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43388-AC5
Crystal Whisker Growth on Porous Coatings

Lorraine F. Francis, University of Minnesota

The overall goal of this research is to understand and control the formation of potassium chloride salt crystals with whisker morphologies.  These unusual crystals are formed during controlled evaporation from one of two set-ups: (1) a substrate with porous nanoparticle silica coating partly immersed in a reservoir of salt solution and (2) a bare glass slide partly immersed in a reservoir of salt solution that contains a small concentration silica nanoparticles.  In set-up 1, the salt solution is pulled into the pores, and then a combination of convection and evaporation leads to supersaturation of the solution in the pores, which establishes the conditions needed for crystallization.  In set-up 2, evaporation causes a particle coating to form and simultaneously the solution becomes supersaturated.  In this final year of research, we systematically explored the effects of experimental conditions on whisker growth, performed visualization studies and used models for transport phenomenon to explain the growth behavior.  We also elaborated on the whisker growth mechanism and compared set-up 1 with set-up 2.

Visualization of the initial rise of the salt solution into the porous coating (set-up 1) led to an interesting result.  For the standard porous silica coating (300 nm thick, 60% porous, 20 nm pore diameter) and a relative humidity of 80%, salt solution rose to the end of the porous coating about 2 cm from the liquid reservoir surface.  Under the same conditions, pure water rose only about 1 mm.  The capillary rise height established by a capillary driven flow (convection) and evaporation was estimated using a mass balance and found to be close to this value for water.  The much higher heights reached by the salt solution and the hazy appearance near the contact line indicates that crystallization assists capillary rise, likely due to a "creeping" phenomenon (Washburn, 1927). 

Systematic studies of whisker growth were carried out with set-up 1.  A larger reservoir was used for the salt solution so that the growth took place from a reservoir of constant concentration.  Relative humidity was varied from 60 to 80%, solution concentration was varied from 0.015 to 0.3 g/ml and porous coatings were prepared from several nanoparticle sizes.  These changes in conditions were made around a standard set of conditions with one variable changing at a time.  The whisker growth initiation time increased and whisker morphologies became straighter and more uniform as concentration of salt solution decreased, relative humidity increased or nanoparticle size in the coating decreased.  There were limits on each of these trends.  With the same changes in variables, the whisker growth zone moved further from the original liquid level.  Models for convection, evaporation and diffusion were used to explain these trends.  In particular, a model developed to understand convective flow and evaporation in colloidal crystal coatings (Brewer et al., Langmuir, 2008) was used to interpret pressure fields, and the Peclet number that establishes a balance between convection and diffusion was estimated using a model from the literature (Puyate et al., Physics of Fluids, 1998).

The evidence gathered in this research points to whisker growth by a base growth mechanism for growth using set-up 1.  First, the pore fluid becomes supersaturated by convection and evaporation.  Second, crystals form and proceed by a creeping mechanism to the surface.  Third, the surface crystal grows, fed by salt solution from the pores.  Growth proceeds on all faces exposed to the salt solution.  At some point, the growth becomes anisotropic and a whisker extends from the surface.  During this fourth stage, salt solution beneath the crystal feeds the growing whisker from the base and the whisker grows upward.  The switch to anisotropic growth likely requires a critical supersaturation, and hence whisker growth occurs in a zone above the liquid level, which can be understood in terms of the transport phenomena.

Whisker growth in set-up 2 occurs below the original liquid level, different than set-up 1.  In addition the growth was found to be faster than set-up 1, and required a lower relative humidity.  The speed of whisker growth is enhanced because the particles assemble into a coating with the salt solution completely filling the pore space and convection drives ions through the solution to the contact line.  No whiskers form when the humidity is above 75%.  The reduced evaporation rate at higher relative humidity results in a decreased rate of convective assembly of particles and perhaps a decrease in the salt concentration gradient.

Through this challenging research problem, the PI has gained experience and knowledge.  The graduate student researcher has also learned a lot of about the research process and the fundamentals of transport and crystallization.  He is currently writing papers and soon will write his thesis.     

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