The difference in cohesive force between different powders is due to the type and strength of interparticle forces (van der Waals forces, capillary forces, electrostatic forces, etc.), and its core influencing factors include particle size, surface roughness, moisture content, and material properties, resulting in cohesive force that can span multiple orders of magnitude (from 10 ⁻⁶ N to 10 ⁻¹ N). This difference can be quantitatively described through the aggregation feature index, surface tension, and roughness correction model.
Particle size: 5um is the key boundary for the strength of cohesive force
Ultra fine powders with a particle size less than 5um have a significantly increased aggregation characteristic index due to their large specific surface area and van der Waals forces dominating the cohesive force. Experiments have shown that when the particle diameter decreases from 10 μ m to 2 μ m, the agglomeration number (the ratio of particle interaction force to gravity) can be increased by three orders of magnitude, resulting in the powder transitioning from a "free flow" state to a "strong agglomeration" state.
For example, the cohesion of nano titanium dioxide (particle size~20nm) is more than 100 times that of micrometer sized titanium dioxide, due to the high proportion of exposed atoms on the surface of fine particles and stronger intermolecular interactions. For powders with a particle size greater than 5 μ m, the gravitational force exceeds the van der Waals force, and the cohesive force is mainly determined by mechanical biting and friction. The agglomeration characteristic index is close to 1, and the flowability is good.
Surface roughness: "cohesive reducer" for dry powder
The cohesive force of smooth surface particles mainly comes from direct intermolecular interactions, while the micro protrusions (roughness>10nm) on the surface of real powders significantly weaken this effect. Theoretical calculations show that the dry adhesion of rough glass beads is only 1/10 of that of smooth spheres, as the micro convex body shields van der Waals forces, reducing the effective contact area to less than 10% of the apparent area. For example, spherical aluminum powder (surface roughness Ra=0.1 μ m) ground by air flow has a 40% lower cohesive force and a more significant improvement in flowability than irregular aluminum powder (Ra=1.2um) ground by mechanical grinding.
Moisture content: capillary force triggers a "step like growth" of cohesive force
A small amount of water (<5%) will form liquid bridges between particles, generating capillary cohesion far beyond the dry state. For glass bead powder, adding 0.5% moisture can increase the cohesive force from 10 ⁻⁵ N to 10 ⁻² N, which is determined by the formula cap-2 πγ LVRcos θ, where surface tension γ - LV and contact angle θ are key parameters. For example, the cohesive force of quartz sand in a dry state is only 0.01N. After adding water to 2%, the cohesive force can reach 0.3N due to capillary bridging, which is sufficient to form a stable "sand castle" structure. But when the moisture content exceeds 15%, the particles are completely enveloped by a water film, and the capillary force decreases, while the cohesive force becomes dominated by buoyancy.
Material properties: surface tension and the regulatory effect of chemical groups
The difference in surface energy of different materials results in different cohesive base values. For example, metal powders (such as copper powder, surface energy γ _SV-1J/m ²) have 30 times higher cohesive strength than polymer powders (such as polyethylene, γ _SV-0.03J/m ²). Powder containing special functional groups (such as hydroxylated silica) has a cohesion that is more than 50% higher than similar non-polar powders due to hydrogen bonding. Water based resins such as SV-6145 can improve coating flowability while maintaining adhesion by reducing cohesion (while retaining anchoring groups). The design principle is to use low surface energy groups to weaken interparticle attraction.
SAT NANO is a best supplier of oxide powder in China, we can offer nano particle and micro particle size. If you have any enquiry of al2o3 powder, tio2 powder and sio2 powder, please feel free to contact us at sales03@satnano.com
