Spontaneous spreading surface roughness

Since real surfaces are not smooth or perfectly flat and most epoxy adhesives are viscoelastic fluids, it is necessary to understand the effects of surface roughness on joint strength. A viscous liquid can appear to spread over a solid surface and yet leave many gas pockets or voids in small surface pores and crevices. Even if the liquid does spread spontaneously over the solid, there is no certainty that it will have sufficient time to fill in all the voids and displace the air. The gap-filling mechanism is generally competing with the setting mechanism of the liquid. 

In general, for liquids, which are normally smooth, the value of i is equal to 1. For solids, i is always greater than 1. The effect of surface roughness on adhesion, penetration and spreading for a rough cube is shown in Table 9.3. The total work for the sum of all of these steps is referred to as the work of dispersion. When the work of dispersion is negative, dispersion is spontaneous. 

A 45° contact angle is given for a liquid on a smooth solid surface. What roughness, i, must the solid surface have to induce spontaneous spreading ... 

Surface Roughness. The wicking of a liquid into surface roughness has already been discussed and in fact may occur even when 0 > 0. In this respect, spontaneous spreading due to roughness differs from that due to surface tension gradients since the latter generally require that 9 = 0 so that a primary film can develop. 

Whene > 0° the liquid is nonspreading, but when 0 = (f the liquid wets the solid completely and spontaneously spreads freely over the surface at a rate that depends upon such factors as the liquid viscosity and roughness of the solid surface, as discussed later. Thus for spontaneous wetting to occur ... 

In Equation (4.70), the spreading of water over a smooth solid surface is spontaneous only for 0 = 0°. However, when the solid surface is rough, even for small contact angles (i.e., 0 > 0°), the spreading may occur. 

Being able to wet every cavity or hole in a rough surface has advantages in bofli nature and man-made devices. When water wets a superhydrophilic surface, it spreads spontaneously with no trap air and no measurable contact angle. In nature, such surface structures have been created in plant leaves so that the plants can adapt... 

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