We thank the referees for their careful review of our manuscript. Both referees deemed this
work publishable in JPC after a revision of the manuscript that addresses the questions that
have been raised. The first referee remarks that “If the authors can revise the paper to
answer these questions/accommodate these remarks, I would be happy to recommend
publication.”, while referee 2 says that “This manuscript is likely publishable, but should be
reviewed again and revised before it is accepted.”
We address the questions and comments that the referees posed in the following.
In addition to the revision outlined below, we uploaded 3 movies files that show the growth
of the slush phase, the subsequent crushing by the upper plate and the growth of the
crystalline phase and 1 figure that shows one example of constant stress measurements on
solidified glycerol. These 3 movies and 1 figure will appear in the supporting information.
Referee 1.
Comment 1: “..The slush phase contains many small glycerol crystallites, as is visible in the
pictures, and evident from the fact that they disappear at the melting point. This is not
clearly laid out in the paper, where it is more or less suggested that the slush corresponds to
a different ‘state’ . What likely happens is that due to the large degree of undercooling, many
crystallites form at the same moment, and one arrives at a wax-like solid. That such a
material has a different Young’s modulus than a single crystal seems evident. More
generally, the same crystalline material can have very different mechanical properties
depending on the size of the crystallites and/or the number, organization and type of defects.
The forging of steel is a well-known example of this. I think all of this merits to be discussed
with a bit more caution, and reference to existing literature.”
Answer: We agree with the referee that our original paragraph was unclear. We have tried
to clarify our point of view in the revised version. We now stress that the slushy phase is
different from the polycrystal which is obtained by seeding the supercooled liquid with a
piece of crystal (see the last two paragraphs in Result section 3). This polycrystal is as hard as
the single crystal. We speculate on the presence of liquid-like parts between the crystallites,
which would explain the reduced shear-modulus and the plastic deformation instead of the
shattering observed in Fig. 6.
Comment 2: “Incidentally, the same holds for ice, and so it is not clear to me how the data of
Fig.1 should be interpreted.”
Answer: The data of Fig.1 shows that it is impossible to measure the shear modulus of ice in
the Couette cell due to tool deformation. We believe that the water in Couette cell at -5 °C
forms normal polycrystalline ice that has a shear modulus on the order of 1GPa. With the
Couette cell, the upper limit of the measurable shear modulus is on the order of 10MPa,
comparable to the shear modulus of the slushy phase of glycerol, but lower than that of the
polycrystalline ice or glycerol.
We have added one sentence in the caption of Fig.1 to stress the limitation of the Couette
cell:
“…due to tool compliance, indicating that the upper limit of the measurable shear modulus
with the Couette cell is on the order of 107Pa.”
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Question 1: “…. it is not clear what happens in steady shear. Are the measurements in Fig.2
before or after solidification, or is the solidification completely reversed by the steady shear?
“
Answer: The measurements in Fig.2 were performed on the quickly cooled liquid state of
glycerol without any prior slow cooling or aging. No deviation from a Newtonian liquid could
be found in such samples. Those measurements were done to confirm that we could
reproduce the literature values of glycerol viscosities at different temperatures.
We have added one sentence in the caption of Fig.2 to emphasize that the viscosity
measurements were performed on the liquid state of glycerol.
“….All the measurements are obtained at steady shear (strain rate controlled) on the quickly
cooled liquid state of glycerol.”
Referee 2:
Q0: On page 3 you state “…which indicates that the slush [simply melts at T_m because it
turns] into a crystal at higher temperatures.” As the sample becomes opaque and shows
what appears to be dendritic growth, what theories do you have to explain anything but
some sort of crystalline phase being present at lower temperatures?
Answer: The reason we believe that the slush is distinct from a crystal is the plateau of the
moduli of the slush phase (Fig.6b). Even though it looks as if the slush is made up of
crystallites, it does not harden any further and the shear modulus remains two orders of
magnitude lower than the polycrystal (see response to comment 1 of referee 1). At this
point we can only speculate to what the origin of this behavior is, although a glacial phase as
seen in TPP could be one explanation. We clarified this in a revised paragraph (see last two
paragraphs in Result section 3).
Q1: You tested two different suppliers of glycerol. Where there any notable differences
between the samples provided by each supplier?
