# Thin-Film Fluid Flows over Microdecorated Surfaces: Observation of Polygonal Hydraulic Jumps

Entry by Leon Furchtgott, APP 225 Fall 2010.

Thin-Film Fluid Flows over Microdecorated Surfaces: Observation of Polygonal Hydraulic Jumps (2010). PRL, 102, 194503.

## Summary

The paper is interested in the behavior of liquid films flowing over rough substrates whose asperities are the same sizes as the film thickness. The flow of thin films on substrates is of interest in many different fields, from industrial processes to biology and vulcanology. There has been some research on how individual isolated features of substrates affect flow, but little on the collective influence of patterns of micron-size asperities. In addition, previous studies have examined films thicker or thinner than the characteristic length scales of the substrate, while this study looks at thin liquid films with thickness on the same order of magnitude as the roughness of the substrate. The authors investigate flows through experimental observations of flows on regular arrays of micron-size pores with different sizes and arrangements of pores. The authors then model the flow using a boundary-layer approach and fit experimental results to the model.

## Experimental Setup

Water is pumped into a nozzle of radius 1.2 mm at a flow rate Q between 0.5 and 2.5 L/min (Fig. 1a). The jet impacts the center of a patterned disc of PDMS of radius 2.5 cm, which is embedded in a smooth clear acrylic plate (Fig. 1b). The patterned surface consists of square or hexagonal arrays of cylindrical posts with height H, radius R and lattice distance D of the order of 100 <math>\mu m</math>. The water spreads radially over the rough substrate, then over the smooth surrounding area, before spilling into the collection reservoir. The water sheet experiences a hydraulic jump at a particular radius. The thickness <math>h(r)</math> of the thin film propagating before the jump is of the order of a few hundreds of microns, whereas the depth of the water layer outside the jump is d = 4.3 mm. Measurements show that d does not depend on the flow rate and the substrate topography.

Intriguingly, patterning of asperities on the substrate can break the symmetry of the typically circular hydraulic jump. In Fig. 1c, the jump adopts an eight-corner star shape. The remainder of the paper examines the role of the lattice geometry and the fluid properties. The paper concentrates on effects on the thin film and assumes that the flow in the outer layer are constant.

## Results

The authors begin by varying the symmetry of the lattice (Fig. 2). They look at jumps of the same mean radius but formed over different micropatterned surfaces, with hexagonal (b) and square (c) shapes. These have an effect on the jump symmetries.

This shape selection is robust in that it is independent of the viscosity and surface tension of the water that is being flowed. This tells us that the symmetry breaking is not caused by viscocapillarity or inertiocapillary instability.

## Discussion

The paper shows/confirms that the most stable small clusters of hard spheres with short-ranged attractions can be determined by geometrical rules: (1) rotational entropy favors structures with fewer symmetry elements; (2) vibrational entropy favors nonrigid clusters, which have half-octahedral substructures sharing at least one vertex; and (3) potential energy favors clusters with both octahedral and tetrahedral substructures, allowing them to have extra bonds.

The paper's description of free-energy landscape is still incomplete because of some of the simplifications it makes. In particular, while the interaction energy in the paper is extremely short-ranged, this is not generally true. For longer-range interactions, the effects will no longer be fundamentally entropic, and they will depend on temperature as well.

## Relation to Soft Matter

This is a neat paper that fits well with the first few lectures and readings in our course. It makes you think about fundamental ideas in statistical mechanics and how to measure them experimentally. It also shows how complex configurations can quickly come about from simple short-range interactions.