PHYSICAL REVIEW E 91, 042407 (2015)
Water transfer and crack regimes in nanocolloidal gels J. T hiery,'-2 S. R odts,1 E. K eita,1 X. C hateau,1 P. F aure,1 D. C ourtier-M urias,1 T. E. K odger,2 and P. C oussot1 1Laboratoire Navier (ENPC-CNRS-IFS'ITAR), Universite Paris-Est, Paris, France 2Experimental Soft Matter Group, Harvard University, Cambridge, Massachusetts 02138, USA (Received 25 November 2014; revised manuscript received 28 March 2015; published 27 April 2015) Direct observations of the surface and shape of model nanocolloidal gels associated with measurements of the spatial distribution of water content during drying show that air starts to significantly penetrate the sample when the material stops shrinking. We show that whether the material fractures or not during desiccation, as air penetrates the porous body, the water saturation decreases but remains almost homogeneous throughout the sample. This air invasion is at the origin of another type of fracture due to capillary effects; these results provide insight into the liquid dynamics at the nanoscale. DOI: 10.1103/PhysRevE.91.042407
PACS number(s): 68.03.Fg, 82.70.Gg, 47.56.+r, 81.40.Np
I. INTRODUCTION
II. MATERIALS AND METHODS
The evaporation o f solvent from colloidal suspensions, gels, or pastes often leads to shrinking, w rapping, and eventually cracking [1] w hich im pacts the m aterial. Cracking during drying raises trem endous technological and scientific concerns as it can drastically im pair the structural properties of concrete [2]; the integrity o f ceram ic films [3], soil, or clays [4]; or the functionalities o f cosm etics, paints, or coatings [5]. A m ong the different physical m echanism s [6] suggested to explain w hy a crack nucleates during drying the most com m on model relies on the fram ew ork o f elastic fracture m echanics [7]. This tensile failure theory considers the propa gation o f cracks triggered once the near tip stress state, w hich intensifies as desiccation proceeds if the m aterial rem ains bound to its substrate, reaches the failure criterion for brittle materials [4,8,9]. By contrast, the opening o f cracks during drying is considered to stem from large capillary stresses resulting from the penetration of irregular air-liquid interface into the m aterial [6,10]. However, even though the exam ination o f fracture patterns as a function o f experim ental conditions is considerable [1 l] ,n o study bears direct w itness o f these effects at local length scale. Additionally, even though drying regim es are already accurately detailed with the spatial distribution o f the liquid in tim e [12,13], the role o f liquid dynam ics on fracturing appears to be som ew hat neglected so far. Yet the tensile failure theory im plicitly assum es the sam ple to rem ain saturated w ith a rather hom ogeneous distribution of water, w hereas, its counterpart brings up the idea o f a partially saturated m edium , at least locally. T herefore, m easurem ents o f the spatial distribution o f w ater during drying w ould greatly enhance the current understanding of crack nucleation. In this paper w e follow both the surface and internal char acteristics, specifically the spatial distribution o f water, ot a drying model nanocolloidal gel w ith the help o f proton nuclear m agnetic resonance (NM R) profiling (a one-dim ensional use of m agnetic resonance im aging) m easurem ents with m icrom e ter spatial resolution. We show that w hether it develops widely open fractures or not during the process, the material remains saturated as it shrinks, and then starts to desaturate alm ost hom ogeneously, w hich induces thin fractures that do not span the gel. These results provide a general view o f w ater transport during drying and cracking, and insight into the thin liquid film dynam ics in nanopores.
