Material left on a cloth after filtering urine can look like proof that nanoparticles were removed. It isn't. This follow-up to my article on nanoparticles in urine explains how fibrous filtration actually works: the five capture mechanisms, why aggregates behave differently from single particles, why capture is a matter of probability rather than pore size, and what the Most Penetrating Particle Size tells us.
It is not only about whether the particle is smaller than the opening
In my earlier article, Why Nanoparticles in Urine Matter — and Why You Shouldn’t Re‑Drink It, I explained how the kidneys filter nanoparticles and why re-ingesting urine reintroduces what the body is trying to remove. Since then, someone suggested, or posed a question to me, that passing urine through a thick cloth could filter the nanoparticles out.
The short answer is no, and a simple analogy shows why. Imagine a chain-link fence. A tennis ball cannot get through, but a grain of sand passes straight through — and making the fence ten feet thick does not change that, because the holes stay the same size.
A tightly woven cloth has gaps of roughly 10–100 micrometers, while nanoparticles measure 1–100 nanometers: about 100 to 100,000 times smaller. A nanoparticle passing through cloth is like a grain of sand passing through a chain-link fence.
But that is not the whole story.
The chain-link fence analogy is useful for understanding the enormous difference in scale between cloth openings and nanoparticles, but real fibrous filtration is more complicated than a simple sieve.
A particle does not travel through a woven material in a perfectly straight line, centered neatly inside each opening.
The liquid follows irregular pathways around and between the fibers. Particles and aggregates move with that flow, but their exact path relative to each fiber is partly determined by their size, inertia, Brownian motion, surface properties and local fluid movement.
As a result, two particles of the same size can have different outcomes.
One may travel through an opening without contacting a fiber.
Another may approach the opening at a slightly different position, collide with a thread and remain attached.
A third may encounter material already deposited on the fabric and become trapped there.
So when material remains on a cloth, it does not necessarily mean that the cloth's openings were smaller than that material.
Several different capture mechanisms can be involved
1.Sieving or size exclusion
This is the mechanism most people imagine.
If a particle or aggregate is physically larger than an available opening, it cannot pass through that particular pathway.
This becomes increasingly relevant for larger aggregates, sediment and other relatively coarse material.
But it is only one mechanism.
2.Interception
A particle may be small enough to fit through the opening and may even follow the liquid stream around a fiber.
However, if its path brings its outer surface close enough to the fiber, it can touch the fiber and become retained.
In other words, the particle did not stop because the “hole was too small.”
It stopped because its trajectory happened to bring it into contact with the fabric.
3.Inertial impaction
Larger or heavier particles and aggregates do not always follow changes in the fluid path perfectly.
When the liquid bends around a fiber, a particle with sufficient inertia can continue along its previous trajectory and collide with the fiber.
Again, the particle may theoretically fit through the space between the fibers but still be captured because of the way it moved through the flowing liquid.
4.Diffusion and random motion
Very small particles do not necessarily follow perfectly smooth trajectories.
At sufficiently small sizes, Brownian motion causes particles to move irregularly relative to the surrounding fluid.
That random movement can increase the chance that a particle encounters a fiber and sticks to it.
This is one reason why fibrous filtration does not behave like a simple kitchen sieve.
5.Surface adhesion
Once a particle or aggregate touches a fiber, whether it remains there depends on interactions between the particle, the fabric and the surrounding liquid.
Electrostatic forces, van der Waals forces, hydrophobic interactions and other surface effects can all contribute.
Some material may therefore adhere strongly.
Other material may contact the fiber briefly and then be washed away again by the moving fluid.
What about aggregates?
This becomes particularly important when nanoparticles form aggregates or agglomerates.
A single nanoscale particle may be far smaller than the spaces between ordinary textile fibers.
But an aggregate composed of many particles may be considerably larger.
A sufficiently large aggregate can therefore:
- be physically excluded from some pathways,
- collide with a fiber through interception,
- impact a fiber because of its greater inertia,
- or become trapped against previously deposited material.
Smaller aggregates, however, may still move through the fabric with the liquid.
So it is more accurate to say:
Individual nanoparticles and small aggregates may readily penetrate ordinary cloth, while larger aggregates have a greater probability of being captured — not only because of their size, but also because their trajectory through the fibrous structure increases the probability of collision and retention.
Capture is probabilistic, not absolute
This is perhaps the easiest way to understand fibrous filtration.
Imagine hundreds of tiny particles flowing toward a woven cloth.
They are not all travelling through exactly the same point in each opening.
Some trajectories pass cleanly between the fibers.
Some bring particles close enough to touch a fiber.
Some particles collide with deposited material.
Some aggregates approach an opening at an orientation that prevents them from passing.
Others of similar size may enter another opening and continue through.
The thicker the fibrous layer becomes, the more opportunities there are for such collisions.
That is why adding layers can increase particle capture.
But increased capture probability is not the same as creating a smaller pore size.
A thick cloth can therefore retain some particles that are much smaller than its nominal openings while still allowing many other particles of similar size to pass through.
The Most Penetrating Particle Size
Filtration science also describes something called the Most Penetrating Particle Size, or MPPS.
Fibrous filters generally capture very large particles relatively efficiently through interception and inertial impaction.
Very small particles can also be captured through diffusion and, in some filters, electrostatic effects.
Between these regimes is a particle-size range where none of these mechanisms is particularly dominant. This produces a minimum in filtration efficiency known as the MPPS.
For many fibrous filters, the MPPS occurs roughly in the 100–400 nanometer range, although the exact value depends strongly on the filter material, fiber structure, airflow or fluid velocity, particle charge and other operating conditions.
The important lesson is that filtration efficiency does not simply decrease continuously as particles become smaller.
Fibrous filtration is governed by several competing capture mechanisms, not by pore size alone.
What this means for filtering urine through cloth
If material remains on a cloth after urine has passed through it, several explanations are possible:
- relatively large sediment was too large to pass through certain pathways;
- aggregates collided with fibers;
- particles were intercepted because of their particular trajectory;
- very small particles underwent random motion and contacted fibers;
- material adhered to the textile surface;
- or material became trapped behind particles that had already accumulated.
At the same time, other individual nanoparticles or small aggregates may have followed different fluid pathways and passed through the cloth.
Therefore:
Material visible on the fabric demonstrates that some material was retained. It does not demonstrate that the cloth reliably filtered nanoparticles from the liquid.
And equally important:
A nanoparticle that becomes retained by ordinary cloth is not necessarily retained because the cloth had a nanoscale opening. It may simply have encountered and adhered to a fiber during its passage through the material.
