Work, Day 4

I spent today reading a few more papers which I will summarize here.
"Laser-Cleaning Techniques for Removal of Surface Particulates" by Tam et al

The paper begins by summarizing what it considers the 3 most important attractive forces between particulates and the varnish/object substrate to which they attach. The three forces are the Van der Waals force, the capillary force, and the force of attraction between an induced double layer of charge. The magnitude of each force exceeds the force of gravity.

The Van der Waals is typically predominant for particulates less than a few microns in diameter, and is the force of attraction between an instantaneous dipole in one body and an induced dipole in another. The most important facet of this attractive force is that it scales as d/z^2, where d is the particle diameter and z^2 is the microscopic distance between the particulate's surface and the surface of whatever substance it is attracted to. Although d will decrease for smaller particles, the particle density will greatly increase, and in general smaller particulates are harder to remove. However, this model assumes no compression/deformation of the particulate in the area of contact. Tam says that the VdW force is much greater if there is compression, but fails to mention by how much.

The capillary force is the "suction" force which may be present if a layer of liquid is trapped between particulate and substrate. It scales as d.

The force resultant from attraction between a double layer of charges results from charge transferral upon contact between particulate and substrate, which creates a contact potential of some magnitude at the border region. It scales as d/z.

Note that each of the attractive forces scale as d. Assuming mass ~d^3, employing force balance and F=ma, one needs an acceleration which scales as 1/d^2 to remove the particulate. In other words, smaller particles are more difficult to remove.

Tam then proceeds to describe a few processes for laser cleaning, defining efficiency as the removal of smaller particles with lower fluence and fewer pulses so as to maintain substrate integrity. The first process was dubbed Dry Cleaning, in which the substrate is hit with a laser wavelength at which it strongly absorbs radiation. The substrate then expands thermally and ejects the particulates. The governing equation is:(I haven't figured out how to use Greek letters on blogger yet)
A second related method involves blasting the object with laser light at a wavelength where the particulates strongly absorb. If the fluence is high enough, the particulate will sublimate or ablate.

The process Tam seemed more excited about he dubbed "Steam laser cleaning" wherein the surface of the object under test is coated with a thin film of water or a solution with a high concentration of water and a low concentration of ethanol and then hit with laser light. There are three types of such cleaning, the most efficient of which was called "Strong substrate absorption." The particulate/solution/substrate mix is hit with a laser pulse of a wavelength such that the water and particulates strongly transmit, but the substrate strongly absorbs. The material then superheats the water in the solution, causing explosive evaporation which removes particulates.
It is this ability to be superheated which makes water preferable to a purely ethanol solution, but high surface tension of the water makes the addition of ethanol to allow liquid diffusion between particulates advantageous.

The other methods of steam laser cleaning were film absorption, which is problematic because absorption occurs mostly at the film/air interface and does not result in the ejection of many particles, and partial substrate absorption, which requires a higher laser fluence and may damage the substrate.

Those are the basic principles. An important side note is that the pulse width (temporal) is limited on the lower end by the necessity to superheat the water and on the higher end by worries about damaging the material with high intensity laser light.

I also read Fotakis et al's "Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward?" and Drakaki et al's "Experimental study on the effect of wavelength and fluence in the laser cleaning of silvering in late Roman coins (Mid 3rd / 4th century AD) ". I'll post a summary of those papers tomorrow morning either before I leave for work or when I get there.