Stick and Slip Behaviour of Confined Oligomer Melts under Shear. A Molecular-Dynamics Study.

The flow behaviour of melts of short chains, confined in molecularly thin Couette flow geometries, is studied with molecular-dynamics simulations. The effect of wall attraction and confinement on the density and velocity profiles is analysed. In these highly inhomogeneous films, a strong correlation between the density and velocity profile is found. Sticking of the interfacial layer on the wall and slip on the wall and inside the film is manifested by changes in the velocity profile. The location of the slip is determined by the strength of the wall attraction.

Function of masticatory muscles during the initial phase of activator treatment

The function of masticatory muscles and the development of the bite force were studied in 15 children before and during the first six months of the treatment of distal occlusion with an activator. Electromyographic recordings of the activity of the anterior and posterior portions of the temporal muscle, the masseter muscle and the anterior belly of the digastric muscle were made bilaterally in the rest position of the mandible and with the activator inserted. Recordings were also made during chewing and swallowing of apple and peanuts and during chewing of chewing gum as well as during maximal bite in the intercuspal position and on the activator. The mean voltage amplitude, the duration and the coordination of the activity were analysed. The maximum bite force was measured at the first molars and at the incisors. In most of the children, the clinical improvement was rapid as evident from a decrease in overjet and in the ANB angle as well as a change in the molar relation. The bite force measured at the incisors increased during the period of treatment. The muscle activity in the rest position was low and was the same with or without the activator. Thus, insertion of the activator did not increase the muscle activity. The activity of the posterior portion of the temporal muscle in the rest position was comparatively high at the start of treatment but decreased during the period of observation. The activity of the masseter and temporal muscles during maximal bite and chewing was influenced by the occlusal instability created during the course of treatment. The decrease of the postural activity of the posterior temporal muscle may reflect an adaptation to a new mandibular positio

A pilot study of the effect of masticatory muscle training on facial growth in long-face children

Daily chewing of a tough chewing material consisting of resin from a pine tree (Mastix from the island of Chios, Greece) was instituted in 13 children (aged 7-12 years) with long-face morphology. The chewing exercise therapy was maintained for one year and aimed at revealing the possibility of strengthening the masticatory muscles and influencing facial growth. Masticatory muscle strength was monitored by measurement of bite force and electromyo-graphic recording of the activity of the anterior temporal and masseter muscles during biting and chewing. The facial morphology was recorded with profile cephalograms and dental casts. During the one-year experimental period, there was a significant increase in bite force and muscle activity during maximal bite. The change was already evident at the first control recording 3 months after the start of the chewing exercise therapy. The muscle activity during chewing of apple and peanuts did not change significantly. The facial growth was characterized by anterior mandibular rotation in 9 of 12 cases while a posterior rotation occurred in 2 cases. The anterior rotation was, on average, 2.5 degrees and thus considerably greater than would be expected during normal growth. There were no signs of reduced vertical growth of the maxilla, a reduced rate of molar eruption or increased growth of the mandibl

Function of masticatory muscles during the initial phase of activator treatment

The function of masticatory muscles and the development of the bite force were studied in 15 children before and during the first six months of the treatment of distal occlusion with an activator. Electromyographic recordings of the activity of the anterior and posterior portions of the temporal muscle, the masseter muscle and the anterior belly of the digastric muscle were made bilaterally in the rest position of the mandible and with the activator inserted. Recordings were also made during chewing and swallowing of apple and peanuts and during chewing of chewing gum as well as during maximal bite in the intercuspal position and on the activator. The mean voltage amplitude, the duration and the coordination of the activity were analysed. The maximum bite force was measured at the first molars and at the incisors. In most of the children, the clinical improvement was rapid as evident from a decrease in overjet and in the ANB angle as well as a change in the molar relation. The bite force measured at the incisors increased during the period of treatment. The muscle activity in the rest position was low and was the same with or without the activator. Thus, insertion of the activator did not increase the muscle activity. The activity of the posterior portion of the temporal muscle in the rest position was comparatively high at the start of treatment but decreased during the period of observation. The activity of the masseter and temporal muscles during maximal bite and chewing was influenced by the occlusal instability created during the course of treatment. The decrease of the postural activity of the posterior temporal muscle may reflect an adaptation to a new mandibular position

