Shearing is widely used in industries such as chemical engineering, mining and waste water treatment. For the particles (200 micrometer in diameter) trapped at the air-liquid interface, the forces acting on them usually include mono-polar forces (gravity, buoyancy force), dipolar forces (due to the electric charge between the interface) and quadrapole force (capillary interaction due to the distortion of liquid interface). Here we introduce a hysteretic and short ranged attractive force by using a second liquid which is immiscible with the bulk liquid to wet particles and form liquid bridges between them. In such a way we create a 2D wet granular media. By applying a shear force, we study the aggregation of 2D wet granular material.

Comparison between dry and wet sample under shear (as the two sample videos shown below) indicates that the capillary force dominates the others. For dry sample, particles aggregation due to the quadrapole force could easily be broken by the viscous drag force. While at the same shear rate, the aggregation formed by wet sample persists.

_{Hydrophobic glass beads trapped at an air-liquid interface are wetted by a secondary oil phase to create a 2D wet granular monolayer for the investigation of its rheological behavior. The image on lower-right is a microscopic top view of the wet glass spheres floating on bulk liquid.}