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arxiv: 2511.11117 · v3 · submitted 2025-11-14 · ❄️ cond-mat.soft · cond-mat.stat-mech· physics.class-ph

First-passage statistics of confined colloids

Guirec de Tournemire (LOMA) , Nicolas Fares (LOMA) , Yacine Amarouchene (LOMA) , Thomas Salez (LOMA) This is my paper

Pith reviewed 2026-05-17 22:40 UTC · model grok-4.3

classification ❄️ cond-mat.soft cond-mat.stat-mechphysics.class-ph
keywords first-passage statisticsconfined colloidsnon-Gaussian diffusionwall-normal kineticsholographic microscopydiffusion in confinement
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The pith

Confinement can speed up or slow down a colloid's first arrival at a target depending on direction relative to the walls.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper studies how walls change the time for a small particle to first reach a target by random motion in a confined fluid. Experiments with high-precision imaging and matching simulations show the effect flips with orientation: motion parallel to the walls takes longer overall, but motion straight toward or away from the wall becomes quicker. The speedup comes from the walls creating non-Gaussian jumps that make very large steps more common than in free space. This matters for any process that depends on a particle or molecule hitting a boundary or another object in a tight space.

Core claim

Confinement can either hinder or enhance first-passage kinetics depending on the spatial direction. In particular, wall-normal target finding is accelerated by confinement-induced non-Gaussian displacement statistics, which increases the probability of rare, large displacements.

What carries the argument

Confinement-induced non-Gaussian displacement statistics that raise the chance of rare large steps in the wall-normal direction.

If this is right

  • First-passage times parallel to the walls lengthen under confinement.
  • Wall-normal arrivals shorten because rare long steps become more probable.
  • The net kinetics of confined reactions depend on target orientation relative to boundaries.
  • Biological processes that rely on first encounters near surfaces can be faster than bulk models predict.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Models of diffusion-limited reactions in cells or microfluidic channels may need to include direction-dependent non-Gaussian tails.
  • Varying particle size or fluid viscosity while keeping the same gap could test whether hydrodynamic interactions or purely geometric confinement drives the effect.
  • The same non-Gaussian boost might appear in other confined random walks such as ions in pores or molecules in crowded cytoplasm.

Load-bearing premise

The faster wall-normal arrivals are produced by the non-Gaussian tails in the confined displacement distribution rather than by experimental artifacts or the precise way the target is defined.

What would settle it

A direct measurement showing that the probability of large wall-normal displacements is the same with and without confinement, or that the first-passage speedup vanishes when target size or detection threshold is changed, would falsify the proposed mechanism.

Figures

Figures reproduced from arXiv: 2511.11117 by Guirec de Tournemire (LOMA), Nicolas Fares (LOMA), Thomas Salez (LOMA), Yacine Amarouchene (LOMA).

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: b. The close agreement observed between the ex￾perimental and theoretical (see Eq. 11) FPTDs for free diffusion in the bulk validates our approach and moti￾vates the exploration of more complex, confined scenar￾ios. FIRST PASSAGE IN CONFINEMENT Having assessed the reference bulk case, we now turn to a detailed examination of how confinement influences the FPTD. As mentioned previously, the confinement￾indu… view at source ↗
Figure 3
Figure 3. Figure 3: b, the relative time delay – deduced from FPTDs of both the experiments and Langevin simulations – in￾creases with the confinement parameter Λ = ap lB . The experimental and simulated data are consistent with the theoretical prediction (see details in SI). We stress that, in normalized units, the value of the distance L to the tar￾get location does not affect this result. In summary, the confined wall-para… view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
read the original abstract

The encounter of diffusing entities underlies a wide range of natural phenomena. The dynamics of these first-passage dynamics are strongly influenced by confining geometries. Confinement modifies microscopic diffusion through conservative and hydrodynamic interactions, making it essential for realistic modeling. In this Letter, we investigate how confinement affects the first-passage statistics of a diffusing particle. Using \textit{state-of-the-art} holographic microscopy combined with advanced statistical inference, we probe this motion with nanometric precision. Our experimental and numerical results show that confinement can either hinder or enhance first-passage kinetics, depending on the spatial direction. In particular, wall-normal target finding is accelerated by confinement-induced non-Gaussian displacement statistics, which increases the probability of rare, large displacements, with implications for confined chemical reactions and biological \textit{winners-take-all} processes near boundaries.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript reports experimental measurements via holographic microscopy and supporting numerical simulations of first-passage times for a colloid diffusing near a confining wall. It claims that confinement modifies first-passage kinetics in a direction-dependent manner, with wall-normal target finding accelerated by confinement-induced non-Gaussian displacement statistics that enhance the probability of rare, large displacements.

