Droplets: sessile

Figure 2.26 Contact angle made by a sessile droplet.

Evaporation of a Sessile Droplet of Solvent at Room Temperature... 

The influence of evaporation on the contact angle of a sessile droplet of solvent has been studied. There are three regimes that a solvent droplet on a smooth substrate undergoes during evaporation. In the first regime, the diameter of the droplet remains constant alternatively one can say that the droplet-substrate area... 

In the case of agar-agar gel, in which the double-sided model represents the hydrophilic moiety, the number of the hydrophilic moieties at the air/gel interface and at the water/gel interface is nearly the same. Consequently, the sessile droplet contact angle of water is low and nearly identical to the sessile bubble contact angle of water. 

Figure 23.6 The effects of the orientation of hydrophilic moieties on the sessile droplet contact angle and sessile bubble contact angle.

Figure 23.8 Advancing and receding sessile droplet contact angle of water on a dry ethylene/vinyl alcohol film.

Figure 23.9 Change of sessile droplet contact angle with water immersion time at 40°C.

LOCALIZED SURFACE CONFIGURATION CHANGE UNDER A SESSILE DROPLET OF WATER... 

Data shown above clearly show that the surface configuration of polymer changes when the contact medium is changed from air to liquid water. The same phenomenon occurs when a sessile droplet of water is placed on the surface of a polymer. In this case, however, the surface configuration change occurs only on the surface, which is under the sessile droplet. When the surface configuration change occurs, it creates the interaction force between the surface and the sessile droplet and holds... 

Figure 23.15 Effect of droplet volume on the contact area of water sessile droplet on a gelatin gel.

The more dramatic demonstration of the creation of interaction force between the surface and a sessile droplet of water can be seen in the measurement of the sessile droplet rolling-off angle, of which principle is depicted in Figure 23.16. If the sessile droplet contact angle is high and no surface configuration change occurs, such... 

Figure 23.16 Schematic presentation of sessile droplet sliding angle measurement.

Figure 23.17 Pictorial view of a sessile droplet of water with high contact angle remaining on the vertical surface.

Figure 25.2 depiets the sessile droplet eontaet angle of water on CF4 plasma-treated PET with varying degree of crystallinity, and the influence of water immersion and the subsequent drying. Water-immersed samples were freeze dried to... 

Figure 25.2 Sessile droplet contact angle of water on CF4 plasma treated PET films subsequently treated differently (1) samples kept in air (no water immersion), (2) samples immersed in water for 120 min, (3) water immersed samples heat treated at 100°C for 10 min, (4) water immersed samples heat treated at 180°C for 10 min.

The decay of hydrophobicity manifested by the sessile droplet contact angles of water as well as of XPS FIs peak intensity is observed in nylon 6 and PET films treated similarly. There is a direct linear correlation between the contact angles of water and the intensity of XPS F Is peaks for each substrate polymer as shown in Figure 25.4. This correlation confirms that the increase and the decrease of the contact angle of water on the surface of plasma-treated films are due mainly to the change in the surface concentration of fluorine-containing moieties, which enable us to deal with the decay and the recovery phenomena by either measurement. 

Figure 25.4 Correlation between sessile droplet contact angle of water and XPS F Is intensity of CF4 plasma treated Nylon 6 and PET films.

In the classical treatment of surface tensions, it is intuitively assumed that the surface tension of a solid, 7s, can be assigned as if it is a material constant. In a practical sense, Eq. (25.3) is valid if the surface tension of the solid does not change after the contact with the liquid (sessile droplet) is made. While Young s equation describes the force balance at the three-phase line, it does not give information relevant to the true interfacial tension at the interface that is beneath the droplet, which is the major concern of surface dynamics. In general cases, 7s and 7sl are... 

An empirical method to estimate the surface tension of a solid is Zisman s plot (cos 9 as a function of yl), which obtains the critical surface tension of wetting. In the absence of specific interaction between the surface and the liquids used for the measurement of contact angles, the critical contact angle of wetting can be accurately estimated and its value used as the surface tension of the surface. However, if a surface interacts with liquids used as the sessile droplet for the contact angle measurement, to the extent that the surface tension is altered, Zisman s plots deviate from the ideal linear relationship. In a strict sense, the plot is applicable only to imperturbable surfaces with which liquid contact does not alter surface configuration, i.e., no surface dynamics applies. 

These observations indicate that a strong attractive force is created between water and the surface under the droplet, and also that the decreased interfacial tension does not influence the force balance at the three-phase line. This implies that Young s equation given by Eq. (25.3) only applies at the three-phase line, and ysl in the equation does not represent the interfacial tension between the surface and liquid water that exists beneath the sessile droplet. 

TIME DEPENDENCE OF SESSILE DROPLET CONTACT ANGLE... 

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