Flow and fracture near the sol-gel transition of silica nanoparticle suspensions.

We analyze the evolution of the mechanical response of a colloidal suspension to an external tensile stress, from fracture to flow, as a function of the distance from the sol-gel transition. We cease to observe cracks at a finite distance from the transition. In an intermediate region where the phenomenon is clearly hysteretic, we observe the coexistence of both flow and fracture. Even when cracks are observed, the material in fact flows over a distance that increases in the vicinity of the transition.

Digital instability of a confined elastic meniscus.

Thin soft elastic layers serving as joints between relatively rigid bodies may function as sealants, thermal, electrical, or mechanical insulators, bearings, or adhesives. When such a joint is stressed, even though perfect adhesion is maintained, the exposed free meniscus in the thin elastic layer becomes unstable, leading to the formation of spatially periodic digits of air that invade the elastic layer, reminiscent of viscous fingering in a thin fluid layer. However, the elastic instability is reversible and rate-independent, disappearing when the joint is unstressed. We use theory, experiments, and numerical simulations to show that the transition to the digital state is sudden (first-order), the wavelength and amplitude of the fingers are proportional to the thickness of the elastic layer, and the required separation to trigger the instability is inversely proportional to the in-plane dimension of the layer. Our study reveals the energetic origin of this instability and has implications for the strength of polymeric adhesives; it also suggests a method for patterning thin films reversibly with any arrangement of localized fingers in a digital elastic memory, which we confirm experimentally.

Synthesizing microcapsules with controlled geometrical and mechanical properties with microfluidic double emulsion technology.

Using lithography-based microfluidic technology, we produce monodisperse single-core microcapsules with UV-cured TPGDA (triprophylene glycol diacrylate) shells. We show that the geometrical and mechanical characteristics of the microcapsules can be predicted on a quantitative basis and tuned by varying the flow conditions. Shell thicknesses are varied by changing the flow rates of the inner or intermediate phases, according to mass conservation constraint. Off-centering of the core with respect to the shell is controlled by varying the shell phase viscosity. The mechanical properties of the capsules can be varied by changing the flow conditions and are quantitatively predicted by a numerical simulation. The simulation moreover provides a correct qualitative description of their rupture. As a whole, the work carried out in the present paper shows, on a quantitative basis, that microfluidic technology allows to finely control the geometrical and mechanical properties of microcapsules generated on chip. The level of control we reach here is not accessible, by far, to conventional technologies. Combined with parallelization, the present work opens routes toward the production of novel families of monodisperse microcapsules with tunable properties.

Transient damage spreading and anomalous scaling in mortar crack surfaces.

The scaling properties of a post-mortem mortar crack surface are investigated. The root mean square of the height fluctuations is found to obey anomalous scaling properties, but with three exponents, two of them characterizing the local roughness ( zeta approximately 0.79 and zetae approximately 0.41 ) and the third one driving the global roughness (zetag approximately 1.60) . The critical exponent zeta approximately 0.79 is conjectured to reflect damage screening occurring for length scales smaller than the process zone size, while the exponent zetae approximately 0.41 characterizes roughness at larger length scales, i.e., at length scales where the material can be considered as linear elastic. Finally, we argue that the global roughness exponent could be material dependent contrary to both local roughness exponents ( zeta approximately 0.8 and zetae approximately 0.4 ) which can be considered as universal.

Dynamics of wing cracks and nanoscale damage in glass.

We investigate initiation, growth, and healing of wing cracks in confined silica glass by molecular dynamics simulations. Under dynamic compression, frictional sliding of precrack surfaces nucleates nanovoids which evolve into nanocrack columns at the precrack tip. Nanocrack columns merge to form a wing crack, which grows via coalescence with nanovoids in the direction of maximum compression. Lateral confinement arrests the growth and partially heals the wing crack. Growth and arrest of the wing crack occur repeatedly, as observed in dynamic compression experiments on brittle solids under lateral confinement.

Anomalous scaling of mortar fracture surfaces.

Scaling properties of mortar crack surfaces are studied from a mode I fracture test. Fracture surfaces initiated from a straight notch are shown to display an anomalous dynamic scaling of the crack roughness emphasizing the different crack developments between the directions parallel and perpendicular to the crack propagation direction. This anomalous roughening involves the existence of two different and independent roughness exponents. The first one, called the local roughness exponent zeta(loc), drives the self-affine scaling properties of the roughness perpendicular to crack propagation direction and can be considered as a universal roughness index zeta(loc) approximately 0.8. The second one, called the global roughness exponent, estimated to zeta approximately 1.3, is used to described the growth of the roughness at large length scales as a function of the distance to the initial notch and appears as a material-dependent parameter. We argue that the anomalous scaling of the roughness development could be an inheritance of the microcracked fracture process zone, quite large in quasibrittle materials. Finally, in the case of such an anomalous roughening, we argue that the fractal dimension appears insufficient to characterize the fracture surface morphology as a whole.