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Introduction to Critical Point Drying (CPD) Incorporating K850 Crtical Point Dryer

Table of Contents:

1. Introduction
2. The Critical Phenomena
3. Critical Point Drying
4. Intermediate Stage
5. Transitional Stage

INTRODUCTION

The advent of Scanning Electron Microscopy (SEM) in the study of surface morphology in biological applications made it imperative that the surface detail of a specimen was preserved.

The air (evaporative) drying of specimens can cause deformation and collapse of structures, the primary cause of such damage being the effects of surface tension. The specimen is subjected to forces, which are present at the phase boundary, as the liquid evaporates. The most common specimen medium, water, has a high surface tension to air, by comparison that for acetone is several times lower. The surface tension could be reduced by substitution of a liquid with a lower surface tension with expectations of reduced damage during air-drying.

However, the occurrence of what is known as 'continuity of state' suggests a drying technique for which the surface tension can be reduced to zero.

If the temperature of liquefied gas is increased the meniscus becomes flatter indicating a reduction in the surface tension. If the surface tension becomes very small the liquid surface becomes very unsteady and ultimately disappears.

When this 'critical point' is reached, it is possible to pass from liquid to gas without any abrupt change in state. If a specimen had been in the liquid it would have experienced a transition to a 'dry' gas environment without being in contact with a surface, avoiding the possibility of the damaging effects of surface tension.

This is termed Critical Point Drying (C.P.D.) the basis of which are the classic experiments carried out over 100 years ago during investigations on the liquefaction of gasses.

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