Acoustic Signature of Evaporation
from Porous Media
Nicholas Grapsas, Nima Shokri
Department of Earth and Environment, Boston University, Boston, MA, USA
o Evaporative AEs were studied for 3 grain sizes of sand (with mean
diameters of 0.34 mm, 0.58 mm, and 0.89 mm) and for 2 sizes of
glass beads (with mean diameters of 0.15 mm and 0.53 mm) all
water-saturated in Hele-Shaw glass columns with dimensions 8cm x
8cm x 1cm.
o AE sensors were mounted on the centers of the cells which were left
to evaporate in an environmental chamber set to 35°C and 30% RH.
o AE sensors could detect amplitudes between -23dB to 117dB. A
26dB minimum threshold was used to filter ambient noise.
o Changes in mass, relative humidity, temperature, and liquid phase
distribution were recorded every five minutes using digital balances,
a HygroClip, and a camera set to image every 20 minutes.
Cumulative fraction (%)
o AE event (hit) characteristics were recorded for each hit.
100
80
60
40
20
0
0.0
S 0.34
S 0.58
S 0.89
GB 0.15
GB 0.53
0.5
1.0
1.5
Particle size (mm)
mm
mm
mm
mm
mm
2.0
FIG. 1. Particle size distributions.
FIG. 2. Experimental setup.
References
o Lehmann, P., S. Assouline, and D. Or (2008), Characteristic lengths affecting
evaporative drying of porous media, Phys. Rev. E, 77, 056309.
o Moebius, F., D. Canone, and D. Or (2012), Characteristics of acoustic emissions
induced by fluid front displacement in porous media, Water Resour. Res., 48 (12),
11507-11519.
o Shokri, N, and D. Or (2011), What determines drying rates at the onset of diffusion
controlled stage-2 evaporation from porous media?, Water Resour. Res., 47.
o Shokri, N., and D. Or (2013), Drying patterns of porous media containing wettability
contrasts, J. Colloid Interface Sci., 391, 135–141.
o Shokri, N., and M. Sahimi (2012), The structure of drying fronts in three-dimensional
porous media, Phys. Rev. E 85, 066312.
0 1 2 3 4 5 6
Time (days)
0
8
4
4
0
0
0
0
2
2 4 6 8
Time (days)
4
6
8
Time (days)
16
14
12
10
8
6
4
2
0
10
FIG. 3. The measured cumulative number of hits and cumulative mass loss during evaporation
versus time for particles (a) S 0.34 mm and S 0.89 mm (b) S 0.58 mm and GB 0.53 mm.
S 0.34
S 0.58
S 0.89
GB 0.15
GB 0.53
40
30
mm
mm
mm
mm
mm
o This data is evidence to
suggest that meniscus jumps
and invasion of the medium
are the source of the
generated AE’s during
evaporation.
FIG. 8. The average hit amplitude during
evaporation for each particle type.
0
2
3
4
5
Mass Loss (g)
6
FIG. 4. The cumulative number of AE
hits during evaporation from each
particle type as a function of mass loss.
Sand
Glass Beads
(b)
103
102
101
S 0.34 mm
S 0.58 mm
S 0.89 mm
30
GB 0.15 mm
GB 0.53 mm
100
40
50
60
Amplitude (dB)
30
40
50
60
70
Amplitude (dB)
FIG. 9. The amplitude distributions of AEs observed during evaporation for each particle type.
The data for (a) S 0.34 mm, S 0.58 mm, and S 0.89 are described by beta values of -0.14,
-0.05, and -0.06 and for (b) GB 0.15 mm and GB 0.53 mm by -0.11 and -0.04 respectively.
o The data are described by the power law N = α*10β where N is the number of hits
observed and the exponent β represents the scaling exponent.
o AE amplitude distributions generated during motions of air-water interfaces have
been shown to exhibit similar power law behavior with beta’s magnitude
covarying with particle size [Moebius et al., 2012].
o AE hits are correlated with
the area invaded by air (IA)
for all particles.
o Smaller particles and rougher
surfaces exhibit larger hit/IA
ratios, trends consistent with
those exhibited by the hits
per unit mass loss analysis.
(a)
102
100
6. Covariation Between AE Hits and Invaded Area
o At the pore scale, air invasion
is underpinned by Haines
jumps. Thus any change in
invaded area necessarily
stems from these meniscus
motions. This implies that
the number of AE hits
recorded directly
corresponds to the number
of Haines jumps that have
occurred in the medium.
Particle Size (mm)
104
103
10
1
0.34 0.58 0.89 0.15 0.53
8. Energy and Amplitude of AE Hits vs. Grain Size
20
0
26
o Since wave energy is proportional to the square of amplitude, AE hits should
display this relationship. A power law with an exponent of 2 emerges when hit
energy is plotted against hit amplitude for each particle size.
101
o Due to the irregular shapes of the
sand grains compared to the
spherical glass beads, more porescale interfacial jumps are expected
during drying of sand particles
compared to glass beads which is
supported by our experimental data.
28
o On the pore scale, a meniscus is pinned to the pore surface by the cohesive forces
acting on the contact line along a meniscus’s perimeter. Thus, more energy is
released when a Haines jump occurs in coarse-textured media due to the longer
contact line.
