6. Observation of the vicinity of ice lens by Raman spectroscopy (Exp. 4)

Knowledge of the ice configuration in the vicinity of a growing ice lens isimportant for clarifying the mechanism of the formation of ice lenses.

In this chapter,the ice configuration in the vicinity of the warmest ice lens in the freezing porous
medium was investigated by combining Raman spectroscopy with the unidirectional
freezing experiments.

Such experiment, which can investigate the ice configuration on

the scale of micrometer, has not carried out yet.

6.1 Sample and method
The sample used here was the sample 2 (d = 9.7 µm).

Water-saturated porous

medium was prepared by placing the glass particles in a desiccator with water,evacuating the desiccator, allowing a few days for the solid-vapor equilibrium to beestablished, adding liquid water, and allowing the system to equilibrate for one day.
Initial water content of the sample was 80 %.

The prepared sample was placed into asample cell shown in figure 14 and set in the unidirectional freezing apparatus equippedwith Raman spectrometer.

Before the unidirectional freezing experiment, the cell was

initially cooled to 2 ˚C.

The sample was frozen using the unidirectional freezing

apparatus with the temperature gradient of 0.33 ˚C/mm, and water in the vicinity of a
growing ice lens was observed by the Raman spectroscopy.
not control the freezing rate same as the Exp. 1.

In this experiment, we did

The temperature of growth surface ofice lens was estimated by temperatures measured by two copper-constantan
thermocouples inserted into the sample cell.

- 95 -Photograph 5. Raman spectroscopy.

- 96 -(a)Stretching vibration

at 3085〜3600 cm-1

Cts/sec

105001000

2000

3000

4000

2000

3000

4000(b)

Cts/sec10

Lattice vibration
at 225 cm-1

5001000νR （cm-1）

Fig. 29.

The Raman spectra obtained from samples at room temperature (a) and thetemperature of liquid nitrogen (b).

The Raman spectrum arising from translationallattice vibrations in ice Ih shows a molecular-optic band peak at 225 cm-1.

- 97 -Fig. 30. A computer-processed image of the vicinity of the growth surface of the icelens monitored using the Raman system. The right side is colder and the left side iswarmer. The warmest ice lens appears black, and the glass particles appear as whitecircles. The diagnostic spot was moved in 0.4-micrometer steps as shown in (b),where d is the distance from the growth surface of the warmest ice lens.

- 98 -

dμm+1000+100+20+10+5±0-5-10-30
100

200

300

400

νR （cm-1）

500

Fig. 31. A series of the Raman spectra obtained from pores at distance d.

- 99 -

6.2 Raman spectroscopy
The Raman spectrum arising from translational lattice vibrations in ice Ihshows a molecular-optic band peak at 225 cm-1 (Wong and Whalley, 1975; Fukazawa etal., 1997).

If ice exists, this peak should be observed.

While the warmest ice lens

was growing, water in the vicinity of the growth surface was investigated using aRaman spectroscope (JOBIN YVON RAMANOR T64000: photograph 5), which wasplaced above the sample cell.

The Raman spectroscope was calibrated to 0.1 cm-1 byrecording the standard emission lines of Neon.

This instrument was equipped with acharge coupled device detector that allows simultaneous recording of the frequency
range between 50 and 400 cm-1.

The excitation energy for Raman emission wasproduced by an Argon ion laser using monochromatic radiation of 514.5 nm with anoutput of 150 mW.

The spectral resolution was 0.45 cm-1 for all spectra.

The

incident laser beam was focused on a spot of the freezing sample 1 µm in diameter,
under a microscope.

The time elapsed during each measurement was 30 sec.

Since

the ice lenses that we observed grew less than 0.5 µm during each measurement, weneglected the growth of the ice lens during the measurement period.

6.3 Experimental results
In order to evaluate the influence of the sample cell and glass particles on theRaman spectra, preliminary experiments were carried out using samples at room
temperature and the temperature of liquid nitrogen.

