Rolling-up mechanism

Removal of Liquid Soil Removal of liquid (oily) soil by aqueous baths is accomplished mainly by a roll-back or roll-up mechanism in which the contact angle that the liquid soil makes with the substrate is increased by adsorption of surfactant from the cleaning bath. 

Roll-up mechanism [Figure 18.1(a)]—when the surface-active agent dispersed in a displacement liquid adsorbs at the solid-water interface, the wettability of the solid surface for water decreases and the area of contact becomes so small that buoyant forces overbalance the adhesion forces and finally cause water droplets to be released. 

Fundamental principles leading to the removal of oily soil from the solid substrate by the so-called roll-up mechanism, in which liquid oil is displaced from the surface by the washing solution in the form of dispersed tiny droplets [60], are essentially the same as those evoked in the attachment of air bubbles onto a mineral surface. 

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. ... 

The tubular nanostructures are formed through a nanosheet roll-up mechanism illustrated in Fig. 3.13. The high catalytic stability of the composites is attributed to the strong chemical interaction between the Keggin unit and the C3N4. The mechanism explaining the photocatalytic activity of these composites, also illustrated in Fig. 3.13, is the same proposed before for the HPA supported on semiconductor materials. 

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... 

Fig. 12.3 Fabrication of the nanocomposite paper units for battery, (a) Schematic of the battery assembled by using nanocomposite film units. The nanocomposite unit comprises LiPF6 electrolyte and multiwalled carbon nanotube (MWNT) embedded inside cellulose paper. A thin extra layer of cellulose covers the top of the MWNT array. Ti/Au thin film deposited on the exposed MWNT acts as a current collector. In the battery, a thin Li electrode film is added onto the nanocomposite, (b) Cross-sectional SEM image of the nanocomposite paper showing MWNT protruding from the cel-lulose-RTIL ([bmlm] [Cl]) thin films (scale bar, 2pm). The schematic displays the partial exposure of MWNT. A supercapacitor is prepared by putting two sheets of nanocomposite paper together at the cellulose exposed side and using the MWNTs on the external surfaces as electrodes, (c) Photographs of the nanocomposite units demonstrating mechanical flexibility. Flat sheet (top), partially rolled (middle), and completely rolled up inside a capillary (bottom) are shown (See Color Plates)...

From this description, it is obvious how agitation and buoyant effects of the soil could speed up this mechanism (in fact, roll up cannot occur at all unless buoyant or agitation forces act on the soil), but soil-removal rate is a kinetic question and will not be pursued further here. 

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