Contact angles as a function of time

Equilibrium at the triple line satisfies Young s equation [Eq. (1)], but if the contact angle as a function of time, 0(0, is greater than 6o, then there is a net spreading force acting per nnit length of triple line and given by [5] ... 

Figure 15 Contact angle as a function of time for polyethylene on alumina substrate at different temperatures. (After Ref. 10.)...

Fig. 15 Water contact angle as a function of time under UV illumination for a polycrystalline Ti02 film on glass.

Y. Oshida, R. Sachdeva, and S. Miyazaki, Changes in contact angles as a function of time on some pre-oxidized biomaterials. Journal of Materials Science Materials in Medicine, 3, 306-312 (1992). 

The stability of the surface wettability was studied by measuring the water contact angles at 24 h., 72 h., and 144 h. after the plasma treatment. There was a general increase in the water contact angle as a function of time after the plasma treatment. The rate of increase in the water contact angle as a function of time after the plasma treatment for the superhydrophilic surfaces was found to be significantly smaller (p < 0.05) than the hydrophilic surfaces (6 < 10°). 

Change in contact angles as a function of storage time was also not studied. Studies on polystyrene indicate that the contact angle does change with time after plasma treatment (11). 

Fig. 3. Dynamic contact angles as a function of water storage time (left) or drying time respectively (right).

Practically, contact angles are measured as a function of time t after deposition of liquid drops. The spreading speed, U, is given by dridt, with r being the drop radius at time t. In the following, the dynamic contact angle will sometimes be expressed as a function of the spreading speed, U [0(0 = 0(0)]. 

To investigate the influence of swelhng of the substrate by the contacting liquid, the contact angle 6 of sessile drops of tricresylphosphate, TCP (drop volume 2 p,L, viscosity t = 70 cP, surface tension = 40.9 mN m ), has been measured as a function of time after deposition, t, on flat, smooth, horizontal surfaces of soft and rigid solids at 20°C. The method of measurement of contact angle is the same as in Section Ill.A. 

As we see in Chapter 6, surface tension and contact angle measurements provide information on liquid-liquid and solid-liquid adhesion energies (Fig. 1.26c). Contact angles measured under different atmospheric environments or as a function of time provide valuable insights into the states of surfaces and adsorbed films and of molecular reorientation times at interfaces. 

Water drops were formed at the mercury-benzene interface by means of a syringe, and the contact angle (measured in the water) was recorded as a function of time. For the interface between Hg and benzene saturated with water, y was measured independently as a function of time. The following table summarizes these data (all measured at 25°C) ... 

Chen and Frank [356] studied the adsorption of a series of fatty acids on to both aluminium oxide and clean glass. In particular they studied the variation of appropriate peaks in the infrared absorption spectra as a function of time in order to monitor the kinetics of adsorption. They also studied the variation of contact angle of both water and hexadecane with time allowed for adsorption. 

The grafting reaction is expected to take place in acidic aqueous media via acetaliza tion between OH groups on the EVA film and the CHO groups of the water-soluble DAS. In Fig. 23 the contact angle of the grafted film against water is shown as a function of time. A decrease in the contact angle, in other words, an increase in... 

Figure 6.37. Contact angle of a Cu-9.5 at.% Ti alloy on AI2O3 as a function of time at 1373K (Kritsalis...

Figure 7.29. Contact angles of Cu on WC plotted as a function of time. Data from work reported in...

Figure 9.4. Contact angle values for SiO2-20Al2O3-1 OCaO-1 OMgO (wt.%) on W at 1400°C plotted as (a) a function of logP02 and (b) as a function of time for logP02 = -15.5, (Ownbyetal. 1995).

Figure 3. Variation of contact angle and atomic percent oxygen as a function of time of treatment ((, 15% Ft, no cage (A., A) 3% Ft, aluminum cage (9, O) 15% F, aluminum cage reaction cortditions 3.0 mm, 40 cc/min, 50 W).

From this review it follows that for practical purposes a variety of Wilhelmy plate methods are available. Which one to choose is often a matter of practicality the choice may also depend on the problem at hand. As a trend the rapid methods are less precise, but for many purposes high precision is not needed. The choice also depends on the chemistry of the liquid. For instance, if a solution, containing slowly diffusing solute is studied, the stationary plate mode is recommended, because then one can measure the force as a function of time. Alternatively, when non-zero contact angles pose a problem, the plate can be made intentionally hydrophobic and the downward force measured. 

On the other hand, the powder tensiometry method can be applied with any Wilhelmy type tensiometer. A special thin glass tube, which has a porous plug at the bottom, is filled with the powder which will be tested and the powder column is then brought into contact with the testing liquid. The mass of liquid absorbed into the porous solid is measured as a function of time by means of an electrobalance. The amount absorbed is a function of the viscosity, density and surface tension of the liquid, the material constant of the solid, and the contact angle of the interaction, as given in the following relationship ... 

Based on mass conservation, measuring d and w for nonvolatile liquids like polymers as a function of time f allows determination of the value of the contact angle 0 and even tracking of it in time and determination of possible variations of 0 in time, without any fitting. For slipping films we get ... 

In this relation, x denotes the fluid front position as a function of time t, system architectures a and b, and intrinsic material properties including contact angles of the fluid upon the hydrophilic and hydrophobic materials 0i and 2, respectively. The solution to this second-order ordinary differential equatimi with respect to meniscus position squared can be solved relatively easy through numerical techniques. The solution determined by the investigators of this approach is in excellent agreement with experimental results [7]. Furthermore, the investigators demonstrated that confluent monolayers of cells could be cultured in their apparatus and proteins could be selectively adsorbed upon the device surface [7]. Consequently, the surface-directed... 

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