Pattern Formation Mechanism via Evaporation of Colloidal Droplet Containing PTFE Particles and NaCl†

doi:10.7503/cjcu20140073

Abstract

Abstract:

The pattern formation via the evaporation of colloidal droplet containing polytetrafluoroethene(PTFE) particle and NaCl salt were investigated. Due to the pinning of the contact line caused by the PTFE particles, a notable “coffee ring” effect was observed in the drying of droplets containing only colloidal particles. Furthermore, because of the stress field built between the inner liquid zone and outside solid zone which is induced by the azimuthal component of the surface tension of the liquid zone, cracks start to emerge in the gel zone and propagate from the edge of the droplet to the centre. The quantity of the cracks decreases with increasing volume fraction of particles. With the presence of NaCl, the outward capillary flow was greatly restricted, leading to a more homogeneous deposition of the colloidal particles. Interestingly, a rough dendrite structure containing both salt and PTFE particles was formed. It was resulted from the release of stress induced by evaporation.

Key words: Coffee ring effect, Capillary flow, Marangoni convection, Gravity sedimentation, Polytetra-fluoroethene, Pattern of evaporation, Colloidal droplet

CLC Number:

O647

TrendMD:

ZHANG Yongjian, LIU Zhengtang, QIAN Yimeng, LI Zhiguang, ZANG Duyang. Pattern Formation Mechanism via Evaporation of Colloidal Droplet Containing PTFE Particles and NaCl^†[J]. Chem. J. Chinese Universities, 2014, 35(6): 1258.

Figures/Tables 13

Fig.1 Final evaporation patterns of PTFE colloidal droplets with different mass fractions(A)—(C) are the optical microscopy images for w(PTFE)=6%, 15% and 30%, respectively; (D)—(F) are the SEM images correspon-ding to the enlarged central zones in images(A)—(C).

Fig.2 Cross section profiles of the final depositions[w(PTFE)=30%] (A) Overview of the section; (B) enlargement of the selected part.

Fig.3 Deposition thickness vs. the distance from droplet center R/R0R0 is the final radius of the deposition.

Fig.4 In situ observation(optical microscopy images) of the evaporation process of a colloidal droplet[w(PTFE)=15%](A)—(H) are the time sequence images showing the propagation of the evaporation fronts(indicated by arrows) and the formation of different zones(G). Time/s: (A) 0; (B) 10; (C) 13; (D) 16; (E) 18; (F) 22; (G) 27; (H) 30.

Fig.5 In situ observation(optical microscopy images) of the evaporation process of a colloidal droplet[w(PTFE)=30%](A)—(H) are the time sequence images. The arrows indicate the wrinkles which reflect the inner stress during evaporation.Time/s: (A) 0; (B) 50; (C) 85; (D) 110; (E) 118; (F) 124; (G) 132; (H) 140.

Fig.6 Number of cracks(N) as a function of particle concentration(w)

Fig.7 In situ observation(optical microscopy images) of the evaporation process of a NaCl solution droplet[w(PTFE)=20%](A)—(H) are the time sequence images; Time/s: (A) 0; (B) 5; (C) 90; (D) 105; (E) 155; (F) 200; (G) 240; (H) 320; (I) the Marangoni convection and gravitation sedimentation in the droplet.

Fig.8 Contact line pinning effect caused by PTFE particles (A) 0.5%PTFE+10%NaCl; (B) 0.5%PTFE+20%NaCl.

Fig.9 Various evaporation patterns(optical microscopy images) obtained with complex droplets

Fig.10 In situ observation(optical microscopy images) of the evaporation process of a complex droplet(6%PTFE+5%NaCl)(A)—(H) are the time sequence images. Time/s: (A) 90; (B) 96; (C) 109; (D) 115; (E) 130; (F) 135; (G) 143; (H) 150.

Fig.11 SEM image of the cross section of the deposition obtained from a complex droplet(6%PTFE+5%NaCl)

Fig.12 Dendrite structure obtained in the evaporation of a complex droplet(A)(6%PTFE+5%NaCl) and EDS analysis of the points b—d marked in image(A)(B—D)

Fig.13 In situ observation(optical microscopy images) of the evaporation process of a complex droplet(6%PTFE+30%NaCl)(A)—(H) are the time sequence images. The arrows indicate the inward Marangoni flow. Time/s: (A) 0; (B) 25; (C) 60; (D) 83; (E) 95; (F) 125; (G) 150; (H) 180; (I) 200; (J) 206; (K) 220; (L) 240.

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