Finger sponge

The textural implications of the above characteristics of the stress-strain relationships are not always clear. When one examines a bread loaf or a roll with the fingers to evaluate its freshness, it seems obvious that the perceived mechanical stimulus is associated with the first region of the curve. Yet, in mastication, the compact s resistance to tearing probably plays a more significant role than the first and second stages of the compression. At the point where the bread crumb is tom, however, the specimen may have already been wetted by saliva so that the relationship between the stress-strain characteristics of the dry sponge and its perceived textural properties is usually obscured. 

Try smacking the spoon down on the oobleck, and then try picking up the oobleck in your hands. It feels fairly solid when you pick it up and squeeze it, but if you hold it in the palm of your hand, it appears to melt into a puddle and you can drip it between your fingers. Oobleck behaves in this manner—not quite solid, not quite liquid—because cornstarch traps water, somewhat like a sponge, but the structure of starch is less rigid so it can flow with its water load. 

Sponges occur in a variety of forms. In the intertidal zone, their shapes are often dictated by the conditions in which they live. Sponges in high-energy water, where the wave action is strong, may be thin and flat. Those in quieter water may grow into vase or finger shapes. 

Figure 7. Photo-micrographs of the casting solution/precipitant interphase at (a) the beginning of the precipitation and after (b) 12 sec., (c) 2 f sec. and (d) 5 min. Series I giving a "finger"-type structure and series II giving a "sponge"-type structure.

Skin Type Membranes With "Sponge"- and "Finger -Like Structures. In... 

The formation of the sponge-structured membranes can be easily rationalized utilizing the description of the precipitation process given above. With finger-structured membranes the formation process is more complex and cannot entirely be described by the thermodynamic and kinetic arguments of phase separation processes. 

The membrane thickness comes to a minimum where permeation has the lowest value. Scanning electron microscope studies on membrane substrate structure revealed that a change from a finely pored sponge structure to a coarsely pored finger structure occurs at the point where the membrane thickness turns to go up with increase in cyclohexanone content as already shown in Figure 6. 

Figure 1.14 Scanning electron micrograph of membrane cross sections with typical structures (a) symmetric microporous membrane without a "skin" (b) asymmetric membrane with a "finger"-type structure and a dense skin at the surface (c) asymmetric membrane with a "sponge"-type structure, a dense skin, and pore sizes increasing from the surface to the bottom side (d) symmetric membrane with a sponge structure, a dense skin and a uniform pore size distribution in the substructure.

The smaller the solubility parameter disparity of solvent and polymer, the better the compatibility of solvent and polymer, and the slower the precipitation of the polymer. Thus, the tendency for a change from a sponge to a finger... 

Skin Type Membranes With "Sponge"- and "Finger"-Like Structures. In skin-type membranes, the two characteristic structures shown in Figure 1.14 are obtained. One is a sponge-like structure and the other is a finger-like substructure underneath the skin. 

In preparing membranes via the phase inversion process for applications in pressure-driven processes, the formation of macrovoids should be avoided completely. These finger-like pores of the type present in the substructure of membranes (b) and (c) of Fig. 3.6-1, severely Hmit the compaction resistance of the membrane. Membranes with a sponge-Hke structure (Fig. 3.6-la) are to be preferred. 

Table 12 Residual Effect of Two Antimicrobial Products on Bacteria Transferred to Fingers from a Sponge...

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