Laurent Lacaze

2025 — Review in Exp. in Fluids 
Experimental Insights into Cross-Shore Morphodynamics of Beaches and Cliffs

Lacaze L., Astruc D. and Moulin F.

Conceptual diagram of the littoral zone subjected to wave forcing. Dissipation coefficient as a function of the Dean number and beach slope β as a function of a dimensionless timescale based on bore height incoming the swash for various experimental configurations. The data originate from facilities of varying sizes (ranging from 10 m to 330 m) under different types of wave forcing.

This review aims to provide an overview of laboratory models of nearshore morphodynamics, focusing specifically on the role of wave action. In this context, shoreline evolution is driven by the direction of sediment flux induced by wave dynamics, corresponding to erosion or accretion processes. These processes, which naturally modify the shape of coastlines, are influenced by factors such as sediment availability, wave climate, and soil strength, among others.
Starting from unconsolidated sandy materials and the equilibrium-based concepts used in natural beach classification, the reanalysis of laboratory experiments shows that they can reproduce natural conditions and serve as conceptual models for understanding nearshore morphodynamics. However, the temporal evolution of the shoreline and the source of available sand are difficult to capture through equilibrium-type physical processes and therefore requires a specific focus on localized zones of the nearshore, depending on the prevailing hydrodynamics and sediment strength, i.e. consolidation.
Accordingly, the local dynamics near the shoreline are closely linked to the behavior of the swash zone and to cliff erosion processes, which constitute a central focus of this review from a laboratory experiment perspective.

2025 — Article in Soft Matter 
Liquid drop impact on granular beds: the influence of drop inertia and grain size

Pontier A., Blosse S., Viroulet S. and Lacaze L.

Top view of a crater formed by the impact of a droplet onto a granular bed (polystyrene beads), obtained experimentally. This configuration highlights the splash regime.

This paper explores crater formation resulting from the impact of a liquid drop on a densely packed granular bed composed of lightweight polystyrene beads. Several regimes based on the drop impact velocity and diameter, and the grain diameter are identified. These regimes are discussed in terms of several dimensionless numbers, including a Froude number, which compares the droplet’s kinetic energy to its potential energy at impact, the Weber number, which compares the inertial to capillary forces, and the grain-to-drop size ratio.
At low Weber, Froude, and grain-to-drop size ratio, the dimensionless crater diameter follows a power-law scaling with Weber, consistent with previous studies on droplet impacts on granular surfaces, where the crater size reflects the maximum droplet spreading observed on a solid surface. This situation is thus analysed using a so-called signature approach. In this situation, the crater size is also shown to quantitatively depend on the grain-to-drop size ratio. When Weber exceeds a critical value, the scaling deviates and the crater size depends mainly on the grain-to-drop size ratio. This transition is discussed in connection with the onset of droplet splashing. For larger grain-to-drop size ratio, a different power-law emerges with an exponent smaller than the previous one, and the splash transition no longer occurs under these conditions. This is consistent with other studies, highlighting the significant amount of energy transfer in crater formation, therefore referred to as the energetic approach. Overall, the final crater size is found to depend strongly on grain-to-drop size ratio among the droplet impact characteristics.
To unify part of these observations, the role of local dissipation due to grain contact friction during crater formation is incorporated. This leads to the definition of a new dimensionless number F, which combines the effects of grain-to-drop size ratio and droplet inertia (via Froude). This parameter enables the collapse of finale crater size data onto a single curve for the range of parameters investigated in this study.

2025 — Article in Phys. Rev. Fluids 
Combined influence of particle friction and inertia on hysteresis in granular media on an inclined plane

Lambert C., Maurin R., Lacaze L. and Fede P.

Inertial number I (left) and inclination angle θ (right) as functions of time within a simulation.

Understanding the transition between fluid-like and solid-like regimes of granular materials requires elucidating its hysteretic behavior. Yet, the origin of this hysteresis, defined as the difference in external stress necessary to induce flow or jamming of a granular medium, is still debated. While the origin of this hysteresis has long been attributed to grain’s inertia, recent studies have shown that it depends on interparticle friction.
To clarify the role of the different effects and possible interplays between them, we study the fluid-solid transition through three-dimensional discrete element simulations of dry and model-immersed granular flows down an inclined plane. In the dry case, a finite hysteresis is observed at the static-flowing transition even for frictionless particles. The hysteresis amplitude is shown to depend both on interparticle friction and particle inertia. Decoupling the effects of friction and inertia allows us to rationalize in a coherent picture the different results found in the literature. In particular, hysteresis is shown to become negligible when both friction and inertia are small, while it cannot be disregarded when at least one of them is present.

The link between hysteresis amplitude and the discontinuous jump of compaction at the jamming transition is discussed. While the presence of hysteresis for frictionless particles is not associated with notable dilatancy and compaction at transitions, the coordination number exhibits discontinuous jumps at both transitions. Also, it is shown that the avalanche angle is directly linked to the static coordination number, and that the hysteresis amplitude is related to the coordination number discontinuity observed at jamming. These results highlight the strong link between the jamming-unjamming transition and the evolution of the granular microstructure through friction and inertia.