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The adhesion force measurement of powders is the measurement of adhesion force for a group of particles on a specific surface, which is mostly determinated via experimental method using centrifuges. Adhesion is the tendency of dissimilar particles or surfaces to cling to one another. The technique pairs the use of an imposed centrifugal force with imaging and subsequent analysis. The centrifuge technique is one example of multi-particle measurement techniques of which there are others; such as the vibration method, the drop test method and the electric field detachment method. The characterisation of more than one particle at once (i.e. possibly large numbers) permits statistical analyses to be carried out on a population of powder particles. Particle adhesion is important in several fields including pharmaceutical formulation and particle contamination.

Theoretical background
Centrifugation causes an object to experience a centrifugal force in the normal direction of the axis of rotation. The magnitude of centrifugal force F on an object of mass m at the distance r from the axis of rotation of a frame of reference rotating with angular velocity $ω$ is:

$$F = m\omega^2 r$$

When the force imposed on a particle using the centrifuge overcomes the adhesion force present between particle and surface, then the particle will detach from the surface.

Experimental technique
Particles are deposited on a solid surface (made of the material of interest) which is then rotated by centrifugation at a known speed. The orientation of the rotation with respect to the particles on the surface is usually perpendicular in which case the force imposed will be normal to the surface and therefore in the direction opposite to the adhesion interaction between particle and surface. It can also be horizontal, with the force will be parallel to the surface, in which case the component of the force must be calculated. In either case the force is directed radially away from the axis of rotation.

Particle detachment with a certain rotation speed gives an upper bound for the force necessary to detach that particle. The more different rotation speeds are tested in the experiment, the more precisely the detaching force can be determined as a lower bound will be determined by a slower rotation speed that does not detach the particle. This limits the error on the adhesion force calculated.

Prior to, and after every rotation step the mass of each particle must be known in order to be able to calculate force. This is often achieved through the use of optical microscopy of the particle distribution on the surface and the application of particle counting software to obtain the particle size distribution present on the surface.

Analysis
In the case of the use of particle counting techniques, particle density and a volume are approximated from the projected area, such that the mass of each particle can be calculated. Using the known mass, distance from the axis of rotation, and the angular velocity that caused detachment of the particle, the detaching force is obtained.

Realistic particles will not have identical adhesion forces even if the population is made up of similar particles due to possible chemical or physical heterogeneity. Statistical analyses are used to extract values of interest. An average value can be calculated using the point 50% of particles of a certain population have been lost. Force distributions can be obtained for a population using more complex analyses.

Other forces may be present in the system which need to be accounted for mathematically. Friction forces will also act when a component of the centrifugal force is parallel to the surface. When the experiment is carried out in air, drag forces will influence particle movement.

Applications
Adhesion forces are of interest to pharmaceutical manufacturers as stable interactions between drug and carrier molecules are a requisite for successful drug delivery. Other applications include paste drying operations, fine particle fluidisation, micro-encapsulation, xerography and printing, and polishing.

The centrifuge technique can be applied under vacuum to simulate the lunar environment for researchers investigating the adhesion of lunar dust.