable dental prostheses because of thin dry atrophic mucosa and lack of a saliva film. This paper describes a new technique of incorporating a salivary reservoir in the maxillary complete denture. The salivary reservoir fabricated by this technique provided good lubrication of the oral tissues and was easily cleansed by the wearer and was fabricated from routine denture materials.

Composition of Enamel Pellicle from Dental Erosion Patients

Oral health is dependent upon a thin mobile film of saliva on soft and hard tissues. Salivary proteins adhere to teeth to form the acquired enamel pellicle which is believed to protect teeth from acid erosion. This study investigated whether patients suffering diet-induced dental erosion had altered enamel pellicles. Thirty patients suffering erosion were compared to healthy age-matched controls. Subjects wore a maxillary splint holding hydroxyapatite and human enamel blocks for 1 h. The acquired enamel pellicle was removed from the blocks and compared to the natural incisor pellicle. Basic Erosive Wear Examination scores confirmed that dental erosion was present in erosion patients and absent from healthy age-matched controls. Erosion patients had half the amount of proteins (BCA assay) within the acquired pellicle forming on splint blocks compared to normal controls (p < 0.05). In particular, statherin, a calcium-binding protein, was 35% less abundant (p < 0.05). Calcium concentration within the acquired pellicle was also reduced by 50% in erosion patients (p < 0.001). In contrast, the natural pellicle on the incisor had similar amounts of total protein in erosion patients and healthy controls. In summary, the formation of new acquired pellicles on surfaces was reduced in erosion patients, which may explain their greater susceptibility to acid erosion of teeth.

Salivary secretion: mechanism and neural regulation.

Maintenance of a film of saliva on oral surfaces is dependent upon nerve-mediated, reflex salivary gland secretion. Afferent signalling arises from taste, olfaction and mastication and is modified by signalling from other centres in the central nervous system before efferent signals are delivered to salivary glands in autonomic nerves. Salivary fluid secretion is largely dependent upon cholinergic signalling from parasympathetic nerves whilst the protein content of saliva is additionally dependent upon signalling by neuropeptides and, in the major (parotid, submandibular and sublingual) salivary glands, by sympathetic nerves and the release of noradrenaline. There have been significant recent advances in our understanding of the membrane transport proteins involved in intracellular calcium signalling in salivary acinar cells in response to nerve stimulation and of the ion transport proteins responsible for acinar cell secretion of saliva. Salivary glands retain an ability to regenerate following extreme atrophy, and autonomic nerves have an important role in both gland development and maintenance of long-term normal function. Continued advances in the understanding of the nerve-mediated regulation of salivary glands should help in the development of strategies for preventing chronic oral dryness resulting from drugs or atrophic disease associated with inflammation and irradiation.

Fabrication of a functional palatal saliva reservoir by using a resilient liner during processing of a complete denture

One of the major problems associated with xerostomic patients is the poor tolerance and retention of removable dental prostheses because of thin dry atrophic mucosa and lack of a saliva film. The use of salivary substitutes can improve lubrication, provide irrigation for dry mucosa, provide significant relief from symptoms, and also improve the retention of removable prostheses. These substitutes can be delivered over prolonged periods by using saliva reservoirs in the prosthesis itself. This report describes a novel technique for the fabrication of a functional palatal saliva reservoir in edentulous xerostomic patients. The technique uses a resilient liner in the floor of the reservoir. The tongue presses against the liner during swallowing, resulting in flow of saliva substitute through a hole in the anterior part of the floor of the reservoir.

Effect of Salivary Factors on the Susceptibility of Hydroxyapatite to Early Erosion

Objective: Salivary pellicle is known to reduce the erosion of enamel and differences in the level of protection exist between individual saliva sources, but which parameters or components are important is not known. The focus of this study was to investigate the relationship between saliva parameters and early erosion of hydroxyapatite (HAp) with an in situ grown saliva film. Methods: Twenty-eight volunteers carried two HAp and one porcelain discs in their buccal sulcus for 1.5 h. Next, the discs covered with pellicle and the attached saliva film were exposed extraorally to 50 mM (pH = 3) citric acid for 2 min and unstimulated and stimulated saliva was collected. Calcium loss from HAp after erosive challenge was measured, corrected for calcium loss from pellicle on porcelain discs and averaged. Several salivary parameters were analysed. Pearson’s linear correlation and multiple regression analysis were used to study the relation between saliva parameters and HAp erosion. Results: Significant correlations were found between HAp erosion and the concentration of phosphorus in unstimulated saliva (r = 0.40, p = 0.03) and between HAp erosion and the concentration of sodium (r = –0.40, p = 0.03), chloride (r = –0.47, p = 0.01), phosphorus (r = 0.45, p = 0.01) and flow (r = –0.39, p = 0.04) of stimulated saliva. Multivariate analysis revealed a significant role in the HAp erosion for sodium, urea, total protein, albumin, pH and flow of unstimulated saliva and for sodium, potassium, urea, and phosphorus of stimulated saliva. Conclusions: Several salivary parameters are associated with the susceptibility of HAp to erosion.