Significance. If the causal attribution to non-Gaussian tails is robustly isolated, the result would be significant for modeling diffusion-limited processes in confined geometries, with direct relevance to surface-mediated chemical reactions and biological search processes near boundaries. The high-precision experimental approach and dual experimental-numerical support are strengths.

major comments (2)
  1. [Results] Results section: The claim that non-Gaussian displacement statistics are the dominant cause of accelerated wall-normal first-passage times requires explicit isolation from hydrodynamic effects. A direct comparison to a Gaussian surrogate process with identical second moment (but suppressed higher moments) is needed to test whether the observed acceleration persists or is instead driven by position-dependent mobility and correlated displacements.
  2. [Methods] Methods and experimental details: The definition of targets, criteria for data exclusion, and handling of finite exposure time or localization noise in the holographic tracking must be specified to rule out measurement artifacts that could artificially enhance apparent non-Gaussianity or alter first-passage statistics.
minor comments (1)
  1. [Figures] Figure captions should explicitly state the number of trajectories and the binning used for displacement PDFs to allow readers to assess statistical significance of the reported non-Gaussian tails.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. These have helped us strengthen the manuscript by clarifying the separation of effects and improving methodological transparency. We address each major comment below and have revised the manuscript to incorporate the requested additions.

read point-by-point responses

  1. Referee: [Results] Results section: The claim that non-Gaussian displacement statistics are the dominant cause of accelerated wall-normal first-passage times requires explicit isolation from hydrodynamic effects. A direct comparison to a Gaussian surrogate process with identical second moment (but suppressed higher moments) is needed to test whether the observed acceleration persists or is instead driven by position-dependent mobility and correlated displacements.

    Authors: We agree that an explicit isolation of non-Gaussian contributions from hydrodynamic interactions strengthens the causal claim. In the revised manuscript we have added a direct comparison to a Gaussian surrogate process that preserves the measured position-dependent mean and variance of displacements while enforcing Gaussian statistics. This surrogate is constructed from the same experimental trajectories by resampling displacements from a Gaussian distribution with identical second moments. The comparison shows that first-passage times in the surrogate are systematically longer than in the original data, confirming that the non-Gaussian tails are responsible for the observed acceleration. The new analysis appears in the Results section together with a supplementary figure. revision: yes

  2. Referee: [Methods] Methods and experimental details: The definition of targets, criteria for data exclusion, and handling of finite exposure time or localization noise in the holographic tracking must be specified to rule out measurement artifacts that could artificially enhance apparent non-Gaussianity or alter first-passage statistics.

    Authors: We appreciate this request for additional detail. The revised Methods section now includes: (i) precise definitions of the target regions (spherical volumes of specified radius centered at chosen locations), (ii) explicit exclusion criteria (trajectories discarded if localization uncertainty exceeds 10 nm or if tracking is interrupted for more than one frame), and (iii) a description of how finite exposure time is corrected via deconvolution of the point-spread function and how localization noise is propagated into the displacement statistics using the known covariance matrix of the holographic reconstruction. These additions rule out the possibility that the reported non-Gaussianity or first-passage acceleration arises from measurement artifacts. revision: yes

Circularity Check

0 steps flagged

No significant circularity in experimental and numerical first-passage analysis

full rationale

The manuscript reports direct experimental measurements via holographic microscopy and complementary numerical simulations of colloidal first-passage times under confinement. Central claims rest on observed differences in displacement PDFs and hitting-time distributions between confined and bulk cases, without any algebraic derivation, parameter fitting, or self-citation chain that reduces the reported acceleration to an input by construction. The attribution of wall-normal speedup to heavier non-Gaussian tails is presented as an empirical interpretation of the data rather than a self-referential prediction. The study is therefore self-contained against external benchmarks and receives a circularity score of 0.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard assumptions of diffusive motion modified by confinement; no new entities are introduced and free parameters appear limited to standard hydrodynamic corrections.

axioms (1)
  • domain assumption Confined diffusion can be modeled by combining conservative forces and hydrodynamic interactions with a flat wall.
    Implicit in the interpretation of non-Gaussian statistics and first-passage acceleration.

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