ORIGINAL PAPER

Controlled production of patterns in iridescent solid filmsof cellulose nanocrystals

Stephanie Beck • Jean Bouchard • Greg Chauve •

Richard Berry

Received: 17 December 2012 / Accepted: 13 February 2013

� Springer Science+Business Media Dordrecht 2013

Abstract A method to produce predefined patterns

in solid iridescent films of cellulose nanocrystals

(CNCs) by differential heating of aqueous CNC

suspensions during film casting has been discovered.

Placing materials of different temperatures beneath an

evaporating CNC suspension results in watermark-

like patterns of different reflection wavelength incor-

porated within the final film structure. The patterned

areas are of different thickness and different chiral

nematic pitch than the surrounding film; heating

results in thicker areas of longer pitch. Thermal

pattern creation in CNC films is proposed to be caused

by differences in evaporation rates and thermal motion

in the areas of the CNC suspension corresponding to

the pattern-producing object and the surrounding,

unperturbed suspension. Pattern formation was found

to occur during the final stages of drying during film

casting, once the chiral nematic structure is kinetically

trapped in the gel state. It is thus possible to control the

reflection wavelength of CNC films by an external

process in the absence of additives.

Keywords Cellulose nanocrystals � Self-assembly �Chiral nematic films � Iridescence � Temperature �Evaporation rate

Introduction

Cellulose nanocrystals (CNCs) are the focus of ever-

increasing interest in a wide range of scientific and

commercial fields. These naturally synthesized nano-

particles are in demand not only because of remark-

able physical and chemical properties that suit them

for many applications, but also because of their

abundance, end-of-life biodegradability and renew-

ability (Habibi et al. 2010).

Cellulose nanocrystals are extracted from native

cellulose sources by controlled acid hydrolysis. Sul-

furic acid imparts negatively charged acidic sulfate

ester groups to the CNC surfaces during hydrolysis

(Marchessault et al. 1961; Revol et al. 1992; Dong

et al. 1998). The rod-like shape and negative surface

charge of CNCs give rise to electrostatically stable

colloidal suspensions which phase separate into an

upper random phase and a lower ordered phase, at

CNC concentrations above a critical value (Onsager

1949; Revol et al. 1992; Dong et al. 1998). The

ordered phase is a chiral nematic liquid crystal

(Marchessault et al. 1959; Revol et al. 1992) in which

the CNCs are arranged in pseudo-planes (de Gennes

and Prost 1993; Revol et al. 1998) as depicted in

Fig. 1. The average CNC axis direction in each plane

S. Beck (&) � J. Bouchard � G. Chauve � R. Berry

Pulp, Paper and Biomaterials Division, FPInnovations,

570 Boulevard St-Jean, Pointe-Claire,

QC H9R 3J9, Canada

e-mail: stephanie.beck@fpinnovations.ca

Present Address:R. Berry

CelluForce Inc., 625 Avenue President-Kennedy, Suite

1501, Montreal, QC H3A 1K2, Canada

123

Cellulose

DOI 10.1007/s10570-013-9888-4

(the director) is rotated at a small angle to the planes

above and below it, producing a helical arrangement

of the directors about a line perpendicular to the planes

(the cholesteric axis). The pitch P of the helix is

defined as the distance required for the average

director to make one full rotation about the cholesteric

axis. The chiral nematic pitch in suspensions of CNCs

is affected by the particle size, longer nanocrystals

giving longer pitches (Revol et al. 1998; Beck-

Candanedo et al. 2005). Depending on the cellulose

source, CNC length varies from *100 nm (wood,

cotton) to several micrometers (algae, tunicates)

(Habibi et al. 2010). The chiral nematic pitch in

aqueous CNC suspensions prepared from cotton or

wood is on the order of tens of microns (Dong et al.

1996; Beck-Candanedo et al. 2005).

Aqueous CNC suspensions can be evaporated to

produce solid semi-translucent CNC films that retain

the chiral nematic order of the liquid crystalline phase

(Revol et al. 1998). The chiral nematic pitch shrinks as

the suspension evaporates to dryness, the film’s pitch

depending on the specific CNC suspension properties

and the film formation conditions. Chiral nematic

CNC films reflect left-handed circularly polarized

light in a wavelength band determined by the pitch

according to k = nPsinh, where k is the reflected

wavelength, n is the average refractive index of the

film (n = 1.55 for CNC), P is the chiral nematic pitch,

and h is the angle of reflection relative to the surface of

the film (Revol et al. 1998). The reflected wavelength

becomes shorter at oblique viewing angles, giving rise

to visible iridescence colours when the value of nP is

around 400–700 nm, such that the chiral nematic pitch

is around 250–450 nm (Fig. 2).