A biomechanical model of swallowing for understanding the influence of saliva and food bolus viscosity on flavor release

After swallowing a liquid or a semi-liquid food product, a thin film responsible for the dynamic profile of aroma release coats the pharyngeal mucosa. The objective of the present article was to understand and quantify physical mechanisms explaining pharyngeal mucosa coating. An elastohydrodynamic model of swallowing was developed for Newtonian liquids that focused on the most occluded region of the pharyngeal peristaltic wave. The model took lubrication by saliva film and mucosa deformability into account. Food bolus flow rate and generated load were predicted as functions of three dimensionless variables: the dimensionless saliva flow rate, the viscosity ratio between saliva and the food bolus, and the elasticity number. Considering physiological conditions, the results were applied to predict aroma release kinetics. Two sets of conditions were distinguished. The first one was obtained when the saliva film is thin, in which case food bolus viscosity has a strong impact on mucosa coating and on flavor release. More importantly, we demonstrated the existence of a second set of conditions. It was obtained when the saliva film is thick and the food bolus coating the mucosa is very diluted by saliva during the swallowing process and the impact of its viscosity on flavor release is weak. This last phenomenon explains physically in vivo observations for Newtonian food products found in the literature. Moreover, in this case, the predicted thickness of the mix of food bolus with saliva coating the mucosa is approximately of 20 μm; value in agreement with orders of magnitude found in the literature.

Lubricating properties of human whole saliva as affected by β-lactoglobulin

The effect of β-lactoglobulin (β-LG) at pH 3.5 and 7.0 on lubricating property of saliva as related to astringency perception was investigated using tribology. Saliva was adsorbed onto surfaces of a rotating poly dimethylsiloxane (PDMS) ball and disc to form a film under conditions that mimic the rubbing contacts in the oral cavity ( Bongaerts, Rossetti, & Stokes, 2007) and the lubricity of saliva films upon exposure to astringent compounds was measured. While addition of non-astringent β-LG at pH 7.0 slowly increased friction of saliva film between tribopair surfaces, β-LG at pH 3.5 rapidly increased the friction coefficients of saliva, similar to other astringent compounds (epigallocatechin gallate and alum). This supports the hypothesis that astringency of β-LG arises from the loss of lubrication of saliva which is in agreement with the well-accepted astringency model of polyphenols. Increasing β-LG concentration at pH 3.5 (0.5–10% w/w) caused a rapid increase in friction coefficient; however, at the highest protein concentration, the friction coefficient, although higher than observed for water, was below the values observed for the lower protein concentrations. This suggests that static tribology testing is different from the dynamic in-mouth system such that a simple relationship between friction and sensory astringency cannot be found for all conditions.

A lubrication analysis of pharyngeal peristalsis: Application to flavour release

After eating a liquid or a semi-liquid food product, a thin film responsible for the dynamic profile of aroma release coats the pharyngeal mucosa. The aim of this article was to analyse the fluid mechanics of pharyngeal peristalsis and to develop a simple biomechanical model in order to understand the role of saliva and food bolus viscosity on the coating of pharyngeal mucosa. We began by analysing the physiology and the biomechanics of swallowing in order to determine relevant model assumptions. This analysis of the literature clarified the types of mechanical solicitations applied on the food bolus. Moreover, we showed that the pharyngeal peristalsis in the most occluded region is equivalent to a forward roll coating process, the originality of which is lubrication by a film of saliva. A model based on the lubrication theory for Newtonian liquids was developed in dimensionless form. The parametric study showed the strong influence of relative saliva thickness on the food bolus coating. A specific experimental device was designed that confirms the model predictions. Two sets of conditions that depend on the relative thickness of saliva were distinguished. The first is characterised by a relatively thin film of saliva: food bolus viscosity has a strong impact on mucosa coating. These phenomena are well represented by the model developed here. The second is obtained when the saliva film is relatively thick: hydrodynamic mixing with saliva, interdiffusion or instabilities may govern mucosa coating. Finally, these results were extrapolated to determine the influence of food bolus viscosity on the dynamic profile of flavour release according to physiological parameters.

