16 research outputs found

    Nonlinear Frictional Energy Dissipation between Silica-Adsorbed Surfactant Micelles

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    The origin of energy dissipation underlies all friction processes, which is crucial in the design of extremely low friction and wear systems. Amontons’s friction law shows that the frictional energy dissipation should be linear with load because of the constant friction coefficient. However, in this Letter, we present the nonlinear behavior of frictional energy dissipation in boundary lubrication with silica-adsorbed surfactant micelles. There exist two completely different friction regimes: a near-zero friction regime with very little energy dissipation and a nonlinear friction regime with a great deal of extra energy dissipation. The additional energy dissipation presents a square (nonlinear) relation with applied load, originating from the elastic deformation of the adsorbed micelle layer on the two friction surfaces, which is tightly linked to the stiffness of the micelle layer and the diameter of the contact area

    Improvement of Load Bearing Capacity of Nanoscale Superlow Friction by Synthesized Fluorinated Surfactant Micelles

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    Although surfactant micelles usually exhibit superlow friction at the nanoscale due to the formation of the hydration layer, the load-bearing capacity (LBC) is limited. In this study, the friction behaviors of two different surfactant micelles (fluorinated and hydrocarbon surfactants, denoted as F-surfactant and H-surfactant) were compared, with the results showing that both can achieve superlow friction (μ = 0.001–0.002) when the self-assembled micelle layers on the two surfaces were not ruptured. Although the two different surfactant micelles have the similar friction behaviors, the LBC of superlow friction for the F-surfactant is 2.5 times larger than that for the H-surfactant. The mechanisms of the superlow friction and the reasons for different LBC were investigated using an atomic force microscopy. The superlow friction can be attributed to the formation of hydration layer on the surfactant headgroups, whereas the higher LBC for F-surfactant originates from the fatness of its carbon chain, which produces the larger hydrophobic attraction and meanwhile increases the stiffness of the micelle layer

    AFM Studies on Liquid Superlubricity between Silica Surfaces Achieved with Surfactant Micelles

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    By using atomic force microscopy (AFM), we showed that the liquid superlubricity with a superlow friction coefficient of 0.0007 can be achieved between two silica surfaces lubricated by hexadecyl­trimethyl­ammonium bromide (C<sub>16</sub>TAB) solution. There exists a critical load that the lubrication state translates from superlow friction to high friction reversibly. To analyze the superlow friction mechanism and the factors influencing the critical load, we used AFM to measure the structure of adsorbed C<sub>16</sub>TAB molecules and the normal force between two silica surfaces. Experimental results indicate that the C<sub>16</sub>TAB molecules are firmly adsorbed on the two silica surfaces by electrostatic interaction, forming cylinder-like micelles. Meanwhile, the positively charged headgroups exposed to solution produce the hydration and double layer repulsion to bear the applied load. By controlling the concentration of C<sub>16</sub>TAB solution, it is confirmed that the critical load of superlow friction is determined by the maximal normal force produced by the hydration layer. Finally, the superlow friction mechanism was proposed that the adsorbed micellar layer forms the hydration layer, making the two friction surfaces be in the repulsive region and meanwhile providing excellent fluidity without adhesion between micelles

    Rate Expressions for Kink Attachment and Detachment During Crystal Growth

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    The central kinetic processes defining layer-by-layer crystal growth or dissolution are the attachment and detachment rates of growth units at kink sites; the net balance of these activated processes leads to either crystal growth or dissolution. Various sets of rate expressions for attachment and detachment processes have been used in the literature, in each case attempting to most appropriately capture the underlying surface chemistry. We examine these proposals with specific attention to thermodynamics and detailed balance criteria and then recommend which expressions to adopt

    Don&apos;t follow your leader: challenging erroneous decisions.

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    A series of novel chiral-bridged atropisomeric monophosphine ligands were synthesized via convenient and simple pathways. The prepared ligands, especially for ligand <b>7d</b>, were found to be highly effective in the Pd-catalyzed Suzuki–Miyaura coupling reaction. The steric hindrance and electronic effect of substrates on the reactivity and enantioselectivity were explored preliminarily

    Table_1_Down-Regulation of PpBGAL10 and PpBGAL16 Delays Fruit Softening in Peach by Reducing Polygalacturonase and Pectin Methylesterase Activity.DOCX

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    <p>β-galactosidases are cell wall hydrolases that play an important role in fruit softening. However, PpBGALs mechanism impacting on ethylene-dependent peach fruit softening was still unclear. In this study, we found that PpBGAL4, -6, -8, -10, -16, and -17 may be required for ethylene-dependent peach softening and PpBGAL10, -16 may make a main contribution to it among 17 PpBGALs. Utilization of virus-induced gene silencing (VIGS) showed that fruits were firmer than those of the control at 4 and 6 days after harvest (DAH) when PpBGAL10 and PpBGAL16 expression was down-regulated. Suppression of PpBGAL10 and PpBGAL16 expression also reduced PpPG21 and PpPME3 transcription, and polygalacturonase (PG) and pectinmethylesterases (PME) activity. Overall, total cell wall material and protopectin slowly declined, water-soluble pectin slowly increased, and cellulose and hemicellulose was altered significantly at 4 DAH, relative to control fruit. In addition, PpACO1 expression and ethylene production were also suppressed at 4 DAH because of inhibiting PpBGAL10 and PpBGAL16 expression. These results suggested that down-regulation of PpBGAL10 and PpBGAL16 expression delays peach fruit softening by decreasing PG and PME activity, which inhibits cell wall degradation and ethylene production.</p
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