On the question of whether lubricants fluidize in stick–slip friction

Intermittent sliding (stick–slip motion) between solids is commonplace (e.g., squeaking hinges), even in the presence of lubricants, and is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick). Using a surface force balance, we measure how the thickness of molecularly thin, model lubricant films (octamethylcyclotetrasiloxane) varies in stick–slip sliding between atomically smooth surfaces during the fleeting (ca. 20 ms) individual slip events. Shear fluidization of a film of five to six molecular layers during an individual slip event should result in film dilation of 0.4–0.5 nm, but our results show that, within our resolution of ca. 0.1 nm, slip of the surfaces is not correlated with any dilation of the intersurface gap. This reveals that, unlike what is commonly supposed, slip does not occur by such shear melting, and indicates that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes.Intermittent sliding (stick–slip) of solids in contact is an everyday effect, such as in the squeak of hinges or the music of violins, when the bow slides past the strings, or, at a different scale, in earthquakes (where tectonic plates slide past each other). Such solid sliding is a major cause of frictional dissipation, and can persist even in the presence of lubricants (1). At a nanotribological level, surface force balance (SFB) measurements, supported by theory and computer simulations, have shown that when simple organic liquids are confined between atomically smooth, solid (mica) surfaces to films thinner than some six to eight molecular layers, they may become solid-like, and are often layered (2–14). Subsequent sliding of the surfaces across such films when they are subjected to shear may then take place via stick–slip motion (15, 16). During the stick part, the surfaces are in rigid contact until the shear force between them exceeds the static friction, at which point they slip rapidly past each other (relaxing the shear stress) and then stick again, in a repeating cycle. The issue of how the confined (lubricant) layer progressively yields and then becomes rigid again during such stick–slip sliding has been intensely studied over the past several decades, not least because a better understanding may result in improved lubrication approaches.The molecular basis of the stick–slip cycle in sheared solid-like lubricant films as described above is not well understood (17–28). This is at least in part because, experimentally, it is very challenging to capture what happens to the lubricant layer during the fleeting, individual slip events taking place in the nanometrically confined film. Even when measured under controlled conditions, as in the SFB, these slip events are not only of very short duration [ca. 20 ms (18)] but generally occupy only a tiny fraction of the stick–slip cycle, with the surfaces in nonsliding contact (stick) for almost the entire cycle period. For this reason, much of our understanding has been derived from theoretical modeling and computer simulation studies (17, 19–25, 27–29). Classically, these almost all suggest that the stick–slip motion involves periodic shear melting transitions and resolidification of the film as it undergoes transition between solid-like and liquid-like phases during sliding. Even where there is some disagreement in the model details [for example, on the precise mechanism by which the films solidify at the end of the slip (22, 25)], they maintain the essential idea of fluidization of the lubricant layers during the slip part of the stick–slip cycle. In the shear-induced solid to liquid transition (fluidization), a density change is also expected because the fluidized phase is less dense than the solid phase. This leads to a volumetric expansion and contraction cycle (corresponding respectively to slip and stick), with a dilation of the thin lubricant film during the slip event (17, 23, 25, 27). Some more recent simulations suggest that slip may occur at the wall–fluid interfaces or via interlayer slip within the film rather than via film melting (19, 27, 28), although the scenario of lubricant fluidization during slip is the generally accepted mechanism.There have been few experimental studies on individual slips during stick–slip sliding across lubricant films, and none where the film thickness in such fleeting events has been examined (15, 16, 18, 30–32). Clues may also be extracted from stick–slip motion of confined granular systems under shear, where numerical simulations (33, 34) and some experiments (35–37) suggest that fluidization and dilation may play a role in the stick–slip instability. While this is suggestive, differences between granular layers and lubricant films include not only five orders of magnitude between size of grains and of molecules but, in particular, the issue of molecular interactions, negligible in granular shear but all-important when shearing lubricants.In the present study, we examine directly the individual slip events during stick–slip sliding across thin lubricant films, and in particular the issue of film dilation during the fleeting slip motion itself. This is done to provide “smoking gun” evidence concerning the issue of film fluidization, where such dilation is expected to be a clear signature. We confine a thin (few nanometers) model liquid film between smooth solid surfaces in an SFB, shear it, and monitor the film thickness during stick–slip sliding via fast video microscopy. To overcome the major challenge presented by the shortness of the slip events, which occupy only some 1% of the stick–slip cycle over which a subnanometer dilation needs to be detected against a comparable level of noise, we analyze our data using tools from classical signal detection theory to correlate the slip events with the instantaneous value of the film thickness.     