Liquid Chromatographic Analysis of Chlorhexidine at Specific Sites in The Saliva Film After Application of a Tooth-Bonded Delivery System

Abstract This paper describes the use of an HPLC assay to investigate whether a chlorhexidine-containing tooth-bonded delivery system produces antibacterial concentrations in the surrounding saliva film. Saliva (<1.5 μl) was collected from ten sites in the mouth at eight times over a period of four days following application of tooth-bonded delivery systems in two dentally healthy subjects. Analysis of chlorhexidine concentrations in these samples showed a non-uniform distribution of chlorhexidine in the saliva film. Antibacterial concentrations were produced in the area immediately surrounding the delivery system (20 ± 4 and 28 ± 12 μg/ml for Subjects One and Two respectively) whereas concentrations at more distant sites remained low.

An Analysis of Factors Influencing Diffusion from Dental Plaque into a Moving Film of Saliva and the Implications for Caries

Recent studies have indicated that saliva in the mouth is present as a film only about 0.1 mm thick, and that this film moves at different rates (about 0.8 to 7.6 mm/min) in different regions of the oral cavity. The clearance rates of KCl, as a model diffusant, from agarose gels at different sites in the mouth have also been found to vary markedly, and it has been proposed that these variations are related to differences in the velocity of the salivary film. A computer model has been developed for prediction of clearance half-times for substances diffusing from plaque of variable dimensions into a film of fluid 0.1 mm thick, moving at different velocities. The results show that over the range of velocities calculated to occur in the mouth, the clearance half-times are directly related to the length of plaque over which the fluid passes, and inversely related to the salivary film velocity. The predictions of the model are in good agreement with experimental results from a physical model. Tests were made of the predicted effect of salivary film velocity on the shape of the pH curve initiated by exposure of plaque to a saturated sucrose solution, followed by normal salivary clearance. With a low salivary film velocity, the fall in pH was greater and more prolonged.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation into the wettability of poly(methylmethacrylate) in vivo.

Abstract One of the principal sources of complete denture retention is the adhesive action of the thin film of saliva between the palate and the surface of the denture. To produce this adhesive effect the saliva must flow easily over the denture surface to ensure its wetting. It has been observed that the retention of a denture base increases with time of wearing. Also it has been demonstrated that there is an immediate increase in retention when a complete denture base is treated with a ‘molecular bonding agent’ (MBA) such as silicon tetrachloride. The following questions are therefore raised: At what rate and to what extent does wettability of a PMMA specimen increase in vivo? Is routine use of an MBA indicated or desirable? The object of this study was to investigate the wettability of poly( methylmethacrylate) in vivo. Standard squares of denture base PMMA were prepared A square was inserted in each of 10 denture bases after final processing and before their insertion into the mouth. A microscope with a goniometer eye-piece was used to measure the contact angles of distilled water on the specimen after various intervals of time in the mouth to monitor wettability. There was a significant increase in wettability after 24 h intra oral exposure. No further increase was detected after 7 d. In vitro experiments with fresh saliva indicate that the increase in wettability occurred within 5 min. The increase was less than that reported by using MBA. No correlation between water absorption and wettability was detected. It was concluded that the wettability of PMMA increased almost immediately on contact with saliva, presumably due to the formation of a film of a salivary component(s), and that water absorption did not cause a measurable increase in wettability.

Base Retention

The role of saliva in the retention of denture bases is discussed in terms of atmospheric pressure, surface tension, and viscosity. There is little evidence that atmospheric pressure is a significant factor. Surface tension contributes to retention only if the film of saliva interposed between the denture base and the underlying tissue is isolated and does not extend beyond the denture borders. This occurs only when the average patient has been breathing through his mouth and evaporation of excess saliva has occurred. In the usual clinical situation, the interposed film of saliva is continuous with the saliva covering the oral mucosa. In this more general situation, the saliva acts as a viscous adhesive and provides a force that resists vertical displacement. Using this explanation, the factors that increase this retentive force are: 1. Close adaptation of the denture base to the supporting tissue. 2. High viscosity of saliva. 3. Short application of displacement forces. 4. Adequate wetting of the denture base material by saliva. This retentive force is especially important in the case of removable partial dentures with precision attachments since the precision attachment minimizes lateral displacement forces which are poorly resisted by viscous forces.