The reflected colours of iridescent CNC films can

be shifted toward shorter wavelengths by increasing

the electrolyte concentration of the suspension prior to

film casting (Revol et al. 1998) or toward longer

wavelengths by high-energy sonication of the suspen-

sion (Beck et al. 2011). Added electrolyte partially

screens the negative charges of the sulfate ester

groups, reducing the interparticle electrostatic repul-

sion and shortening the pitch in a predictable manner.

This method of blue-shifting CNC film iridescence is

limited by the amount of salt which can be added

before the colloidal suspension is destabilized and

forms a gel (Dong et al. 1996; Revol et al. 1998). We

have found that sonicating CNC suspensions can

increase the chiral nematic pitch and shift the peak

reflection wavelength of films cast from these suspen-

sions toward longer wavelengths in a controllable

manner (Beck et al. 2011). It is proposed that this

effect is electrostatic in nature (Beck et al. 2011).

Fig. 1 Schematic illustration of the organization of the isotropic

and chiral nematic phases of a biphasic CNC suspension at

equilibrium. Half the chiral nematic pitch, P/2, is shown

1 cm

Fig. 2 Solid CNC film in diffuse light, showing its iridescence

when viewed normal to the surface (left) and at an oblique angle

(right)

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123

Composite CNC/silica films with tunable peak

reflection wavelengths have also been produced

(Shopsowitz et al. 2010); in this case, increasing the

amount of silica precursor added to the initial CNC

suspension results in a red-shift of the reflectance

peaks of the films. The red-shift is also preserved in the

final mesoporous silica films.

In the present paper, we propose a new procedure to

locally shift the reflection band in solid chiral nematic

CNC films, toward either longer (red-shift) or shorter

wavelengths (blue-shift) without the use of additives.

Our method uses pattern-forming objects (PFOs)

during film casting to modify the heat transfer kinetics

to different areas of the evaporating CNC suspension.

Experimental methods

CNC suspension preparation and characterization

Aqueous CNC suspensions were prepared in FPInno-

vations’ pilot plant by sulfuric acid hydrolysis of a

commercial bleached softwood kraft pulp according to

a procedure modified from the literature (e.g., Dong

et al. 1998). The sulfate ester content (240 ± 10

mmol/kg CNC) was measured by inductively coupled

plasma spectroscopy–atomic emission spectroscopy

(ICP–AES). The average CNC dimensions (length,

90–110 nm; cross-section, 5–10 nm) were measured

by scanning transmission electron microscopy (results

not shown).

Thermally stable neutral CNCs (Na-CNC)

were obtained by titrating as-produced acidic CNC

(H-CNC) suspensions with solutions of aqueous

NaOH (Anachemia) to a pH of 7. CNC suspension

samples were sonicated using a Sonics vibra-cell

130-W 20-kHz ultrasonic processor with a 6-mm

diameter probe: Typically, 15 mL of a 2–3 wt%

aqueous CNC suspension were sonicated at 60 % of

the maximum power output.

CNC film casting

A known mass (5–40 g) of 2–3 wt% aqueous CNC

suspension was poured into polystyrene Petri dishes

and allowed to evaporate at ambient conditions or in

an oven (VWR gravity convection, 141.6 L) at a

controlled temperature (30–75 �C). Solid CNC films

of basis weight 60–70 g/m2 and an average thickness

of 50–80 lm were obtained. To produce a pattern, the

Petri dish containing the CNC suspension was placed

on top of a metal object of specific shape (the pattern-

forming object, PFO) which was heated or cooled.

UV–visible spectroscopy of CNC films

Light transmittance (%T) properties of the CNC films

were measured with a Cary 100 Bio UV–visible

spectrophotometer (Agilent Technologies Canada

Inc.). For this semi-quantitative study, the wavelength

shift of the minimum transmittance (%Tmin) was used

as an indicator of the red-shift of the peak reflection

wavelength induced in the CNC films and will be

referred to as the peak reflection wavelength; the

reflected light shows up as an absorbance peak in

transmission mode (Revol et al. 1998). The films were

held in place by magnetic strips in the scanning film

holder, perpendicularly to the light beam. The %T was

measured at 0� incidence with air as a reference,

scanning over several locations on each film. Trans-

mittance spectra acquisition parameters were as fol-

lows: scan range 800–200 nm, 0.1 s signal averaging

time, 1.0 nm data interval, 600 nm/min scan rate,

2.0 nm spectral band width, double beam mode, no

baseline correction.

