How does particle size affect drag force in colloids?

[Wiemster]

How does one evaluate the drag force on a particle moving through a colloid? And how does this depend on the size of the particle compared to the constituents making up the collloid?

For particles much larger than the constituents I can imagine one can just do the same as for a normal fluid and use the various correlations that exist. If the rheology of the colloid is non-Newtonian I can imagine you evaluate the viscosity for a shear, characteristic to the moving particle. But I can also imagine the particle size matters. I would say for particle sizes much smaller than the colloid constituents, the viscous drag experienced by the particle is that of the liquid phase, the interaction with the constituents playing little role. Is that correct?

If so, how does the viscosity in the drag force expressions change with particle size relative to the colloid constituents?

Any experience, refererences, thoughts, comments are welcome!

 

[Wiemster]

No colloidal chemists / physicists on this forum with knowledge on the subject?

The specific case I have in mind is that of particles moving through blood, anybody knows of any experiments/simulations reporting on this issue?

 

[oswaler]

I can provide a response to your questions regarding the effect of particle size on drag force in colloids. The drag force experienced by a particle moving through a colloid is influenced by several factors, including the size of the particle compared to the constituents making up the colloid.

Firstly, it is important to note that colloids are heterogeneous systems, consisting of particles dispersed in a continuous medium (usually a liquid). The interaction between the particles and the medium plays a crucial role in determining the drag force experienced by the particles.

In general, for particles much larger than the colloidal constituents, the drag force can be evaluated using standard fluid dynamics equations and correlations. This is because the particles are significantly larger than the surrounding medium, and their motion is largely unaffected by the interactions with the colloidal constituents. In this case, the drag force can be calculated using the same methods as for a normal fluid.

However, for particles much smaller than the colloidal constituents, the situation is different. In this case, the drag force experienced by the particle is primarily determined by the viscosity of the surrounding medium. The interaction between the particle and the colloidal constituents becomes less significant, and the particle behaves more like a particle in a homogeneous fluid.

The viscosity in the drag force expression does change with particle size relative to the colloidal constituents. In general, as the particle size decreases, the viscosity of the medium increases, resulting in a higher drag force experienced by the particle. This is because smaller particles experience more resistance from the surrounding medium due to their higher surface area to volume ratio.

It is important to note that the rheology of the colloid also plays a significant role in determining the drag force experienced by the particles. Non-Newtonian behavior, such as shear-thinning or shear-thickening, can affect the viscosity of the medium and therefore impact the drag force experienced by the particles.

In terms of evaluating the drag force on a particle moving through a colloid, various experimental and theoretical methods can be used. These include direct measurements using techniques such as particle tracking, or indirect methods such as rheological measurements and modeling. The choice of method will depend on the specific properties of the colloid and the particles being studied.

I hope this response has provided some insight into the influence of particle size on drag force in colloids. For further reading, I would recommend looking into literature on colloid dynamics and rheology, as well as specific studies