We use aqueous dispersions o f hydrophilic nanosized silica particles (r = 6 nm radius) obtained from Sigm a-A ldrich (Ludox H S-40) in w hich 0.5 m o l/L o f sodium chloride is added to trigger aggregation. The initial stable suspension of particles has a solid volum e fraction o f 23% . Since according to the DLVO [14] theory the electrical double layer thickness around a single particle is governed by the concentration o f the surface charge counterions in the liquid phase o f the suspension, the higher the salt concentration in this electrolyte, the higher the aggregation rate o f colloids. T herefore, in order to obtain a fairly hom ogeneous final gel structure, i.e., avoiding substantial structural heterogeneity possibly caused by local gradients o f rate o f aggregation— sedim entation not applying here— we dissolved the quantity o f salt needed into the additional necessary w ater (thus m ixing tw o hom ogeneous phases together) to w ork at initial volum e fraction o f particles (
Water transfer and crack regimes in nanocolloidal gels J. T hiery,'-2 S. R odts,1 E. K eita,1 X. C hateau,1 P. F aure,1 D. C ourtier-M urias,1 T. E. K odger,2 and P. C oussot1 1Laboratoire Navier (ENPC-CNRS-IFS'ITAR), Universite Paris-Est, Paris, France 2Experimental Soft Matter Group, Harvard University, Cambridge, Massachusetts 02138, USA (Received 25 November 2014; revised manuscript received 28 March 2015; published 27 April 2015) Direct observations of the surface and shape of model nanocolloidal gels associated with measurements of the spatial distribution of water content during drying show that air starts to significantly penetrate the sample when the material stops shrinking. We show that whether the material fractures or not during desiccation, as air penetrates the porous body, the water saturation decreases but remains almost homogeneous throughout the sample. This air invasion is at the origin of another type of fracture due to capillary effects; these results provide insight into the liquid dynamics at the nanoscale. DOI: 10.1103/PhysRevE.91.042407
PACS number(s): 68.03.Fg, 82.70.Gg, 47.56.+r, 81.40.Np
I. INTRODUCTION
II. MATERIALS AND METHODS
The evaporation o f solvent from colloidal suspensions, gels, or pastes often leads to shrinking, w rapping, and eventually cracking [1] w hich im pacts the m aterial. Cracking during drying raises trem endous technological and scientific concerns as it can drastically im pair the structural properties of concrete [2]; the integrity o f ceram ic films [3], soil, or clays [4]; or the functionalities o f cosm etics, paints, or coatings [5]. A m ong the different physical m echanism s [6] suggested to explain w hy a crack nucleates during drying the most com m on model relies on the fram ew ork o f elastic fracture m echanics [7]. This tensile failure theory considers the propa gation o f cracks triggered once the near tip stress state, w hich intensifies as desiccation proceeds if the m aterial rem ains bound to its substrate, reaches the failure criterion for brittle materials [4,8,9]. By contrast, the opening o f cracks during drying is considered to stem from large capillary stresses resulting from the penetration of irregular air-liquid interface into the m aterial [6,10]. However, even though the exam ination o f fracture patterns as a function o f experim ental conditions is considerable [1 l] ,n o study bears direct w itness o f these effects at local length scale. Additionally, even though drying regim es are already accurately detailed with the spatial distribution o f the liquid in tim e [12,13], the role o f liquid dynam ics on fracturing appears to be som ew hat neglected so far. Yet the tensile failure theory im plicitly assum es the sam ple to rem ain saturated w ith a rather hom ogeneous distribution of water, w hereas, its counterpart brings up the idea o f a partially saturated m edium , at least locally. T herefore, m easurem ents o f the spatial distribution o f w ater during drying w ould greatly enhance the current understanding of crack nucleation. In this paper w e follow both the surface and internal char acteristics, specifically the spatial distribution o f water, ot a drying model nanocolloidal gel w ith the help o f proton nuclear m agnetic resonance (NM R) profiling (a one-dim ensional use of m agnetic resonance im aging) m easurem ents with m icrom e ter spatial resolution. We show that w hether it develops widely open fractures or not during the process, the material remains saturated as it shrinks, and then starts to desaturate alm ost hom ogeneously, w hich induces thin fractures that do not span the gel. These results provide a general view o f w ater transport during drying and cracking, and insight into the thin liquid film dynam ics in nanopores.
We use aqueous dispersions o f hydrophilic nanosized silica particles (r = 6 nm radius) obtained from Sigm a-A ldrich (Ludox H S-40) in w hich 0.5 m o l/L o f sodium chloride is added to trigger aggregation. The initial stable suspension of particles has a solid volum e fraction o f 23% . Since according to the DLVO [14] theory the electrical double layer thickness around a single particle is governed by the concentration o f the surface charge counterions in the liquid phase o f the suspension, the higher the salt concentration in this electrolyte, the higher the aggregation rate o f colloids. T herefore, in order to obtain a fairly hom ogeneous final gel structure, i.e., avoiding substantial structural heterogeneity possibly caused by local gradients o f rate o f aggregation— sedim entation not applying here— we dissolved the quantity o f salt needed into the additional necessary w ater (thus m ixing tw o hom ogeneous phases together) to w ork at initial volum e fraction o f particles (