Assessment of Tribological Properties of Ti3C2 as a Water-Based Lubricant Additive

Water-based lubrication has attracted remarkable interest due to its environmental and economic advantages. However, practical applications of water-based lubrication are often limited, mainly because of low viscosity and corrosivity. The use of additives has been proposed to overcome these limitations. In this work, the tribological characteristics of titanium carbide (Ti₃C₂) MXenes, as additives for water-based lubrication, were systematically investigated for contact sliding between stainless steel under various normal forces and Ti₃C₂ concentrations. Both friction and wear were found to decrease with increasing Ti₃C₂ concentration up to 5 wt%, and then increased when the concentration was larger than 5 wt%. The results suggest that Ti₃C₂ flakes hindered direct contact, particularly at the edges of the contact interfaces. It was further shown that the agglomeration of Ti₃C₂ flakes may have reduced the hindering when an excessive amount of Ti₃C₂ (e.g., 7 wt%) was applied. The decreases in the friction coefficient and wear rate with 5 wt% of Ti₃C₂ concentration w approximately 20% and 48%, respectively. The outcomes of this work may be helpful in gaining a better understanding of the tribological properties of Ti₃C₂ as a feasible water-based lubrication additive.     

Comparative binding and disintegrating property of Echinochloa colona starch (difra starch) against maize,sorghum, and cassava starch

Context: Starch obtained from different botanical sources exhibit different characteristics due to variation in amylase–amylopectin ratio, which results in different binder substrate interactions.

Objective: The present study characterized Echinochloa colona (L.) Link (Poaceae) starch and evaluated its compressional characteristics for use as tablet excipient against commonly used maize, sorghum, and cassava starch.

Materials and methods: Three experimental design studies were performed to determine the effects of the maize starch and povidone on physical properties of paracetamol (250?mg) tablets. The effect of superdisintegrants sodium starch glycolate and croscarmellose sodium on the optimized composition obtained in the preceding experiments was evaluated in two factorial experimental studies. The maize starch in the optimum formulations was replaced with difra, sorghum, and cassava starch, and tablets prepared from these starches were compared for their compressional characteristics, lubrication sensitivity, moisture uptake, and drug release.

Results: Tablets prepared from maize starch and povidone (30:9, w/w) blend which was previously mixed for 8?min disintegrated (DT) in 10?min. Superdisintegrants decreased DT of tablets significantly (p??sorghum?>?maize starch) and swelling property was 1.5?min and 2.5?min, respectively, with a friability of 0.32%. The effect of lubrication on the DT and friability of tablets containing maize and difra starch was significant (p?in vitro dissolution studies.

Conclusion: Difra starch can replace maize and sorghum starch as tablet excipient.     

丁卡因润滑止痛凝胶预防全麻术后咽喉疼痛20例效果观察

目的:观察丁卡因润滑止痛凝胶对全麻术后咽喉疼痛的防治效果。方法:40例病人随机分为两组,对比观察病人术后咽痛VAS评分及拔管不良心血管反应。结果:丁卡因凝胶组术后咽痛程度明显轻于对照组,且有效抑制拔管时的不良心血管反应,无明显术后不良反应。结论:气管导管前端涂布丁卡因润滑止痛凝胶可以有效防治病人全麻术后咽痛,对病人无其他不良影响。     