Answer: We did not find any noticeable difference in behavior between the two glycerol
samples. The viscosity values we measured with both samples agreed within measurement
accuracy. We have added one sentence in Experimental setup section to clarify that.
“…, Invitrogen and Fluka, and no significant differences were found between these two
samples.”
Q2: Are the results (specifically the point at which the fluids solidify) plotted in figures 3.B –
3.D reproducible?
Answer: The solidification in the Couette cell is reproducible, but the aging time at which the
moduli start to increase can show some variations. We repeated the solidification at
Ta=220K, which started around ta=25 hr, the same time as the run shown in Fig. 3(d). For
another run at Ta=240K, the moduli started to increase around ta =1.8hr, while for the run
shown in Fig.3(b) the increase started somewhat earlier at 1.2hr. However, this probably
depends whether the nuclei form in the gap or at the bottom of the Couette cell. In the
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latter case, it will take longer for the slush to grow from the cup to the inner cylinder or vice
versa.
Q3: In figure 3.C (T_a = 230 K), the G’ and G’’ data only show scatter at times larger than 5
hours. Why is this, when experiments with at lower- or higher-T_a do not result in similar
phenomena?
Answer: The origin of the scatter in figure 3c) is indeed somewhat puzzling. Most likely it is
due to internal stresses building up during the process of solidification. There are
presumably several nucleation points within the sample. This may lead to fluid flow due to
the density difference between the solid and liquid phase. If there is such a creep motion
within the sample it would certainly lead to scatter in the oscillation experiment since the
strain amplitude was chosen to be small.
We know that after constant stress measurements in the solidified state subsequent
oscillation measurements become noisy. Below we show an example. This left plot shows
the same oscillation test as shown in Fig.3(b) when the sample was aged at T=240K. The right
plot is a zoom in at aging beyond 8 hr.
Before ta=10hr (first dotted vertical line - region (a)), we did the oscillation test at f=0.1Hz,
and strain amplitude 0.00053. In region (b), we decreased the strain amplitude by a factor 5
and the measurements start to become noisy. In region (c), we changed the frequency to
f=0.01Hz and the strain amplitude back to 0.00053 and the two data points for G’ and G’’
agree with the previous measurement. At the beginning of region (d) (3rd dotted line), we did
a constant stress test at 8777 Pa (the maximum stress we can apply in this geometry) for
100s. We then continued the oscillation test with the same parameters as in region (a)
(f=0.1Hz, and strain amplitude 0.00053). However, now the data is very scattered and it
persists for hours. This can only be due to stresses that have built up during the constant
stress test, but it is unclear why they persist for so long.
We have added one sentence in Caption 3.
“…(c) Ta=230K (scatter of points for t > 7h is not well understood and may be due to internal
stresses building up upon solidification)…”
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Q4: Why was such a low heating rate used in Fig. 4? Wouldn’t a higher heating rate provide
a clearer picture about the system present at low temperatures- as heating more quickly
would provide less time for changes in morphology?
Answer: The low heating rate was chosen to prevent temperature overshoots. The
temperature in the chamber is regulated by heating elements that ensure equilibrium. At
high heating rates the temperature increases beyond the desired temperature and this
would have made it impossible to determine the melting point accurately, which was the
sole purpose of the plot.
Q5: In Fig. 5, the data is representative of a viscoelastic response, where a viscous response
is present at the lowest frequencies. If the material was fully crystallized there wouldn’t be a
viscous response at low temperatures. Within this framework, how can one interpret that
the higher-frequency limit of G’ is approximately equal to the modulus of crystalline glycerol?
Answer: Fig.5 represents data from liquid supercooled glycerol, not the solidified glycerol.
Therefore, the viscous response at low frequency is expected. We have added two words in
Caption 5 to make it clear.
“Relaxation peak of liquid supercooled glycerol near the glass transition.”
Q6: On page 13, in reference to the insert in Fig. 7, you state that “although for larger strain
the elastic modulus decreases while the loss modulus increases due to slip.” What evidence
do you have this is not flow induced rejuvenation (breakdown of the solid phase) rather than
slip?