Temperature measurement

Temperature differences between zones of the Petri

dish in contact with air and with the PFO were

measured as follows: Hermetically sealed type K

thermocouples (Omega HSTC) were connected to a

4-input thermometer/datalogger (Omega HH1384).

The thermocouples were fixed to the inside surface of

the Petri dishes. The Petri dish was placed on a PFO

or suspended over air at the same height above the

oven shelf. Pattern-forming objects included 20 mm

thick aluminum blocks in the shape of a half- or full-

circle corresponding to the Petri dish dimensions.

Experiments were performed with empty and full

(water only) Petri dishes to determine the tempera-

ture difference between the areas of suspension

exposed to metal and to air (DTPFO). In addition,

experiments were performed in which the water

was allowed to evaporate completely to determine

the relative evaporation rates with and without

PFOs. Temperatures were recorded and equilibrium

temperatures determined.

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123

Results and discussion

Pattern formation by local heating

When CNC suspensions are evaporated with a section

of the container in contact with a pattern-forming

object (PFO) at a higher temperature than the

surrounding material, they produce an iridescent solid

film having a discernible pattern in the shape of the

PFO. The pattern is almost identical in dimensions to

the PFO and reflects longer wavelengths than the

surrounding film areas, indicating that the self-assem-

bled chiral nematic structure has a larger pitch.

Pattern formation by local heating can be achieved

either by heating only the PFO, or by heating the

suspension/container/PFO assembly in an oven.

A PFO that is heated or cooled will heat or cool the

suspension above it, relative to the surrounding

suspension. When casting CNC films above a PFO

in an oven, the vessel containing the CNC suspension

is spaced from the heat source, such that air acts as a

heat transfer medium to parts of the suspension that

are not in contact with the PFO (see Fig. 3). The rate of

the heat transfer to the evaporating suspension will

control its temperature. A good thermal conductor

such as a metal will transfer heat to the suspension

more rapidly than the surrounding air at a given oven

temperature, creating a zone of higher temperature in

the suspension above the metal object.

A typical experimental set-up is illustrated in

Fig. 3. The side view shows the CNC suspension in

a Petri dish on top of a thermal conductor PFO half-

circle, set on a metal shelf in an oven. The arrows

indicate the relative rates of heat transfer to the

suspension. The peak wavelength in the pattern area

A of the final film is red-shifted relative to the

surrounding areas.

Local temperature differences

Measurements with thermocouples have confirmed that

the local temperature of the Petri dish in contact with a

thermally conductive PFO is higher than the surround-

ing areas. At an oven temperature of 43 �C, temperature

differences DTPFO of *2 and *4 �C were measured

between areas in contact with a full-circle aluminum

PFO and with air, for empty and water-containing Petri

dishes, respectively. The temperatures in the dishes

containing water are lower than those in the empty

dishes due to evaporative cooling effects (DTevap).

The difference in temperature DTPFO between the

areas of CNC suspension exposed to a metal PFO and

top

side

HTair HTmetal<

Petri dish

conducting PFO

oven shelf

heat sourcekT(air) kT(metal)<

Tsusp(air) < Tsusp(metal)

A

CNC suspension

Fig. 3 Schematic

illustration of pattern

formation in a CNC film by

heating a CNC suspension in

an oven on a metal PFO. The

top view shows the higher

temperature area A in the

suspension. Colours are

indicative of the relative

red-shift of the heated area.

The size of the arrowsindicates the relative rates of

heat transfer (HT) to the

suspension. The relative

thermal conductivities kT of

the heat transfer media are

indicated

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123

air depends on the oven temperature. Thermocouple

measurements show that DTPFO in water-containing

Petri dishes increases from ca. 1 �C to ca. 5 �C as the

oven temperature increases from 33 to 72 �C (Fig. 4).

Full-circle PFOs appear to give higher DTPFO values

(by 0.6–1.1 �C) than the half-circle PFOs (data not

shown). When casting films from CNC suspensions,

we observed that the portion of the CNC film forming

above the PFO evaporates more quickly, reaching the

gel state and dryness before the surrounding areas of

suspension.

Reflection wavelength and film thickness

UV–visible transmittance spectra of patterned CNC

films illustrate the red-shifts induced by metal PFOs.

