Less is more: Unstable foams clean better than stable foams

Abstract

Hypothesis

Foamed surfactant solutions can clean surfaces! We hypothesise that the cleaning efficiency depends on the liquid fraction and on the stability of the foam. We also hypothesise that the cleaning efficiency is the better the smaller the average bubble size is.

Experiments

The double syringe technique was used to generate foams with varying liquid fractions but the same, very small bubble sizes with and without perfluorohexane in the gas phase. We performed cleaning tests in which the foams were applied to glass substrates contaminated with a fluorescent oil.

Findings

We found that unstable foams clean better than stable foams. Three cleaning mechanisms were identified: (1) imbibition at low liquid fractions, (2) wiping, i.e., shifting of the contact line between oil, foam and glass, at all liquid fractions, and (3) drainage at high liquid fractions. The change of the liquid fraction and of the foam stability lead to different combinations of these mechanisms and thus to different cleaning results.

Introduction

In the last 20 years colloid and material science have made significant contributions to the development of cleaning processes for sensitive surfaces such as historical surfaces of artistic and cultural assets. Cleaning sensitive surfaces is quite a challenge because a tailor-made cleaning method needs to be developed for each individual surface. As outlined by Baglioni et al. [1] the most promising methods for cleaning sensitive surfaces include new “green” surfactant-based self-assembled systems, different types of emulsions and microemulsions, responsive hydro- and organogels, nanoparticle dispersions in apolar solvents, and hybrid organic-inorganic nanocomposite systems [1], [2]. Our goal is to develop innovative, foam-based cleaning methods for sensitive surfaces. Foams can be material-preserving and environmentally friendly alternatives for the cleaning of surfaces, especially for those that should not be exposed directly to the cleaning solution. Using foams, one can reduce the amount of surfactant by up to 90% compared to the use of the respective non-foamed cleaning solution. Furthermore, foam-specific physical mechanisms can make the cleaning process much more efficient than that of the non-foamed cleaning solution. We consider the cleaning process as efficient if (a) no cleaning solution drains out of the foam, (b) contaminations (e.g. oils, soot, pesticides, microbes) are soaked up by the foam, (c) the cleaning process is fast.

Note that foams are commonly considered to have no cleaning effect but to be only a by-product of the cleaning process and an aesthetic add-on for the user [3], [4]. Nevertheless, current research shows that foamed surfactant solutions can clean far more efficiently than the non-foamed ones due to two reasons [3], [5], [6], [7]: (1) presence of air–liquid interfaces and (2) imbibition. Both effects are briefly discussed in the following.

• That interfaces play a significant role in cleaning processes was demonstrated in two studies. In the first study Andreev et al. [5], [6] showed a positive influence of the meniscus on the cleaning of silicon wafers contaminated with strongly adhered Si₃N₄ particles in an immersion/withdrawal cell. It was shown in [5] that in the case of non-foamed suspensions the cleaning occurs only in the narrow meniscus region (70% efficiency), while a foamed particle suspension leads to significantly better cleaning in the bulk of the cell (90% efficiency) [6]. Jones et al. [3] found an improved removal of a model lipid mixture (human sebum) from silicon wafers in the presence of an air-liquid interface and discussed two reasons for this observation. (a) The concentration of surfactant molecules is increased at the interface, allowing impurities/oil to be more easily absorbed into micelles [3]. (b) The sliding of the contact line over the solid interface (due to wettability defects) is helpful in removing oil and dirt from the solid interface.

• The second reason for improved cleaning with foams is capillary imbibition. Mensire et al. [8], [9] put dry aqueous foams into contact with either a miscible (glycerol containing surfactant solution) or an immiscible liquid (olive or sunflower oil). For both types of liquid, a remarkable ability of the foams to soak up the liquids into the Plateau border network of the foam was observed. For both types of liquid, the imbibition becomes slower as time increases. The authors describe numerically the details of the experimental imbibition process, which is driven by the capillary pressure and resisted by viscous and gravity forces in the Plateau borders [9]. They identify two parameters that control the imbibition efficiency: (i) the ratio between the oil–water and the air-water interfacial tension and (ii) the Bond number B = g ρ_l R² σ⁻¹, which measures the relevance of gravitational stresses g ρ_l R (g - gravity, ρ_l - liquid density) with respect to the capillary pressure σ/R (σ - surface tension)) using the mean bubble radius R as the characteristic length. The first parameter predicts the imbibition strength. The second parameter tells us that the smaller the bubble size the larger is the capillary pressure and thus the imbibed liquid amount. Note that, reducing the bubble size one observes a slowing-down of the imbibition process, since at the same liquid fraction, Plateau borders are smaller and hence viscous dissipation becomes important. The average bubble radius was about 1–2 mm and the liquid fraction ε in the foam did not exceed 0.5% in the studies carried out by Mensire et al.) [8], [9].

As just mentioned, foams with low liquid fractions and small bubble sizes are needed to maximise imbibition, i.e. to optimise the cleaning process. Furthermore, the smaller the bubbles the more interfaces and meniscii are created in contact with the solid surface which further improves the cleaning process. The double-syringe technique is an ideal tool for generating well-controlled, small-bubble foams (see Section 2.3.). With this technique foams with bubble sizes as small as 10–20 μm and liquid fractions of 3–30% can be generated [10]. In contrast to other methods, the liquid fraction of the foam can be varied without impacting the bubble size. The aim of our study is to understand how the structure and stability of foams affect the cleaning efficiency of a foamed surfactant solution. For this purpose, we carried out cleaning tests in which foams with different liquid fractions but same bubble size were put on glass substrates contaminated with a fluorescent oil. Cleaning foams with predefined liquid fractions were produced with the double-syringe technique. For foams with different liquid fractions both the dynamics of the cleaning process and the oil imbibition ability were studied. The same experiments were carried out with perfluorohexane-containing foams to avoid Ostwald ripening during the cleaning process [11], [12], [13], [14], [15]. We will show and discuss that three different mechanisms, namely imbibition, shifting of the contact line (wiping), and drainage contribute to the cleaning process with foams and how a wise choice of the foam properties leads to an optimum interplay between the three effects and hence to optimal cleaning.