# Geometrical selection in growing needles

## TL;DR

This paper models the growth dynamics of needles from a substrate, analyzing how random directions and interactions influence their large-time behavior through exact and Boltzmann equation methods.

## Contribution

It introduces a comprehensive analysis of needle growth with random directions and interactions, including cases on finite intervals and two-dimensional substrates.

## Key findings

- Needles exhibit specific large-time growth patterns.
- Interactions lead to freezing of some needles, affecting overall growth.
- Different geometries influence the growth evolution.

## Abstract

We investigate the growth of needles from a flat substrate. We focus on the situation when needles suddenly begin to grow from the seeds randomly distributed on the line. The width of needles is ignored and we additionally assume that (i) the growth rate is the same for all needles; (ii) the direction of the growth of each needle is randomly chosen from the same distribution; (iii) whenever the tip of a needle hits the body of another needle, the former needle freezes, while the latter continues to grow. We elucidate the large time behavior by employing an exact analysis and the Boltzmann equation approach. We also analyze the evolution when seeds are located on a half-line, on a finite interval. Needles growing from the two-dimensional substrate are also examined.