Figure 5 shows the transmittance spectra of the two

areas of a CNC film made by casting 5 g of 2.5 wt%

H-CNC (sonicated to 1,335 J/g CNC) in a 50-mm

diameter Petri dish with half the surface in contact with

a 20-mm thick aluminum PFO in an oven at 53 �C.

A PFO-induced red-shift of about 100 nm was found.

The thickness of different areas of various patterned

CNC films was measured with a digital micrometer. The

CNC density is not uniform throughout the film. It was

found that the red-shifted portions of patterned CNC

films produced on metal PFOs are thicker and hence less

dense, greater thickness corresponding to areas of

longer chiral nematic pitch. As expected, thicker

portions of the film show smaller minimum transmit-

tance values (larger absorption peaks in the transmit-

tance spectra as seen in Fig. 5, indicating more intense

reflection). Experiments (not shown) suggest that CNC

film thickness in itself has little or no effect on the peak

reflection wavelength for a uniform film of nominal

chiral nematic pitch P. Table 1 shows the average peak

reflection wavelength and film thickness values for the

patterned film shown in Fig. 5. A distinct shift in optical

properties is observable between the portions of the film

formed on the half-circle aluminum PFO and over air.

Similar results were seen for all CNC films. The same

type of experiment shows that the average peak reflec-

tion wavelength and the average film thickness also show

an increase when produced on a full-circle PFO at

identical conditions compared to films produced over air.

Behaviour in the presence of a plasticizer

Pure CNC films are quite brittle; plasticizers are often

added to facilitate their handling by improving their

Fig. 4 Interior surface temperature of water-containing Petri dishes in contact with air or with a full-circle aluminum PFO as a function

of oven temperature. Inset: DTPFO values as a function of oven temperature

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123

flexibility. It is known that plasticizers such as

poly(vinyl alcohol) (PVA) do not alter the iridescence

properties of these films (Zou et al. 2010). A 2.6 wt%

Na-CNC suspension containing 3.5 wt% PVA on

CNC dry weight was sonicated to an energy input of

1,540 J/g CNC. The PVA is present as a plasticizer to

improve CNC film flexibility and does not interfere

with the self-assembly of the CNCs into the chiral

nematic texture of the films, nor with the development

of iridescence colour in the films by means of

sonication (Revol et al. 1998; Beck et al. 2011).

A film was then cast by heating the suspension in an

oven at 60 �C in a Petri dish on an aluminum wire in

the shape of the FPInnovations logo (Fig. 6a). The film

displays a distinct FPInnovations logo pattern

(Fig. 6b). The pattern itself is orange–yellow in colour

while the surrounding areas are yellow–green to blue.

The presence of plasticizer (PVA) clearly does not

interfere with iridescence and pattern formation. This

experiment also demonstrates the sensitivity of the

pattern-forming method, as the area of direct contact

between the wire and the Petri dish is minimal.

Pattern formation by local cooling

The formation of patterns by changing the tempera-

ture of the PFO alone was exemplified by cooling a

PFO to below room temperature. A 15-g aliquot of

2.6 wt% H-CNC suspension was placed in a Petri dish

set on a metal pipe through which cold (8–12 �C)

water was continuously flowed, as illustrated in

Fig. 7. Upon evaporation at ambient conditions

(23 �C), a film showing a distinct blue-shift of

reflection wavelength in the PFO area B is produced.

The film shows thickness differences analogous to

those found in heat-patterned films; the blue-shifted

area is thinner because of its shorter pitch. At ambient

conditions, the temperature of a CNC suspension cast

in a Petri dish placed on a metal PFO with *8 �C

water flowing through it was around 10 �C lower in

the PFO area. The CNC gel evaporated to a solid film

more slowly above the cooled PFO. Despite the use of

a cooled (instead of heated) PFO in this case, the

mechanism of pattern formation remains the same; the

non-PFO area is at a higher temperature and is

‘‘heated’’ relative to the PFO area, resulting in a

longer pitch in that area.

