Water-soil-fiber contact angle

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

Figure 6. Dependence of Detergency on Soil-Fiber-Water Contact Angle.

For quite hydrophilic surfaces like cotton, yws is smaller than yos, and a contact angle greater than 90 is commonly achieved. In this case, the roll-up mechanism is operative the water preferentially wets the fabric, causing the oily stains to be entirely lifted off the fibers into the washing solution. This behavior, shown schematically in Figure 12.13b for soil removal from a flat surface, is enhanced on cotton fabric due to swelling of the cotton fibers with water, which increases the hydrophilicity of the fabric surfaces. ... 

For low energy surfaces, i.e. hydrophobic materials such as polyester, a contact angle of less than 90 is usually observed, and small amounts of the oily soil may be removed by hydraulic forces at the soil-water interface, as shown in Figure 12.13a. If the fabric surface is completely covered by oily soil, no location is available for the surfactant solution to reach the fiber surface and undercut the soil. Observation of this emulsification mechanism has been made by many investigators for mineral oils and mineral oil/polar soil mixtures on hydrophobic flat films and... 

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