Matching the nano- to the meso-scale: Measuring deposit–surface interactions with atomic force microscopy and micromanipulation

Abstract

Many researchers have studied the effects of changing the surface on fouling and cleaning. In biofouling the 'Baier curve' is a well-known result which relates adhesion to surface energy, and papers on the effect of changing surface energy to food fouling can be found more than 40 years ago. Recently the use of modified surfaces, at least at a research level, has been widespread. Here two different ways of studying surface-deposit interactions have been compared. Atomic force microscopy (AFM) is a method for probing interactions at a molecular level, and can measure (for example) the interaction between substrate and surfaces at a nm-scale. At a μm-mm level, we have developed a micromanipulation tool that can measure the force required to remove the deposit; the measure incorporates both surface and bulk deformation effects. The two methods have been compared by studying a range of model soils: toothpaste, as an example of a soil that can be removed by fluid flow alone, and confectionery soils. Removal has been studied from glass, stainless steel and fluorinated surfaces as examples of the sort of surfaces that can be found in practice. AFM measurements were made by using functionalized tips in force mode. The two types of probe give similar results, although the rheology of the soil affects the measurement from the micromanipulation probe under some circumstances. The data suggests that either method could be used to test candidate surfaces. 2 INTRODUCTIONThe cleaning of process plant is a difficult multiscale problem; metre-scale plant becomes clean as a result of fluid and chemical action on surfaces of individual plant items, acting at the meso-and nano scale. A sketch of the different length scales involved in cleaning process plant is given as Figure 1. To remove deposit from surfaces is difficult both to understand scientifically and to do industrially (Wilson, 2005; Fryer et al., 2006)

Fouling and Cleaning Studies in the Food and Beverage Industry Classified by Cleaning Type

“…A hygienic surface needs to be smooth, easy to clean, able to resist wear, and retain its hygienic qualities. Stainless steel is the most common food contact material used in the industry, being stable at a variety of temperatures, inert, relatively resistant to corrosion, and it may be treated mechanically or electrolytically to obtain a range of finishes (Akhtar and others ). The wettability of a surface is dependent on its surface energy.…”

“…Wetting is determined by the nature of both the liquid and the solid substrate. The cleanability (and disinfectability) of stainless steel has been compared with those of other materials, and is comparable to glass when cleaning microbes, and significantly better than polymers, aluminum, or copper (Akhtar and others ).…”

“…Akhtar and others () compared adhesion of a range of food and personal care foulants to different surfaces. Particle tips of different materials were attached to an atomic force microscope (AFM) cantilever to study the detachment from toothpaste and some confectionery components: Turkish delight, caramel, and sweetened condensed milk (SCM).…”

“…Force of attraction between stainless steel, PTFE (fluorinated low energy surface), and glass particles and different food materials obtained using AFM (from Akhtar and others ). F / R is the force/probe radius with units of N/m.…”

Identification of residual nano-scale foulant material on stainless steel using atomic force microscopy after clean in place

“…In cleaning research, the size of deposit is an important variable to define, especially when comparing deposits of different magnitudes. 'Length' scales to describe the size of deposits in cleaning and fouling research have been previously presented (Akhtar et al, 2010). The presented scale describes fouling layers ranging from millimeter lengths (e.g.…”