Fig. 5 Transmittance

spectra for a film cast from

H-CNC on an aluminum

half-circle PFO at

Toven = 53 �C (film shown

in photo). Each spectrum is

an average of 4–7 scans

taken at different points on

the film surface

Table 1 Average reflection wavelength, film thickness and

%Tmin of a patterned film cast from 5 g of 2.5 wt% H-CNC

sonicated to 1,335 J/g CNC on a half-circle aluminum PFO at

Toven = 53 �C

Heat transfer

medium

Average

wavelength

(nm)

Average

thickness (lm)

Average

%Tmin

Aluminum 563 ± 10 73 ± 2 44 ± 2

Air 466 ± 8 44 ± 3 54 ± 1

Cellulose

123

Pattern formation mechanism

The PFO does not need to be present during the entire

process of film casting. It is only necessary that there

be a difference in temperature DTPFO when the chiral

nematic phase has already been formed but a certain

minimum amount of water still remains in the

structure, beginning when the suspension no longer

flows (later referred to as a ‘‘gel state’’). A nearly dry

CNC suspension which is still in the gel state produces

a patterned film when placed on a PFO for the last

stages of drying at 60 �C (Fig. 8). Correspondingly, if

the suspension is removed from the PFO when the

suspension is almost dry, a pattern is present in the

final film. However, if a CNC suspension is removed

from a PFO while it is still in a flowing liquid state, the

final film does not show a pattern.

It appears to take a certain amount of time for the

full pitch difference (the red-shift) between the pattern

(PFO) area and the surrounding film to be created (see

Fig. 8). That is, there is a moment during the drying

process at which the pattern formation can begin. If

2 cm 1 cm

(a) (b)

Fig. 6 a Aluminum wire (2.4 mm cross-section), bent in the shape of the FPInnovations logo. b Film produced by evaporating an

Na-CNC suspension containing PVA while heating on a wire as shown in a

Fig. 7 Pattern formation in a CNC film by cooling a PFO. The

top view shows the lower temperature area B. Colours are

indicative of the relative blue-shift of the cooled area. Arrows

indicate the relative directions of heat transfer (HT). The

photograph shows a CNC film produced in this way

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123

the PFO is added prior to this crucial moment, the

maximum attainable red-shift is reached before com-

plete dryness is achieved; if the PFO is added later, a

pattern with a smaller red-shift may still be created,

since the cellulose nanocrystals would be locked into

position before the maximum red-shift can occur.

Once the CNC suspension approaches a solid film

state, a pattern will not be created if the PFO is added

thereafter. This is an important observation regarding

the industrial application of this method, as the greater

part of the water evaporation does not need to be

performed in the presence of the PFO.

We propose two mechanisms for pattern formation

in cellulose nanocrystal films during casting, both of

which are controlled by the value of DTPFO. The first

mechanism deals with the kinetics of evaporation and

self-assembly, the second with the relative thermal

motion of the CNC particles in the distinct areas of the

suspension. It is worth noting that these patterns do not

form or develop in the sense of a separate self-assembly

process occurring in different areas of the CNC film.

Evaporation kinetics

Pattern formation in CNC films is correlated with the

relative rates of water evaporation of the different

zones of suspension. As the water in the CNC

suspension evaporates to give a gel and then a film,

the chiral nematic pseudo-planes approach each other

and the pitch P decreases with increasing CNC

concentration. The water in the CNC gel evaporates

faster in areas of higher temperature (e.g., above the

PFO) due to the higher heat transfer rate and its lower

surface tension. Due to the higher viscosity in these

areas at a given time, the chiral nematic CNC structure

attains a fixed pitch sooner than in the surrounding

suspension, generating a larger spacing between the

pseudo-planes, and hence a longer pitch and a red-

shifted film reflection wavelength. That is, faster

evaporation in areas of the nearly dry CNC gel ‘‘locks

in’’ the structure at a longer pitch; a thicker, less dense

film structure in the red-shifted area is created. The

CNC concentration is uniform in the initial aqueous

suspension; thus, when the gelation point is reached,

the CNC density is uniform throughout the gel.

However, as the remaining water continues to evap-

orate, it evaporates faster above the (thermal conduc-

tor) PFO, locking in the chiral nematic structure and

fixing the film thickness first. The gel exposed only to

air continues to become more compact beyond this

time, until its structure and thickness is fixed as well.

This mechanism explains the observations in Fig. 8;

Fig. 8 Schematic representation of pattern formation kinetics in CNC films produced by heating (not to scale). Adding a PFO at

different stages of drying results in red-shifts of different magnitudes; colours are indicative of the relative red-shift of the film areas

Cellulose

123

alternatively, removing the PFO before or very shortly

after the gel stage is reached will not result in a red-

shift in the final CNC film.

The kinetics of the heat supply control the CNC

suspension temperature and therefore its evaporation

rate. The evaporation rate was indeed found to be

higher in Petri dishes in contact with aluminum blocks

than in those in contact only with air, as shown in

Fig. 9. The temperature of the Petri dish surface

increases once all the water it contained evaporates;

this occurs for 10 mL water after 560–660 min in a

Petri dish on a full-circle aluminum PFO in an oven at

51 �C, whereas it takes 760–870 min to evaporate in a

Petri dish exposed only to air under the same

conditions.

