# Immobilization of water drops on hydrophobic surfaces by contact line pinning at non-lithographically generated polymer microfiber rings

## TL;DR

This paper introduces a nonlithographic method to immobilize water drops on hydrophobic surfaces using contact line pinning at polymer microfiber rings, enhancing stability for lab-on-chip applications.

## Contribution

The study presents a novel, nonlithographic technique to immobilize water drops on hydrophobic surfaces via polymer microfiber rings that pin the contact line.

## Key findings

- Polymer microfiber rings effectively immobilize water drops.
- The method prevents dewetting and drop rolling on hydrophobic surfaces.
- Water drops of 50 microliters are stably held by 6.5 mm diameter rings.

## Abstract

Water drops used as reaction compartments are commonly immobilized on hydrophilic areas bordered by hydrophobic areas. For many applications, such as the trapping of non-adherent cells, it is desirable to exploit the inertness and the anti-fouling behavior of hydrophobic surfaces as well as their repulsive behavior towards adsorbates in lab-on-chip configurations. However, the immobilization of water drops on hydrophobic surfaces has remained challenging. We report a nonlithographic approach to arrest water drops on hydrophobically modified macroporous silicon (mSi) with per uorinated surface. Contact line pinning at rings of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) fibers protruding from the mSi macropores immobilizes water drops when the hydrophobically modified mSi is moved or tilted and prevents dewetting within the PS-b-P2VP fiber rings. Without PS-b-P2VP fiber rings, water drops readily roll off. The PS-b-P2VP fiber rings were prepared by dropping PS-b-P2VP solution onto hydrophobically modified mSi. Selective swelling of the P2VP in the thus-formed circular PS-b-P2VP films with hot ethanol followed by detachment of the latter yielded hydrophobically modified mSi exhibiting annular areas, in which ruptured PS-b-P2VP fibers protruded from the mSi macropores. For example, PS-b-P2VP fiber rings with diameters of 6.5 mm and widths of about 0.2 mm immobilize water drops with a volume of 50 microliters.