Dilatant sediments

Although the production of highly deflocculated suspensions is a primary objective for formulation of suspension concentrates, these systems tend to settle under gravity forming dilatant sediments (clays). The latter must be prevented either by controlled flocculation or by the addition of a second disperse phase to the continuous medium (1). One method which may be applied to sterlcally stabilised dispersions, is to add a free (ie. non-adsorbing) polymer to the continuous medium. 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

Normally, in sediment volume measurements, the initial volume (or height Hjj) is compared with the ultimately reached value V (or H). A coUoidally stable suspension gives a close-packed structure with relatively small sediment volume (dilatant sediment, referred to as clay). A weakly flocculated or structured suspension gives a more open sediment, and hence a higher sediment volume. Thus, by comparing the relative sediment volume V/V or height H/H, it is possible to distinguish between a clayed and flocculated suspension. 

Sedimentation of Suspensions and Prevention of Formation of Dilatant Sediments (Clays)... 

At sufficiently high volume fraction of the suspensions and high volume fraction of free polymer a 100% sediment volume is reached and this is effective in eliminating sedimentation and formation of dilatant sediments. 

When starch is added to cold water (below 29°C, 85°F), only negligible swelling will occur. However, the suspension volume expands, since the insoluble starch replaces water. Addition of starch to water at a concentration of 10% will increase the volume by 13%. The maximum in suspension solids is 40-45%. Various methods are used to determine the solids content of the starch slurry aerometer,92 density cells, densitometer, attenuation of vibration (Dynatrol) or a radiation-type density meter. Concentrated starch slurries have high viscosity and shear thickening (dilatent) rheology. Settling of starch from the slurry produces densely packed sediments that are difficult to disperse. 

As shown in Equation (4.81), suspended particles eventually settle and form dilatant (cake) sediments, which are difficult to redisperse. Several methods to control sedimentation and cake formation will be described here. 

A previously healthy 63-year-old man, who had taken quinidine gluconate 972 mg/day for 9 months, developed diffuse edematous erythema on the extensive surfaces of the hands, arms, and face, with marked accentuation over the joints. His nail-fold capillaries were dilated and the shoulder abductors were shghtly weak. His erythrocyte sedimentation rate was shghtly raised (29 mm/hour) and there was a positive ANA titer (1 640) with a speckled pattern. There were no antibodies to Sm, ribonucleoprotein, SSA or SSB antigens, or histones. There was no evidence of inflammatory myopathy on electromyography, and a skin biopsy showed a mild, superficial, perivascular, lymphocytic inflammation with positive direct immunofluorescence for IgG and IgM at the dermoepidermal junction. There was no evidence of malignancy. All these abnormalities resolved rapidly after quinidine withdrawal. 

A strong dilatation occurs in clay subject to the clay smear process. Previously, dilatation has typically been perceived as a characteristic of strong rock e.g., during shear of two rough surfaces in strongly lithified or cemented sediments. 

To Detect Chalk in Milk. Dilate the milk with water the chalk, if there bo any, will settle to the bottom in an hour or two put to the sediment an acid, vinegar for instance and if effervescence take place it is chalk. 

The thickness of LVZ (waveguides) can reach kilometers. Their low electric resistance is connected with filling of dilatant voids with brines or graphite or other sediment films. 

If the suspension also contains particles smaller than 1 pm (the colloidal fraction of the suspension), these particles will not settle but will continue to be suspended in the liquid so that the liquid above the sediment remains cloudy. The sediment exhibits dilatant flow behaviour. 

Flow behaviour sediment dilatant Flow behaviour sediment plastic... 

With case (c), consisting of a wide distribution of particle sizes, the sediment may contain larger proportions of the larger size particles, but a hard clay is still produced. These clays are dilatant (i.e. shear thickening) and they can be detected easily by inserting a glass rod in the suspension. Penetration of the glass rod into these hard sediments is very difficult. 

Fig. 13.5. An 80 year old lady, 3 years after fundoplication presented with severe solid food dysphagia, weight loss and heartburn. Endoscopy revealed severe esophagitis. Upper G1 series showed a constant level of mucus and sedimented contrast material as a sign of delayed oesophageal clearance (white arrow). Small parts of the stomach are above the diaphragmatic hiatus (black arrow), indicating a partial migration of the fundoplication without disruption of the wrap itself. pH monitoring did not reveal reflux and the oesophagitis was suspected to be due to retention. Dilatation of the wrap was successful...

A much less common type of behaviour is that of the shearthickening or dilatant liquid, which has a consistency curve of the shape of curve 4 in Fig. 6.3. The apparent viscosity increases with shear rate and, at sufficiently high shear rates, the apparent viscosity may become so high that flow ceases, an effect known as shear blocking. Shear-thickening behaviour is associated with tightly-packed, lightly-wetted or unwetted disperse substances such as oil paint sediment and wheat starch in water. 

In our explanation of dilatancy the condition of stability is essential, because only stable Suspensions can give a sediment which is dense enough (see 5b, p. 357) to show dilatancy. In a flocculated system the concentration of particles remains too low. 

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