104
50
30
o Results show that AE hit energy (averaged over all recorded hits) trends with
particle size and is higher for glass beads than sand.
5. Number of AE Hits vs. Mass Loss
o AE hits per unit mass loss trend
inversely with particle size. Since the
mass that can be contained within a
pore co-varies with the pore’s
volume, under a same evaporative
mass losses, more AE hits are
generated in the case of a medium
with finer texture.
Sand
Glass Beads
32
AE Hits
0
34
FIG. 7. The average hit energy during
evaporation for each particle type.
AE hits (1000)
2
8
AE hits (1000)
4
12
GB 0.53 mm
36
Particle Size (mm)
Mass loss
AE hits
6
0.34 0.58 0.89 0.15 0.53
5
S 0.58 mm
8
10
Sand
Glass Beads
Average hit amplitude (dB)
0
20
Average hit energy (aJ)
0
2
3
4
Time (days)
(b)
AE hits (1000)
2
AE hits (1000)
Mass loss (g)
8
4
140
120
100
80
60
40
20
0
AE Hits
3. Materials and Methods
4
30
o Beta’s magnitude also tends to increase with particle size during evaporation and
is greater for rough surfaces than for smooth surfaces. This is consistent with the
notion smaller Haines jumps preferentially produce fainter, less energetic AEs.
9. Summary and Conclusions
 Acoustic emission techniques can be used to non-invasively to detect the
evaporative water losses and general drying behavior of porous media.
FIG. 5. Typical invasion time progression. White, red,
green, and blue regions represent areas invaded by air
5, 10, 15, and 25 hours after the onset of evaporation.
3.0
S 0.58 mm
6
2.5
Invaded area
AE hits
2.0
4
2
0
0.0
6
4
2
0
GB 0.53 mm
0
1
2
Time (days)
0.5
1.0
1.5
Time (days)
4
3
2
1
0
1.5
1.0
0.5
IA (1000 mm2)
2. To identify the source mechanism of evaporative AEs and link their
characteristics to the texture of porous media potentially revealing
non-invasive methods to investigate drying of porous media
10
1
S 0.89 mm
IA (1000 mm2)
1. To study the acoustic signature of the evaporation process from
porous media
0
40
12
AE hits (1000)
2. Objectives
o Our results indicate that the
observed cumulative number
of AE hits is strongly
correlated to the mass loss
and drying curves.
50
Mass Loss
AE Hits
Mass loss (g)
o The air invasion of liquid-filled pores underpinned by pore-scale
interfacial jumps generates a crackling noise that consists of acoustic
“hits” which can be detected using acoustic emission (AE) methods.
o In stage two evaporation, the
hydraulic connectivity is
disrupted, leading to slower
invasion of the medium and as
a result fewer AE hits.
7. Energy and Amplitude of AE Hits vs. Grain Size
S 0.34 mm
(a)
AE Hits (1000)
o Displacement of the drying front at pore-scale proceeds in discrete,
rapid interfacial bursts (Shokri and Sahimi, 2012) called “Haines
jumps.” In a Haines jump, the liquid meniscus spanning a pore space
destabilizes, retreats, and re-stabilizes across another stable
orientation due to changes in capillary pressure.
o In stage one evaporation,
hydraulic connectivity with
the surface causes a drying
front to propagate relatively
fast through the medium
generating more AE hits.
12
10
8
6
4
2
0
AE hits (1000)
o This process occurs in two distinct stages. During stage one, hydraulic
connectivity between the receding drying front (i.e. the interface
between saturated and unsaturated zone) and the surface causes
liquid vaporization to occur at the surface leading to high
evaporation rates. In the second stage, the hydraulic connectivity
breaks once the drying front reaches a characteristic, mediumdependent depth inducing much lower evaporation rates (Lehmann
et al., 2008; Shokri and Or, 2011; Shokri and Or, 2013).
o Across all particle sizes and
roughnesses, typical stage one
and stage two evaporation
were observed.
Mass loss (g)
o Evaporation from porous media is a key process in mass and energy
exchange between land and atmosphere, affecting various
hydrological processes as well as biodiversity in the vadose zone.
4. AE Hits and Cumulative Mass Loss
Mass loss (g)
1. Introduction
0.0
2.0
FIG. 6. The time evolution of AE hits during evaporation
plotted alongside the corresponding invaded area for
particles S 0.58 mm and GB 0.53 mm. Similar
correlations were found for all particles evaluated.
 The texture of porous media significantly impacts the acoustic signature of
drying in porous media.
 Rapid interfacial bursts due to the invasion of saturated zone by air are the
source of individual AEs.
 This research demonstrates the potential of AE as a technique to non-invasively
investigate the drying of porous media.
10. Future Direction
 Effort is needed to quantitatively relate the mass lost per AE hit for a given pore
geometry, potentially a non invasive method to determine evaporative fluxes.
 To apply these findings in field scenarios, the influence of fluid properties such
as viscosity on the observed AEs must be better understood.
 The exact moment of AE generation remains unknown. If this can be pinpointed,
source inversion models can be applied to deduce information about the porescale dynamics of individual meniscus displacements.