Buckling of a monolayer of plate-like particles trapped at a fluid-fluid interface*

We investigate experimentally the deformation mechanics of monolayers of rigid plates trapped by capillary forces at an air-liquid or liquid-liquid interface. This study is motivated by the need to understand the compression of 2D nanoparticles (e.g., graphene) at a fluid-fluid interface, the stability of Pickering emulsions and the fundamental differences between spherical particles and plates in these applications as anisotropic particles exhibit different interactions. To investigate when the particle-laden interface buckles, and to study the wavelength of buckling and the force required for compression, we carry out experiments using a one-dimensional chain of flat rigid plates of length L compressed by a moving barrier with an assigned displacement. The main observations are: i) a critical buckling force and a characteristic post-buckling wavelength of about 2 particle lengths (different from spheres); ii) the buckling wavelength is independent of the particle Bond number and the particle length; iii) the existence of gravity-dominated and surface-tension-dominated regimes in which the buckling force (F_b) scales as L³ and L¹, respectively.  To explain these observation, we develop a mathematical model by considering the capillary forces acting on each particle, the particle weight and buoyancy forces. Good agreement between the theory and the experiments is found. Our observations show that an interfacial monolayer of rigid plates does not have a bending rigidity, unlike a monolayer of spheres. Implications for jamming and wrinkling of 2D monolayers of thin rigid disks trapped at a fluid interface are discussed.