Lubrication properties of non-adsorbing polymer solutions in soft elastohydrodynamic (EHD) contacts

The in-use performance and processing of many consumer products in the food, home and personal care industries are dependent on their tribological properties. A major component of these products is often a high molecular weight polymer, which is typically used to thicken aqueous systems. Polymer solutions tend to be non-Newtonian, and in particular their viscosity varies with shear rate, such that it is difficult to predict their friction or hydrodynamic film-forming behaviour. The present work relates the tribology of aqueous polymer solutions to their rheological properties in thin films in ‘soft’ contacts at high shear rates. The friction properties of three types of polymers in aqueous solution, polyethylene oxide, PEO; xanthan gum, XG; and guar gum, GG, have been studied as a function of polymer concentration over a wide range of entrainment speeds in a point contact formed between silicone rubber and steel. This has enabled the boundary lubrication and isoviscous-elastic lubrication properties of the solutions to be investigated using both hydrophilic and hydrophobic silicone surfaces.It is found that the friction vs. entrainment speed dependence follows the shape of a classical Stribeck curve. In general, a lower friction is observed with increasing polymer concentration in the mixed-regime. Using scaling factors for the entrainment speed, we have shown that this decrease in friction is likely to be due to viscous effects and that the scaling factors represent effective high shear rate viscosities. In the case of PEO and XG, and GG at low concentrations, a good correlation is found between this effective viscosity and the apparent viscosity measured at the highest shear rates attainable with the available rheometer. However, for GG at concentrations above 0.2%, the effective viscosity decreases with increasing polymer content.The three polymers do not significantly reduce friction in the boundary regime and in general give essentially the same response as water when an effective viscosity is taken into account. However, a slight increase in friction in comparison to pure water has been observed for XG and GG on hydrophobic surfaces. It is suspected that this may be due to a blocking of fluid entrainment, or possibly exclusion of polymer from the contact, due to the large hydrodynamic volume and rigid nature of the two biopolymers. Finally, for PEO solutions with full-film elastohydrodynamic conditions were reached, the measured friction coefficient of the film correlated quite well with the value calculated from the effective viscosity.     

Effects of lithium carbonate on the memory and motor speed of bipolar outpatients.

We examined the effect of lithium on the memory and motor speed of 22 outpatients with affective disorders in remission. Patients were assessed weekly over a 5-week period starting at their current lithium dosage, twice during administration of a blind placebo, and twice after their lithium was blindly reinstated. Motor speed was assessed using the finger tapping test. Memory was assessed using the Buschke selective reminding protocol. Mood was assessed at each session to ensure remitted status by clinical interview, the Hamilton Rating Scale for Depression, the Longitudinal Rating of Manic States Scale, and a subjective state questionnaire. Weekly blood samples were also drawn to assess plasma lithium level by means of atomic absorption spectrophotometry. The results indicated that lithium had significant detrimental effect on memory and motor speed: Performance improved when lithium was discontinued and declined when lithium was reintroduced. The implications for patient management and diagnosis in bipolar disorder are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mobile Design as Neighborhood Acupuncture: Activating the Storytelling Networks of South Los Angeles

Abstract

A delicate touch is required to empower neighborhoods using civic media. Funding is persistently scarce. Especially in marginalized neighborhoods, blunt designs can be counterproductive and even entrench complex problems. New metaphors may be needed to guide design and empower local neighborhoods. Urban acupuncture is used as the basis for this study, emphasizing a light-touch strategy that has shown success in Brazil with urban transit, and more recently in Europe with urban design. We specifically propose “neighborhood acupuncture” to address the local level, tapping the sociology of place-based communication. To investigate the implications for systematic design, a case study is probed in South Los Angeles using mobile media for community mapping. Using qualitative methods, three tactics were investigated for the potential to “poke” the network into action, including one to bridge diverse storytelling networks. Each tactic ultimately seeks to build the capacity for collective action around neighborhood issues. Acupuncture is broadly argued to sustain two design shifts: first to help approach neighborhoods as ecosystems, and second, to design for circulation rather than any single technology platform.     

Influence of ionic strength on the tribological properties of pre-adsorbed salivary films

We have studied tribological properties of pre-adsorbed salivary films formed in vitro on compliant hydrophobic surfaces. The adsorbed salivary film significantly decreases boundary friction under physiological ionic strength, which is related to a hydrophilic character of the adsorbed film and its structure. Decrease in the ionic strength below physiological conditions affects film’s structure, but it does not significantly affect boundary lubrication at low loads. Applications of high loads led to a gradual loss of lubrication due to shear-induced wear of the films. The wear became more extensive as the ionic strength of the solvent was lowered below physiological conditions.