Tensions in Practice: Useful passage in tension with unwanted passage¶
Idealized porous boundary · a size-only rule
A boundary should pass small and large useful carriers while keeping another carrier out. In this toy, the useful large carrier and the unwanted carrier have the same size. An opening of 1.5 passes only the size-1 carrier. Widening it to 2.5 also passes both size-2 carriers. The size lever cannot distinguish those two simply because one is wanted.
Pass the useful carriers
Allow both intended sizes through the boundary when their passage is needed.
Restrict the unwanted carrier
Avoid admitting the unwanted size-2 carrier through the same pathways.
Why these aims pull against each other
When the selected mechanism responds only to size, equally sized carriers share its admission result. Widening that pathway gains useful passage and unwanted passage together.
Choose an arrangement to see what changes and what remains difficult.
Finite illustrative comparisons. Labels carry the meaning; color does not establish a preference or measured effect.
What this choice protects
What it costs
When it fits
Compare the arrangements
Keep the narrow opening
Use the stipulated size-only opening of 1.5. The toy rule permits a carrier only if its size is smaller than the opening.
| Toy size | Intended? | Passes? | |
|---|---|---|---|
| Small carrier | 1 | Yes | Yes |
| Large carrier | 2 | Yes | No |
| Other carrier | 2 | No | No |
- What it protects
- The unwanted size-2 carrier is excluded while the useful size-1 carrier still passes.
- What it costs
- The useful size-2 carrier is excluded too.
- When it fits
- Fits a context where the small carrier’s passage is sufficient and the missing larger useful carrier is an acceptable cost.
Illustration note: These are invented geometric eligibility values, not a physical membrane model, flux measurement or filtration rating.
Widen the opening
Use an opening of 2.5 with the same rule and unchanged carriers.
| Toy size | Intended? | Passes? | |
|---|---|---|---|
| Small carrier | 1 | Yes | Yes |
| Large carrier | 2 | Yes | Yes |
| Other carrier | 2 | No | YesUnwanted passage |
- What it protects
- Both intended carriers can pass.
- What it costs
- The equally sized unwanted carrier can pass as well; widening has not added kind discrimination.
- When it fits
- Fits a context where the larger useful carrier is needed and the unintended passage can genuinely be tolerated or handled outside this boundary.
Illustration note: A separate chemical, charge or identity-selective mechanism could change the tradeoff, but is not silently supplied by a larger opening.
What this illustration does—and does not—establish
Permeability: Bottleneck versus Leakage (sign/direction) supplies the coupled bottleneck/leakage conflict; Core names mechanism-specific selectivity. The finite carrier set shows why a size adjustment cannot perform an unencoded kind distinction.
- The boundary persists and still filters by the chosen rule; neither option claims unrestricted mixing.
- Passage eligibility is not a quantitative permeability or flux estimate. Gradients, material properties and rates are omitted.
- Real carrier shape, deformation and interactions may defeat this simple size rule.
Source entries
Permeability
Permeability: Bottleneck versus Leakage (sign/direction) supplies the conflict examined here.
Bottleneck versus Leakage (sign/direction)
Permeability fails in two opposite directions: too little passes the intended carrier (bottleneck) or too much passes an unintended one (leakage), and tuning toward one risks the other. The tension is that opening pathways to relieve congestion can open them to the wrong carrier.
Core Idea
Second, it is *selective by mechanism*: which pathways exist, and their geometry, gating, charge, or chemistry, determines what can pass and at what rate, so permeability to one carrier need not imply permeability to another.