Faceting–roughening transition of a Cu grain boundary under electron-beam irradiation at 300 keV

In this study, we examined the beam-irradiation effect on the structural evolution of the grain boundary (GB) in a Cu bicrystal at room temperature using a Cs-corrected, monochromated transmission electron microscope at an acceleration voltage of 300 keV. Faceting of the GB was observed at a low current density of the electron beam. With increasing current density, the GB became defaceted. The faceting–roughening transition was shown to be reversible, as the process was reversed upon decreasing the current density. The structural transition is explained by inelastic scattering effects by electron-beam irradiation.

Roughening transition as a driving factor in the formation of self-ordered one-dimensional nanostructures

Peculiar scenarios in the dynamics of BCC and FCC 1D-nanostructures leading to the formation of ultra-short, and sometimes stable, high-amplitude surface modulations are analysed and the means of achieving the desired periodicity are discussed.

Morphology of scallop patterns in erosion by dissolution

<p>Erosion by dissolution is a decisive process shaping small-scale landscape morphology [1]. For fast dissolving minerals, the erosion rate is controlled by the solute transport [2] and characteristic erosion patterns can appear due to hydrodynamics mechanisms. Among the diversity of the dissolution patterns, the scallops are small depressions in a dissolving wall, appearing as cups with sharp edges. Their size varies from few millimeters to around ten centimeters. The scallops occur typically as the final steady form of ripple patterns created by the action of a turbulent flow on a dissolving surface [3,4]. Moreover, very similar shapes are also met, without imposed external flow, when the fluid motion results from the solutal convection induced by the dissolution [2,5,6]. Finally, scallop patterns resulting from similar mechanisms appear also on ice surfaces by melting in presence of a turbulent flow [7] or a convection flow [6]. <br>Using three-dimensional surface reconstruction, we characterize quantitatively the scallop patterns mainly for experimental samples patterned by solutal convection. The temporal evolution of the scallop shape, of their spatial distribution and of the induced roughness are specifically investigated, in order to determine mechanisms explaining the generic aspects of dissolution patterns. </p><p>[1] P. Meakin and B. Jamtveit, Geological pattern formation by growth and dissolution in aqueous systems, <strong>Proc. R. Soc. A 466</strong> 659-694 (2010)</p><p>[2] J. Philippi, M. Berhanu, J. Derr and S. Courrech du Pont, Solutal convection induced by dissolution, <strong>Phys. Rev. Fluids, 4,</strong> 103801 (2019)</p><p>[3] P.N. Blumberg and R.L. Curl, Experimental and theoretical studies of dissolution roughness,  <strong>J. Fluid Mech. 65</strong>, 735 (1974)</p><p>[4] P. Claudin, O. Durán and B. Andreotti, Dissolution instability and roughening transition,  <strong>J. Fluid Mech. 832</strong>, R2  (1974)</p><p>[5] T.S. Sullivan, Y. Liu and R. E. Ecke, Turbulent solutal convection and surface patterning in solid dissolution, <strong>Phys. Rev. E 54</strong>, (1) 486, (1996)</p><p>[6] C. Cohen, M. Berhanu, J. Derr and S. Courrech du Pont, Erosion patterns on dissolving and melting bodies (2015 Gallery of Fluid motion), <strong>Phys. Rev. Fluids, 1,</strong> 050508 (2016)</p><p>[7] M. Bushuk, D. M. Holland, T. P. Stanton, A. Stern and C. Gray. Ice scallops: a laboratory investigation of the Ice-water interface, <strong>J. Fluid Mech. 873</strong>, 942 (2019)</p>

Dissolution instability and roughening transition

We theoretically investigate the pattern formation observed when a fluid flows over a solid substrate that can dissolve or melt. We use a turbulent mixing description that includes the effect of the bed roughness. We show that the dissolution instability at the origin of the pattern is associated with an anomaly at the transition from a laminar to a turbulent hydrodynamic response with respect to the bed elevation. This anomaly, and therefore the instability, disappears when the bed becomes hydrodynamically rough, above a threshold roughness-based Reynolds number. This suggests a possible mechanism for the selection of the pattern amplitude. The most unstable wavelength scales as observed in nature on the thickness of the viscous sublayer, with a multiplicative factor that depends on the dimensionless parameters of the problem.