Effects of diet consistency on mandibular growth. A review

Το παρόν άρθρο είναι μια ανασκόπηση που διαπραγματεύεται την επίδραση της σύστασης της διατροφής στην μορφολογία της κάτω γνάθου. Πολλές δημοσιευμένες εργασίες έχουν επικεντρωθεί στη μελέτη της σχέσης της μασητικής λειτουργίας και της αύξησης της κάτω γνάθου, εξαιτίας της θεώρησης ότι η αύξηση της κάτω γνάθου εξαρτάται απο τις φορτίσεις που ασκούνται απο τους μασητήριους μύες. Επιπρόσθετα ο σύγχρονος τρόπος διατροφής με μαλακές τροφές έχει ενοχοποιηθεί για την αύξηση των ορθοδοντικών ανωμαλιών. Ακόμα και στους επίμυς η διατροφή με μαλακή τροφή θεωρείται ένας απο τους παράγοντες που συντελεί σε ανωμαλίες της σύγκλεισης. Όλες οι δημοσιευμένες εργασίες είναι πειραματικές, κυρίως σε τρωκτικά ζώα, επειδή είναι αδύνατον να εφαρμοσθούν ανάλογες μελέτες σε ανθρώπους σε σύντομο χρονικό διάστημα. Οι περισσότερες πειραματικές μελέτες συμπεραίνουν πως οι μασητικές φορτίσεις επηρεάζουν την οστική μάζα, τη ποσότητα του οστού, τη πυκνότητα του οστού, το μήκος και το πλάτος του οστού, το βαθμό της επιμετάλωσης, την γενετική έκφραση, και την ανοσοιστοχημική αντίδραση του κολλαγόνου, όπως και τη δράση των χονδροκυττάρων στο χόνδρο του κονδύλου της κάτω γνάθου. Έχει διατυπωθεί πως το βάρος και η πυκνότητα των γνάθων επίμυων που είχαν διατραφεί με μαλακή τροφή ήταν μικρότερα απο τα αντίστοιχα των ζώων που είχαν διατραφεί με σκληρή τροφή. Επίσης οι γνάθοι και οι κόνδυλοι επίμυων που έχουν διατραφεί με μαλακή τροφή ήταν μικρότεροι σε μέγεθος και παρουσιάζουν μικρότερη πυκνότητα σε σύγκριση με τις γνάθους και τους κονδύλους των ζώων ελέγχου. Επιπρόσθετα το μήκος και το πλάτος των κονδύλων των ζώων που είχαν διατραφεί με μαλακή τροφή ήταν μικρότερα απο τα αντίστοιχα των κονδύλων των ζώων που είχαν διατραφεί με σκληρή τροφή. Η μαλακή διατροφή διαπιστώθηκε ότι επηρεάζει τέλος το βαθμό επιμετάλλωσης όπως και τη δράση των χονδροκυττάρων στο χόνδρο.This article is a review that focuses on the diet consistency and how this affects mandibular morphology. Various published studies focused on the relationship between mastication and growth of the mandible because it is considered that mandibular growth is dependent on the loads exerted by the function of the masticatory muscles. Moreover it has been pointed out that the increase of orthodontic anomalies is due to the modern softer diet. Even in rats, soft diet is one of the factors causing malocclusions. All of the studies have been experimental, mainly in rodents, since this research is impossible to be applied on humans in a short period of time. Most experimental studies suggested that occlusal loading affects bone mass, bone amount, bone density, the length and the width of the bone, the degree of mineralization, the genetic expression, the collagen immunoreaction and the chondrocytes action on the cartilage. It is stated that bone volumes and thickness of the mandible of rats fed with soft diet were smaller when compared to animals fed with hard diet. Also the mandibles and condyles were smaller and less dense in the rats of soft diet as compared to controls. Furthermore the length and the width of the condyle in the soft diet group of animals were smaller as compared to the condyle of the hard diet group of animals. Soft diets affect also the degree of mineralization, and the action of the chondrocytes on the cartilage

Molecular dynamics of flows in the Knudsen regime

Novel technological applications often involve fluid flows in the Knudsen regime in which the mean free path is comparable to the system size. We use molecular dynamics simulations to study the transition between the dilute gas and the dense fluid regimes as the fluid density is increased.Comment: REVTeX, 15 pages, 4 EPS figures, to appear in Physica

Adsorption-desorption kinetics in nanoscopically confined oligomer films under shear

The method of molecular dynamics computer simulations is employed to study oligomer melts confined in ultra-thin films and subjected to shear. The focus is on the self-diffusion of oligomers near attractive surfaces and on their desorption, together with the effects of increasing energy of adsorption and shear. It is found that the mobility of the oligomers near an attractive surface is strongly decreased. Moreover, although shearing the system forces the chains to stretch parallel to the surfaces and thus increase the energy of adsorption per chain, flow also promotes desorption. The study of chain desorption kinetics reveals the molecular processes responsible for the enhancement of desorption under shear. They involve sequences of conformations starting with a desorbed tail and proceeding in a very fast, correlated, segment-by-segment manner to the desorption of the oligomers from the surfaces.

A Molecular-Dynamics Study

Abstract. -The flow behaviour of melts of short chains, confined in molecularly thin Couette flow geometries, is studied with molecular-dynamics simulations. The effect of wall attraction and confinement on the density and velocity profiles is analysed. In these highly inhomogeneous films, a strong correlation between the density and velocity profile is found. Sticking of the interfacial layer on the wall and slip on the wall and inside the film is manifested by changes in the velocity profile. The location of the slip is determined by the strength of the wall attraction. Even though the macroscopic phenomena of friction, lubrication and adhesion have been studied for a long time now, their molecular mechanisms have yet to be unveiled. Recent novel experimental techniques such as the surface forces apparatus together with the scanning probe microscopies are capable to give a vast amount of information on the nanometer level; these combined with computer simulations will provide further insight into the nanoscopic dynamics of friction, lubrication and adhesion. The structural and dynamical properties of ultrathin confined films between atomically flat surfaces undergoing shear are studied here with molecular-dynamics simulations. Simulations are performed on a fluid of short chains in a planar Couette flow geometry realized by confining the polymer system in the x-direction between two parallel planar f.c.c. (111) planes. Periodic boundary conditions are imposed in the other two directions. The Couette flow is introduced by moving the two walls with equal and constant velocities towards opposite directions keeping the wall-to-wall distance constant. In this way, a steady shear rate is introduced with the direction of flow parallel to the x-axis and the velocity gradient parallel to the x-axis, i.e. normal to the walls El]. The chains consist of six segments which are connected in a linear freely joined topology. Of course, the model is not expected to capture the real monomer response and segments correspond to several chemical monomers [2]. The segments of the same chain as well as segments belonging to different chains interact via a pairwise purely repulsive, shifted an