II. Particles Removal

II.1 Ultrasonic cleaning

To achieve removal of particles from the substrate, an easily implemented and efficient process is the use of ultrasonic cavitation in an aqueous surfactant solution.  Despite its ubiquity and common use, it is helpful to exercise care in applying this technique.  The basic approach is simple.  An aqueous surfactant solution, generally with concentration in the range 1-5% is prepared.  The sample is placed in the surfactant solution and the latter is immersed in an ultrasound bath for a period of time ranging from one minute to fifteen minutes. This cleaning is followed by rinsing and drying of the substrate.

A second tip is that the cavitation size decreases with increasing frequency.   Thus, an ultrasonic bath operating at 68 kHz will generate smaller impacts with a higher density.   This will contribute to less damage of fragile structures and particles with smaller sizes will be removed.   This information becomes important when the particle sizes to be removed become of the order of one micron and if the sample to be cleaned bears a fragile surface structure.   

Generally, low frequencies, in the range 35 - 45 kHz, have been found to be appropriate for cleaning a wide range of industrial components, where the material is solid and rapid cleaning is desired. 

II.1.1 Ingredient for good cleaning

For efficient cleaning in an ultrasonic bath, three essential ingredients must be controlled: the presence of a surfactant solution at adequate concentration, the operating temperature, and the degassing of the solution.   The first of these is required to efficiently transmit the ultrasonic cavitation energy.   The presence of surfactant also influences the cavitation threshold, which must be below the minimum amount of energy available in the bath.   The temperature of the solution has a strong influence on the cleaning efficiency.   Finally, degassing of the solution is critical to achieving uniform cleaning.   In fact, gas bubbles will absorb cavitation energy, leading to poor cleaning efficiency.   This may be again observed using a thin sheet of aluminum foil dipped into the ultrasound bath.   For a uniform "white noise" ultrasonic bath, the foil is emerges uniformly puckered.   The presence of areas where the foil has remained smooth indicates an incomplete degassing of the solution.   Degassing is achieved by leaving the ultrasonic bath running for an extended period of time.   Last, but not least, ultrasonic agitation is efficient for heating liquids and stirring liquids.

On a more practical level, ultrasonic cleaning may be practiced as follows.   Deionized water is mixed with a standard ultrasonic cleaning surfactant, such as Chem-Crest 14, from Crest ultrasonics, in the ratio of 3% surfactant by weight.   This is poured into the ultrasound bath.   Ultrasonic agitation and heating (if present) are started.   The temperature may be set to 45°.   The solution is then allowed to warm up and degas for thirty minutes to one hour.   

II.1.2 Surfactants choice

Next, a surfactant is chosen for cleaning the glass substrate.   This surfactant should be chosen according to the substrate and the desired cleaning effect.   Alkaline surfactant solutions may cause a slight etching of the glass surface.   For example, Optical 1789, from NGL Cleaning in Switzerland, gives a solution of pH 9 at 3% concentration.   This solution was found to lightly etch a Pyrex glass surface.   To avoid etching, an acidic surfactant may be used.   Some of these surfactants are highly efficient at eliminating dust and organic surface residues.   Should the primary focus be on the removal of particles, a charged polymer surfactant may be used.   Such an example is BYK 154, from BYK-Chemie, Wesel, Germany.   
These charged polymer surfactants, designed for dispersing pigments in paints, are considered to be highly efficient at suspending particles in solution.   The chosen cleaning surfactant is diluted into pure water (see below) at the manufacturer's recommended concentration (typically 2-5%, by weight).   This solution chosen is then poured into a glass beaker or other rigid container.   Plastic containers should not be used, as these will not transmit ultrasonic energy effectively.   Container wall materials should be rigid, such as glass or metal.   The container filled with the cleaning surfactant solution is then placed in the ultrasonic bath, the ultrasonic agitation is activated, and the solution is allowed to heat and degas for 30 minutes to one hour.

II.1.3 Water purity

Pure water in this context is defined as deionized water that has received a subsequent organic removal, such as that from an activated carbon cartridge.   Water purifying systems are available from Millipore, Barnstead, or Elga, and the production of 18 megaohms x cm resistivity water free of organics is a routine laboratory installation.   
One caveat to be noted is that high resistivity (18 megaohms x cm) water is deionized (free of ions), but the organic molecule contaminants have not necessarily been removed.   In fact, the organic contamination is not measured by the resistivity of the water.   While ions are generally not considered strong contaminants for glass surfaces, the organic molecules contained in water may deposit on the glass substrate, hence re-contaminating the surface in a random manner.

The specification for pure water, particularly as relating to the cleaning of glass surfaces, implies the absence of organic molecules that may strongly adsorb to the glass surface, rendering it non-uniform and "contaminated."   This implies that the water should not come into contact with materials that may release organic molecules.   A simplistic approach would then be to only used cleaned inorganic containers (such as pyrolysed glass (see section on dry cleaning below), or carefully cleaned (possibly with Hellmanex, as described in wet cleaning section below) metal containers.   This restriction may be lifted for some plastic containers.   Nalgene plastic bottles may be used to store and dispense pure water, provided they are "cured" with pure water for several days before use.   This curing involves filling the bottles with pure water and replacing this water twice per day for one week.   The reasoning behind this procedure is that the majority of molecules that could leach out of the plastic bottles in a short time are removed by the water curing.   The cured bottle is not expected to contaminate pure water during short storage times.  This has not been confirmed by specific measurements.   One final caveat regarding pure water is to avoid storage or stagnation.   This is due to the water being a suitable medium for the growth of organisms.   Essentially, the water will grow organisms in a similar manner to a piece of bread being allowed to mold.   This is particularly critical for water purifying systems, which should be kept running and not be switched off for more than 24 or 48 hours.   If they are switched off, they may require complete cleaning and disinfecting.