Spreading kinetics

Within the framework of this general description, two limiting cases can be defined depending on the rate of the chemical reaction at the triple line compared to the rate of diffusion of reactive solute from the drop bulk to the triple line (or of a soluble reaction product from the triple line to the drop bulk). Note that this description does not need the knowledge of the detailed (and complicated) configuration close to the triple line, i.e., inside a small volume a few nm (or tens of nm) in diameter. 

An example is the behaviour of Cu-Si alloys on vitreous carbon (Dezellus et al. 1998). In this system, wetting is promoted by the formation of a continuous layer, a 

The Arrhenius activation energy AG in equation (2.20) obtained for Cu-Si alloys of constant composition is close to 250 kJ.mol which is of the order of magnitude expected for a chemical reaction. 

For systems exhibiting linear wetting, it is expected that the spreading rate will be sensitive to the structure of the solid. Indeed, in the Al/C system, the spreading 

In general, solutes are transported in the liquid by convection and diffusion and the governing equation is Fick s second law written in the referential of the triple line moving with a velocity U (U = dR/dt)  

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Figure 6 presents the variation of the contact angle, 6(0, on the same rubber, but this time the elastomer had not been preswollen by TCP. In comparison with the spreading kinetics on rigid solids (Teflon PFA, fused silica), we observe the same qualitative behavior as before, i.e., a relatively slower variation of the contact angle 6(t) on the elastomer. However, after the main variation of the contact angle occurring within the first 30 minutes, we observe a very slow, yet continuous, decrease of 6(t) over a period of several hours before a stable value for Go is obtained. 

Cuvelier D, Rossier O, Bassereau P et al (2003) Micropatterned adherent/repellent glass surfaces for studying the spreading kinetics of individual red blood cells onto proteindecorated substrates. Eur Biophys J Biophys Lett 32(4) 342-354... 

Changes in experimental temperature between 510°C and 920°C have only a small effect on U0 and spreading time (a few tens of %). The relatively weak effect of temperature on spreading kinetics appears to be a common feature in spreading of non-reactive liquid metals on metallic solids. 

Unfortunately, there are no specific experimental studies of spreading kinetics of alloy/oxide systems in which only adsorption occurs without formation of a 3D compound. However, when analysing R(t) curves in reactive systems, it may be useful to calculate the spreading rate in the case where the decrease of the contact angle is due to the lateral extension of an adsorption layer. This needs diffusion of interface-active solute from the drop bulk to the liquid adjacent to the adsorption layer at the triple line, followed by a transfer from the liquid to the adsorption... 

At high temperatures, there is always some dissolution of the oxide in the liquid metal but to what extent can this reactivity affect wettability Useful insight can be obtained by studying spreading kinetics the time for millimetre size droplets to reach capillary equilibrium is less than I0-1 second for non-reactive systems (see Section 2.1.1), so much slower spreading kinetics are a strong indication of control by interfacial reactions. 

Figure 8.8. Spreading kinetics for C-presaturated Ni (squares) and pure Ni (full circles) on polycrystalline graphite at 1460°C. From data reported in (Naidich et al. 1972).

These conclusions are also valid for vitreous carbon and diamond although some differences exist in the final contact angle for non-reactive metals and the spreading kinetics for reactive metals. 

The grating coupler has been used for monitoring drugs, pesticides and metal ions, for studies of affinity reactions and for measurement of adhesion and spreading kinetics of living cells. 

The quality of energy tells us the form of the energy (potential or kinetic). The quantity of energy tells us how much. The total amount (or quantity) is conserved. However, when concentrated potential energy is converted to spread kinetic energy, we say that the quality is decreasing. 

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