(232ac) Penetration of a Non-Swelling Liquid into a Tableted Porous Medium

The imbibition of a liquid into porous substrates is a phenomenon of interest in a wide array of processes both in nature and industrial applications. As for the latter, several examples can be cited, among which are inkjet printing, 3D printing, fabric dying, painting or coating of porous materials [1]. In these and other activities, the final quality of the product is in many cases directly related to whether or not the penetration of the liquid into the porous medium takes place and at which rate this phenomenon takes place.

Liquid penetration has been widely studied in loosely packed powder beds, where the liquid penetration time up to a certain depth has been quantified [2, 3]. Also, liquid spreading on thin and thick porous substrates [4] and fiber mats [5] has been reported. These studies employed setups based on the utilization high-speed cameras to monitor with precision the fast occurring phenomenon of the interaction between both phases.

Tablets constitute another porous medium widely used in the pharmaceutical, food and consumer goods industries among others, in which the assessment of liquid penetration and uptake is of importance to evaluate processes like coating.

The properties of the substrate and the liquid as well as the type of interaction will determine the behaviour that imbibition may display. Depending on such characteristics, swelling may occur simultaneously to the imbibition [6]. For this reason, characterisation of the inner structure of the porous medium and the liquids is necessary to take into account or dismiss these side phenomena.

The work presented herein intends to show some assessments on the penetration of an inert fluid into a tableted porous substrate of different degrees of porosity as well as some methodologies to characterise the physical and structural properties of the materials used.

Methodology

The experimental workflow followed throughout the investigations consisted of a series of steps:

1. First, tablets were prepared from 0.25 g of powder of a composition relevant to the manufacturing process of detergents (The Procter & Gamble Company). For this purpose, an Instron Microtester 5848 apparatus was utilised using a load cell of up to 2000 N.
2. The surface roughness of the tablets prepared under different force conditions was measured with a KLA Tencor MicroXAM2 interferometer.
3. The tablets produced were analysed by means of an X-ray micro-computed tomography (μ-CT) Bruker Skyscan 1172 device. Further to visualization of the porous structure of the solid via a non-invasive technique, reconstruction and 3D analysis of the scanned images was performed to characterise the internal structure of the tablet.
4. In this work, the penetration studies have been made using n-hexadecane (purity â?¥ 99%, Sigma Aldrich) as a non-volatile liquid showing neither swelling nor dissolving interaction with the solid substrate. The surface tension (Krüss K100 tensiometer) and viscosity (TA Instruments AR-G2 rheometer).
5. Liquid penetration experiments of n-hexadecane into tablets of different porosities have been performed using image-based techniques. On the one hand, the evolution of the contact angle and penetration of a droplet placed on tabled throughout time has been measured with a Krüss DSA-30 goniometer. On the other, videos of the liquid uptake of the tablets placed in contact with a pool of the liquid were taken with a Photron Mini UX100 Fastcam high-speed camera at a frame rate of 150 fps.

Preliminary results

With the aid of the microtester, powder could be pressed to make consistent tablets of 13mm of diameter and thicknesses varying from 1.10 to 1.49 mm over a force load from 2000 to 600 N, respectively.

Using these pressing conditions, interferometry characterisation of the porous substrates led to the conclusion that they possess a very high smoothness owing to the fact that their mean absolute heights are in the range of 1.5 to 4.0 μm. In addition, 3D image analysis after μ-CT scanning retrieves total varying porosities in the interval 15 to 44%.

For its part, n-hexadecane viscosity varied from 3.09 to 1.84 cP and surface tension from 27.34 to 25.29 over the range of temperatures from 25 to 50 C.

Finally, deposition of n-hexadecane droplets on the tablets yield initial contact angles of 24.8 ± 1.5 °.

Acknowledgement

The authors would like to thank the Advanced Manufacturing Supply Chain Initiative (AMSCI) for funding the CHARIOT Consortium. AMSCI is a government supply chain fund which is helping to rebuild British manufacturing prowess.

References

1. Gambaryan-Roisman, T., Liquids on porous layers: wetting, imbibition and transport processes. Current Opinion in Colloid & Interface Science, 2014. 19(4): p. 320-335.

2. Hapgood, K.P., et al., Drop penetration into porous powder beds. Journal of Colloid and Interface Science, 2002. 253(2): p. 353-366.

3. Nguyen, T., W. Shen, and K. Hapgood, Drop penetration time in heterogeneous powder beds. Chemical Engineering Science, 2009. 64(24): p. 5210-5221.

4. Starov, V.M., et al., Spreading of liquid drops over porous substrates. Advances in Colloid and Interface Science, 2003. 104: p. 123-158.

5. Wang, Z., et al., Water droplet spreading and imbibition on superhydrophilic poly(butylene terephthalate) melt-blown fiber mats. Chemical Engineering Science, 2016. 146: p. 104-114.

6. Anderson, D.M., Imbibition of a liquid droplet on a deformable porous substrate. Physics of Fluids, 2005. 17(8).