3. Results and discussion
3.1.Modification of dispersed phase flow behaviour
3.1.1.Effect of transmembrane pressure
Fig. 2 shows the resulting droplet size of the emulsion at different transmembrane pressures across a range of rotational speeds. The trend observed in general was that with increasing pressure, the droplet size decreased to a minimum before gradually increasing upon further pressure application. This behavior is more prominent when the rotational speed, and hence the shear rateislow (i.e.100RPM).It is highly likely that to some extent coalescence is a contributing factor to the relatively large droplet sizes under these processing conditions, since these droplet sizes vary between 8.6 and17.2 times larger than the pore diameter i.e. the upper end of the ratio values suggested by other authors [4,10]. Furthermore, droplet size distribution span values (d90 d10/d50) across the whole data set range dramatically between 0.720 and 2.210 depending on processing conditions. Within literature, the effect of transmembrane pressure(TMP)in membrane emulsification is contentious with the majority suggesting the droplet size increases with increasing transmembrane pressure [12–14]. Abrahamseetal. [15] found the droplet size to decrease with increasing transmembrane pressure. Within this study, it is observed that there is an initial decrease followed by an increase agreeing with the work of Vladisavljevic et al. [16].It is therefore logical to suggest that there are multiple ,contrasting factors affecting the droplet size produced. Droplet size increase with increasing pressure is attributed to higher interfacial tension values at small droplet formation times [13] and significant mass transfer via the droplet neck during detachment [14]. On the other hand, explanations for size decrease are that steric hindrance aids detachment of droplets from the membrane surface [15] and formation occurs from smaller diameter pore channels that were previously inactive at lower pressure [16]. The graph presented in Fig. 2 can be essentially divided into three regions with regards to TMP; a droplet decrease to a minimum(o0.5 bar), a gradual increase in droplet size(0.5–1.5bar) and then a rapid increase with polydisperse droplet sizes formed(41.5bar).The exact transitional point between the regions depends on the rotational speed so approximations are stated to enable clarity during the discussion. Although not observed for the experimental conditions studied for systems shown in Fig. 2, in the later parts of the discussion and for systems of higher IFT values, the existence of a fourth region, in which a droplet size plateau is observed, will be proposed.
3. ผลลัพธ์ และสนทนา3.1.Modification พฤติกรรมขั้นตอนระยะที่กระจัดกระจาย3.1.1.Effect ดัน transmembraneFig. 2 shows the resulting droplet size of the emulsion at different transmembrane pressures across a range of rotational speeds. The trend observed in general was that with increasing pressure, the droplet size decreased to a minimum before gradually increasing upon further pressure application. This behavior is more prominent when the rotational speed, and hence the shear rateislow (i.e.100RPM).It is highly likely that to some extent coalescence is a contributing factor to the relatively large droplet sizes under these processing conditions, since these droplet sizes vary between 8.6 and17.2 times larger than the pore diameter i.e. the upper end of the ratio values suggested by other authors [4,10]. Furthermore, droplet size distribution span values (d90 d10/d50) across the whole data set range dramatically between 0.720 and 2.210 depending on processing conditions. Within literature, the effect of transmembrane pressure(TMP)in membrane emulsification is contentious with the majority suggesting the droplet size increases with increasing transmembrane pressure [12–14]. Abrahamseetal. [15] found the droplet size to decrease with increasing transmembrane pressure. Within this study, it is observed that there is an initial decrease followed by an increase agreeing with the work of Vladisavljevic et al. [16].It is therefore logical to suggest that there are multiple ,contrasting factors affecting the droplet size produced. Droplet size increase with increasing pressure is attributed to higher interfacial tension values at small droplet formation times [13] and significant mass transfer via the droplet neck during detachment [14]. On the other hand, explanations for size decrease are that steric hindrance aids detachment of droplets from the membrane surface [15] and formation occurs from smaller diameter pore channels that were previously inactive at lower pressure [16]. The graph presented in Fig. 2 can be essentially divided into three regions with regards to TMP; a droplet decrease to a minimum(o0.5 bar), a gradual increase in droplet size(0.5–1.5bar) and then a rapid increase with polydisperse droplet sizes formed(41.5bar).The exact transitional point between the regions depends on the rotational speed so approximations are stated to enable clarity during the discussion. Although not observed for the experimental conditions studied for systems shown in Fig. 2, in the later parts of the discussion and for systems of higher IFT values, the existence of a fourth region, in which a droplet size plateau is observed, will be proposed.
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