Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction

When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.</p

Local Shearing Force Measurement during Frictional Sliding Using Fluorogenic Mechanophores

[Image: see text] When two macroscopic objects touch, the real contact typically consists of multiple surface asperities that are deformed under the pressure that holds the objects together. Application of a shear force makes the objects slide along each other, breaking the initial contacts. To investigate how the microscopic shear force at the asperity level evolves during the transition from static to dynamic friction, we apply a fluorogenic mechanophore to visualize and quantify the local interfacial shear force. When a contact is broken, the shear force is released and the molecules return to their dark state, allowing us to dynamically observe the evolution of the shear force at the sliding contacts. We find that the macroscopic coefficient of friction describes the microscopic friction well, and that slip propagates from the edge toward the center of the macroscopic contact area before sliding occurs. This allows for a local understanding of how surfaces start to slide

Nonmonotonic Friction due to Water Capillary Adhesion and Hydrogen Bonding at Multiasperity Interfaces

Capillary adhesion due to water adsorption from the air can contribute to friction, especially for smooth interfaces in humid environments. We show that for multiasperity (naturally oxidized) Si-on-Si interfaces, the friction coefficient goes through a maximum as a function of relative humidity. An adhesion model based on the boundary element method that takes the roughness of the interfaces into account reproduces this nonmonotonic behavior very well. Remarkably, we find the dry friction to be significantly lower than the lubricated friction with macroscopic amounts of water present. The difference is attributed to the hydrogen-bonding network across the interface. Accordingly, the lubricated friction increases significantly if the water is replaced by heavy water (D2O) with stronger hydrogen bonding

Utilização da nebulização e ventilação forçada sobre o desempenho e a temperatura da pele de suínos na fase de terminação Effect of nebulization and artificial ventilation on the performance and skin temperature of finishing swine

O experimento foi realizado com o objetivo de avaliar os efeitos da nebulização e ventilação forçada sobre o desempenho e a temperatura da pele de suínos na fase de terminação. Utilizaram-se 137 animais (Landrace x Large White x Duroc) machos castrados e fêmeas, distribuídos em um delineamento de blocos ao acaso, com dois tratamentos de oito repetições por tratamento, com média de oito animais/baia. Os tratamentos utilizados foram: T1 = sem nebulização de água e ventilação forçada e T2 = com nebulização de água e ventilação forçada. Durante o período experimental, a temperatura média observada foi de 26,32ºC (variando de 23,32 a 30,72ºC) e a umidade relativa do ar média, de 64,5%. Os resultados não demonstraram diferenças significativas entre os tratamentos utilizados para as variáveis ganho de peso médio diário, consumo de ração médio diário e conversão alimentar. Entretanto, a nebulização de água associada à ventilação forçada apresentou menores valores de temperatura da pele, indicando melhor conforto aos animais. Não é necessário o uso de nebulização e ventilação forçada em instalações para suínos na fase de terminação, pois, apesar da melhoria no conforto dos animais (temperatura da pele), não trouxe benefício sobre o desempenho dos mesmos.<br>An experiment was carried out to evaluate the effects of nebulization and artificial ventilation on the performance and skin temperature of finishing swine. Castrated males and females (n = 137; Landrace x Large White x Duroc) were allotted to two treatments by using a radomized bolck design, with eight replicates: T1 = pens without either nebulization or artificial ventilation; T2 = pens with both nebulization and artificial ventilation. There was an average of eight animals per each pen. During the experimental period, average air temperature was 26.32ºC (from 23.32 to 30.72ºC) and humidity, 64.5%. No signifgicant effects of treatments on average daily weight gain, average daily feed intake and feed conversion were observed. Water nebulization and ventilation improved animal confort (skin temperature). The use of nebulization and ventilation is not necessary for finishing swine because, although there was less stress to animals, such management strategy did not increase their performance