Biomedical subjects

Terence D Blake

Publications and source records attributed to Terence D Blake.

3 recordsLinked to original sources

PubMed2006

Nonlocal hydrodynamic influence on the dynamic contact angle: slip models versus experiment.

Experiments reported by Blake [Phys. Fluids., 11, 1995 (1999)] suggest that the dynamic contact angle formed between the free surface of a liquid and a moving solid boundary at a fixed contact-line speed depends on the flow field and geometry near the moving contact line. We examine quantitatively whether or not it is possible to attribute this effect to the bending of the free surface due to hydrodynamic stresses acting upon it and hence interpret the results in terms of the so-called "apparent" contact angle. It is shown that this is not the case. Numerical analysis of the problem demonstrates that, at the spatial resolution reported in the experiments, the variations of the "apparent" contact angle (defined in two different ways) caused by variations in the flow field, at a fixed contact-line speed, are too small to account for the observed effect. The results clearly indicate that the actual (macroscopic) dynamic contact angle--i.e., the one used in fluid mechanics as a boundary condition for the equation determining the free surface shape--must be regarded as dependent not only on the contact-line speed but also on the flow field and geometry in the vicinity of the moving contact line.

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PubMed2006

The physics of moving wetting lines.

Scientists tend to think in terms of their most familiar models. It is not accidental that the earliest descriptions of the moving wetting line and its associated dynamic contact angle were in terms of displaced equilibria (chemists), friction (physicists) and viscous bending of the liquid-vapour interface (engineers and mathematicians). Each of these approaches has progressed since its inception, but, while each reflects a different facet of the underlying physical mechanism, and each offers at least a semi-empirical route to its description, none is complete. There is, as yet, no fully agreed treatment that is consistent with all three viewpoints and provides an effective basis for prediction -- though at least one new hydrodynamic approach has emerged that goes some way in this direction. This paper seeks to offer a status report: to briefly review each of the current approaches, to illustrate their successes and limitations as revealed by experiment and simulation, and to suggest ways in which the different aspects of wetting dynamics might be investigated in the future.

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PubMed2004

Wetting at high capillary numbers.

The coating of liquids onto solids is an important industrial process. A prerequisite for successful coating is that the liquid dynamically wet the surface of the solid. One of the limits to high-speed coating is the onset of dynamic wetting failure, which leads to air entrainment. In simple experiments in which a tape or fibre plunges vertically into a pool of liquid, air entrainment usually occurs at capillary numbers Ca < 1. However, this limit is not immutable. Indeed, the term "hydrodynamic assist" has been coined to emphasise the fact that coating flows may be manipulated to promote wetting and so postpone air entrainment. Commercial curtain coating typically operates in the range 0.5 < Ca < 10. Flow visualisation of this process has shown that hydrodynamic assist leads to a reduction in the dynamic contact angle for a given wetting speed and it is this that permits the higher coating speeds. Methods of coating optical fibres have evolved to the point where comparatively viscous liquids can be coated successfully at very high speeds with Ca of order 1000. In a recent paper Jacqmin, (D. Jacqmin, J. Fluid Mech. 455 (2002) 347) has suggested that in this case air entrained into the coating dissolves under the high fluid pressures found in the coating die, which are of order 1 MPa. Here we report successful curtain coating over the interval 0.5 < Ca < 50. The new study supports an alternative hypothesis that the postponement of air entrainment to very high capillary numbers is the result of intense hydrodynamic assist.

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