Answer: We agree with the referee. Even though at some strain slip is expected, we cannot
exclude the possibility that shear rejuvenation takes place beforehand. We therefore
rephrased the sentence: “… modulus increases. This is either due to slip or shear-induced
rejuvenation of the solid”
Q7: Would a fictive temperature or the density as a function of time at temperature provide
further insights regarding what happens while the samples is cooled to T approx T_g + 10K?
Answer: The density as a function of time could very well give insight into this phenomenon.
However, this measurement would require a different setup. As we remain well above the
glass transition, the concept of a fictive temperature might not be applicable.
Q8: On page 14 you quite logically state that the differences between the Couette and
parallel plate rheology are likely related to the stark differences in surface areas. Are the
surface types and roughness also factors? Why did you not include results using a series of
other geometries? I know experiments with other geometries might lack rheology data, but
shouldn’t the transition be observable optically?
Answer: After failing to reproduce the slush-like state in the plate-plate geometry, we did
roughen the surface of the upper plate with sand paper in the hope of inducing nucleation
points. However, this also did not help to create the slush-like state.
We agree that a systematic study of the influence of geometry and surface properties on the
nucleation of a slush-like (or crystalline) state is desirable and important. However, due to
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the aging time and the high consumption of liquid Nitrogen in the environmental chamber
this kind of study was cost prohibitive in the rheometer setup. A cryostat setup with optical
access may be more suitable for this.
Q9: On page 15, why do you think that the material from the Couette cell experiments is
“also slush-like” when it appears to exhibit a modulus similar to the solid glycerol and can
also be used for “seeding a quenched glycerol sample”?
Answer: Even though we do not have optical access in the Couette cell during the
experiment, we believe that the sample is slush-like, since we applied the same slow cooling
protocol as in the plate-plate geometry where we can measure the moduli reliably (Fig.6).
The moduli measured in the Couette cell, which are about 106-107 Pa (Fig. 3b-d) in the solid
phase cannot be trusted due to the compliance problem. When the sample in the Couette
cell (that has undergone slow cooling and aging) is heated to just below the melting point of
the crystal, the sample does have a crystalline appearance (looking from the top at the end
of the experiment) and we harvested crystallites that were used for seeding. However, as
stated in the answer to Q0, at the aging temperature, the slush state persists and the sample
does not harden any further which is not expected in ordinary crystal growth (Fig.6b). The
moduli plateau 2 orders of magnitude below the shear modulus of the crystal. One
explanation could be a glacial phase (see answer to Q0), which is known to turn into a
crystalline phase at higher temperatures. However, in the absence of Neutron/X-ray data,
we cannot determine the nature of the slush state conclusively.
Note 1: In Fig. 2, if the data is indeed from steady shear experiments, please provide
representative details on the applied shear rates for your measurements… if a low enough
shear rates were not applied, the sample would be shear thinned, and thus a lower value of
eta would be observed (as is seen in your data). Also, the units used in the VFTH equation
should not be italicized.
Answer: We now provide the range of shear rates we applied in this experiment in the
caption of figure 2. In the Couette cell the typical shear rates where   10  100s 1 at room
temperature and   10 4 s 1 at 200K. In the plate-plate geometry, the range of shear rates
was   10 6  10 4 s 1 in the temperature range 185-200K. At each temperature we
measured the viscosity for a range of different shear rates, so we are quite certain that there
is no shear-thinning effect. The slightly lower viscosity values we obtain in plate-plate are
therefore most likely due to the edge effect and the hyperbolical shape of the sample
between the plates.
The font in the VFTH equation has been corrected.
Note 2: While the solidification time can eventually be inferred, it is never clearly defined.
Perhaps in Figure 3.A you could indicate what t_a = 0 represents?
Answer: We now define the aging time t_a in the caption of Fig.3.
Note 3: The dashed lines in Fig. 3.A are not defined.
Note 4: The reference temperature in Fig. 5.B should be defined.
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Note 5: In Fig. 6.D you show four photos, but do not document the differences between each
of them.
Note 6: the Kelvin on page 15 should not be squared.
We provided the missing information in the relevant captions. The Kelvin squared was a
label for a reference and has been moved to avoid confusion. In addition, we have
rephrased the description of Fig.5.B to make it clear.
“ … (b) Collapse of the data of (a) with the temperature-dependent rescaling of the
frequency axis according to the VFTH fit … ”.
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