Dynamic wetting behaviors on soft solids are important to interpret complex biological processes from cell-substrate interactions. Despite intensive research studies over the past half-century, the underlying mechanisms of spreading behaviors are not clearly understood. The most interesting feature of wetting on soft matter is the formation of a "wetting ridge", a surface deformation by a competition between elasticity and capillarity. Dynamics of the wetting ridge formed at the three-phase contact line underlies the dynamic wetting behaviors, but remains largely unexplored mostly due to limitations in indirect observation. Here, we directly visualize wetting ridge dynamics during continuous- and stick-slip motions on a viscoelastic surface using X-ray microscopy. Strikingly, we discover that the ridge spreads spontaneously during stick and triggers contact line depinning (stick-to-slip transition) by changing the ridge geometry which weakens the contact line pinning. Finally, we clarify 'viscoelastic-braking', 'stick-slipping', and 'stick-breaking' spreading behaviors through the ridge dynamics. In stick-breaking, no ridge-spreading occurs and contact line pinning (hysteresis) is enhanced by cusp-bending while preserving a microscopic equilibrium at the ridge tip. We have furthered the understanding of spreading behaviors on soft solids and demonstrated the value of X-ray microscopy in elucidating various dynamic wetting behaviors on soft solids as well as puzzling biological issues.

A B S T R A C TThe ratcheting behaviour of Inconel 718 was investigated at 649 • C under uniaxial cyclic loading. Stress-control tests have been conducted at various combinations of stress amplitude and mean stress. The ratcheting strain at failure increases with increasing mean stress for a given stress amplitude and with decreasing stress amplitude for a given mean stress. Fatigue lives were correlated using three mean stress models: the Goodman equation, the Smith-Watson-Topper (SWT) parameter and the Walker parameter. It has been shown that the Goodman equation and the SWT parameter do not correlate life data, while the Walker parameter yields acceptable correlation. The SWT parameter was modified to incorporate the ratcheting effect. The new parameter is found to yield correlation similar to that of the Walker parameter.Keywords Goodman equation; mean stress effect; ratcheting strain; SWT parameter; uniaxial fatigue; Walker parameter. N O M E N C L A T U R Eσ a = stress amplitude σ m = mean stress σ eq a = equivalent stress amplitude σ max = maximum stress R = stress ratio (R = σ min /σ max ) ε r = ratcheting strain N = number of cycles N f = number of cycles to failure σ f = fatigue strength coefficient b = fatigue strength exponent γ = Walker exponent I N T R O D U C T I O NMany engineering components are subjected to cyclic loading in which the fatigue process takes place under stress-control conditions. Fatigue cycles may not be fully reversed. In these circumstances, the mean stress effect has to be taken into account in fatigue life prediction. A phenomenon that occurs in the presence of mean stress in the low-cycle fatigue regime is ratcheting, which results from the accumulation of plastic strain. Ratcheting causes fatigue damage of the material and shortens the life of engineering components significantly, unless a shakedown state is reached in the early stage of life. Naturally, this important phenomenon has received the attention of many researchers, and numerous studies have been carried out on the deformation aspect of the phenomenon. The Armstrong-Frederick nonlinear kinematic hardening rule 1 is widely used in ratcheting analysis. The decomposed model by Chaboche 2 and the multilinear model by Ohno and Wang 3 belong to this category. These models have been successful for predicting uniaxial ratcheting, but they tend to overpredict ratcheting under multiaxial loading. Despite many modified models suggested 4,5 to resolve the shortcomings, the prediction of ratcheting strain under multiaxial loading still remains a challenging problem. Not only that, there are many other variables influencing ratcheting behaviour which need to be further investigated. For all these and other reasons, the 1076

Cylindrical asymmetry of an rf gun cavity excites the multipole rf field which will result in emittance growth of electron beams. With demanding of electron beams of ultralow emittance for the x-ray free electron laser, it is necessary to study the effect of the multipole rf field to the emittance growth and to eliminate the multipole field effect on electron beam emittance in an rf gun. In this paper, we present both theoretical analysis and numerical simulations of the multipole field effect to the emittance growth. We proposed a four-hole scheme to minimize the emittance growth from the multipole field.

A fast microtomography system for high-resolution high-speed imaging has been developed using bright monochromatic x-rays at the BL29XU beamline of SPring-8. The shortest scan time for microtomography we attained was 0.25 s in 1.25 μm effective pixel size by combining the bright monochromatic x-rays, a fast rotating sample stage, and a high performance x-ray imaging detector. The feasibility of the tomography system was successfully demonstrated by visualization of rising bubbles in a viscous liquid, an interesting issue in multiphase flow physics. This system also provides a high spatial (a measurable feature size of 300 nm) or a very high temporal (9.8 μs) resolution in radiographs.