润滑止痛胶在前列腺增生症患者导尿术中的应用

目的评价在前列腺增生症患者导尿中应用润滑止痛胶的效果。方法将180例患者分为两组,每组各90例,常规组采用常规导尿法,润滑止痛胶组在常规导尿基础上使用润滑止痛胶,比较两组患者的疼痛程度、一次性成功率和尿道损伤率。结果两组疼痛程度比较χ2=34.1,P<0.01;一次性成功率比较χ2=10.98,P<0.01;尿道损伤率比较χ2=5.27,P<0.05。结论润滑止痛胶可减少前列腺增生患者导尿中对尿道的损伤及疼痛,提高导尿成功率。     

周期间歇卷吸条件下弹流油膜的实验研究

应用光干涉方法对周期间歇卷吸条件下的弹流油膜进行了测量，结果显示了三种不同的局部油膜凹陷。第一种油膜凹陷产生于固体表面周期性的运动和停止，纯滚动的凹陷封油量大于纯滑动的封油量。第二种凹陷产生于启动过程的速度振荡瞬时峰值，该类局部凹陷以卷吸速度通过赫兹接触区。纯滑动条件下，界面滑移作用产生第三种凹陷，该凹陷位于入口附近，与表面速度和运动时间有关。实验中观察到的现象可由油膜的挤压效应，卷吸效应和界面滑移效应进行解释。     

基于动载荷的齿轮传动系统瞬态弹流润滑分析

综合考虑了齿轮传动系统振动影响、润滑流体的可压缩性 ,以及滑动速度和曲率半径随时间和坐标的变化 ;进行了动载荷下的直齿轮传动弹流润滑数值分析 ,获得了齿轮传动沿啮合线的中心油膜及摩擦系数的分布 ,并给出了 7个特殊啮合点上的压力分布及油膜形状 .     

医用高分子材料表面的润滑改性进展

综述了医用高分子材料表面的润滑改性方法，对影响表面润滑性的因素进行了讨论，简述了材料表面润滑性的测定方法，润滑机理及润滑表面的形态，概述了表面润滑的医用高分子材料在临床中的应用。     

一种肠镜润滑消泡剂的细胞毒性和遗传毒性生物学评价

目的 对一种以二甲基硅油、甘油、羧甲基纤维素钠为成分的肠镜润滑消泡剂进行细胞毒性和遗传毒性评价,为这种肠镜润滑消泡剂在临床使用提供生物安全依据。方法 选择琼脂扩散试验对肠镜润滑消泡剂进行体外细胞毒性检测,选择细菌回复突变试验（Ames试验）、用小鼠淋巴瘤细胞进行的体外哺乳动物细胞基因突变试验（MLA试验）、体外哺乳动物细胞染色体畸变试验进行遗传毒性检测。细胞毒试验,将样品放置在固化的琼脂层上与L929细胞间接接触24 h,用中性红对细胞进行染色后显微镜下观察细胞毒性。Ames试验选用TA97a、TA98、TA100、TA102、TA1535五种组氨酸缺陷型鼠伤寒沙门氏菌株,观察肠镜润滑消泡剂对细菌回复突变率的影响;MLA试验采用小鼠淋巴瘤细胞与样品溶液接触3 h和24 h,通过计算其平板效率和大小集落形成数目计算突变频率,判断受试样品对小鼠淋巴瘤细胞突变率的影响;体外哺乳动物细胞染色体畸变试验,使样品溶液与中华地鼠肺细胞（CHL细胞）接触6 h和24 h,通过对处于有丝分裂中期的CHL细胞的染色体畸变情况进行分析,评价肠镜润滑消泡剂对CHL细胞潜在的致突变性。结果 在细胞毒试验中,受试样品与阴性对照相比,细胞数量相对较少且受试样品显微镜下观察有少量畸形和退化的细胞,为轻微的细胞毒性;Ames试验样品组回变菌落数均未比阴性对照回变菌落数增加1倍或者1倍以上且无重复性;MLA试验样品组MF值与阴性对照相比无超过126×10-6的增长;染色体畸变试验受试样品组染色体结构畸变率与阴性对照相比,差异无统计学意义（P＞0.05）。结论 以二甲基硅油、甘油、羧甲基纤维素钠为成分的肠镜润滑消泡剂无潜在的细胞毒性和遗传毒性,可进一步用于临床研究。