No remarkable peaks arising fromthe sample cell and the glass particles were observed in the frequency range between 50
and 400 cm-1 from either sample (figure 29).
When different temperatures were applied to either end of the sample cell, ice
lenses formed similar to the ice lenses shown in figure 15.

The temperature gradientnear the warmest ice lens shown in the sample was 0.25 ˚C/mm and the temperature at- 100 -the growth surface of the ice lens was estimated to be -0.06 ˚C.

Then, we measured

the Raman spectra to observe the peak at 225 cm -1 in the vicinity of the warmest ice lensthat had grown for 300 minutes.

Figure 30a shows a computer-processed image of thevicinity of the growth surface of the ice lens monitored using the Raman system.

Theright side is colder and the left side is warmer.

The warmest ice lens appears black,and the glass particles appear as white circles.

To measure each pore, the diagnostic

spot was moved in 0.4-micrometer steps, as shown in figure 30b, where d is the distancefrom the growth surface of the warmest ice lens.
Figure 31 shows a series of the Raman spectra obtained from pores at distanced.

The spectrum in the ice lens (d < 0) had a strong peak at 225 cm-1, which indicatedthe existence of ice Ih.

The peak was also observed in the spectrum at the boundary

between a glass particle and the ice lens (d = 0).

On the other hand, the spectrum at awarmer pore more than 5 µm away from the ice lens (d > 5) had no remarkable peak at
225 cm-1, and was very similar to that of the sample at room temperature (figure 29a).

6.4 Discussion
If enough water reaches the growth surface of an ice lens, the ice lens will keep
growing.

If there is not enough water, however, the ice lens will stop growing andanother ice lens will start to form in a new location.
surface is important for ice lens formation.

Therefore, water near the growth

In our experiment, no ice larger than 1 µm

was observed in pores at locations warmer than the growing ice lens.

In nucleation,

only aggregates larger than a critical size are stable and grow to become the solid phase.
The size of the aggregates depends on the temperature and decreases with thetemperature.

Assuming a cylindrical solid in liquid at a temperature of T, the criticalradius r* is given by the Gibbs-Thomson effect:- 101 -

r* =γTmρ sq Tm - T,(29)

where γ is the solid-liquid interfacial free energy, ρs is solid density, q is the latent heatof melting, and Tm is bulk melting temperature.

With γ = 29 × 10 -3 J/m, ρs = 0.917 ×

103 kg/m3 and q = 3.33 × 105 J/kg (Ishizaki et al. 1996), we obtain r* = 0.026 / (Tm - T),
where r* is measured in µm.

Consequently, the calculated critical diameter at the pores

warmer than the growth surface of the warmest ice lens, which had an estimatedtemperature was -0.06 ˚C, is more than 0.87 µm.

In addition, ice formation is moredifficult in a pore than in bulk water because of surface forces from surrounding
particles (Handa et al., 1992; Ishizaki et al., 1996).

The estimated diameter only gives

the smallest value and suggests that no ice smaller than about 1 µm exists in pores at
positions warmer than the warmest growing ice lens.

Therefore, no pore ice exists in

any location warmer than the growth surface of the warmest ice lens in the porous
medium consisted of glass particles with diameter of 9.7 µm and the ice lens grows
without penetrating the warmer pores.

6.5 SummaryUnidirectional freezing experiments were carried out on an unconfined, watersaturated, uniform, porous medium consisting of micro glass particles with diameter of
9.7 µm.

The formation of ice lenses was observed in the porous medium as same asshown in the Exp. 1.

The Raman spectrum of water was used to clarify the ice

configuration in the vicinity of the growing ice lens.

No ice was found in any porewarmer than the warmest ice lens in the porous medium and the ice lens grew without
penetrating the warmer pores.

This result indicates that the growth surface of active

ice lens correspond to the freezing front in the porous media.

- 102 -