Thermal motion

The thermal motion of particles depends on their

temperature. Although CNCs lie roughly parallel to

the pseudo-planes in the chiral nematic liquid crystal

phase, many lie with their long axes somewhat out-of-

plane, and will continuously vibrate, rotate and tilt in

and out of the plane like a see-saw due to Brownian

motion when in aqueous suspension. In areas of higher

temperature, the CNCs will experience greater thermal

motion and will therefore lie more out of plane,

causing an increase in separation of the pseudo-planes.

When the structure is kinetically trapped during film

drying, this results in a longer pitch and the film

structure being thicker and less dense in the red-shifted

PFO area.

Reflection peak width

In all CNC films cast at a given oven temperature, the

portions of the film formed over a metal PFO give

broader reflection peaks as measured by transmission

mode UV–vis spectroscopy than the portions formed

over air. For example, the red portion of the film shown

in Fig. 5 gives a reflection peak with a width at half-

height of around 175 nm, whereas the blue portion of

the film gives a peak with a width at half-height of

around 95 nm. The peak width increases with increas-

ing temperature for CNC films formed over metal

PFOs. However, peak width increases with reflection

wavelength regardless of the film casting temperature,

for both ‘‘metal PFO’’ and ‘‘air’’ regions (results not

shown). Analogous spectra of CNC films in which the

red-shift is induced by sonication treatment of the

original CNC suspension (Beck et al. 2011) also show

broader peaks as the chiral nematic pitch increases.

These results suggest that the broader reflection peaks

observed for film areas of longer pitch is a general

effect and may be due to the greater disorder of the

chiral nematic domains in these regions. As the CNC

suspension evaporates, tactoids coalesce to give

domains of varying cholesteric axis orientation that

are preserved in the final film structure (Revol et al.

1992). Domains of longer pitch are less compact, and

thus have a wider distribution of orientations as they

cannot align as easily during film formation. Thus,

light passing perpendicularly through longer-pitch

regions of the CNC film will strike the randomly

oriented chiral nematic domains at a larger range of

angles, resulting in a larger range of reflected wave-

lengths (i.e., k = nPsinh). Broader reflection peaks are

thus associated with these portions of the CNC films.

Effect of temperature on pattern formation

When an Na-CNC film is formed in a Petri dish on a

3-mm-thick metal ring at ambient conditions (23 �C),

a very faint pattern results. Based on the proposed

mechanisms of pattern formation, it is to be expected

that simply increasing the temperature would result in

a gradual increase in the red-shift and distinctness

observed in the pattern. When similar films are

produced in the oven, distinct patterns form at

Fig. 9 Temperature profiles for Petri dish surfaces at

Toven = 51 �C. The Petri dishes (50 mm diameter) contain

10 mL water and are in contact with air or with an aluminum

full-circle PFO

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123

temperatures above *30 �C. As the temperature

increases, the patterns become more distinctly red-

shifted as shown in Fig. 10.

As the temperature of the surroundings increases,

the difference in temperature DTPFO between the PFO

and non-PFO areas of the suspension also increases

(see Fig. 4). The kT values of the PFO materials and of

air are nearly constant over the temperature range

studied. At higher temperatures, it is increasingly

difficult for air to replenish the heat absorbed by the

suspension as quickly as a metal PFO. Thus, for a heat

transfer system consisting of air and a PFO of

conductivity kT, a minimum temperature Tmin is

required for a detectable red-shift between the PFO

area and the surrounding film to be obtained. In the

case of a stainless steel ring at 23 �C, the metal and air

temperatures are almost equal, giving a very small

DTPFO which produces a faint pattern in the final film

(thus, Tmin & 23 �C). As the oven temperature is

increased, DTPFO becomes large enough that the

difference in evaporation rates and thermal motion

in the zones also increase, leading to patterns with

larger wavelength differences (red-shifts).

Conclusions

Providing a higher temperature to a specific area of an

aqueous CNC suspension during evaporation and film

formation causes a red-shift of the minimum trans-

mittance (i.e., peak reflection) wavelengths or colour

in the corresponding area of the final iridescent chiral

nematic CNC film, resulting in a watermark-like

pattern incorporated into the film structure. Depending

on the desired application, the patterned films may be

dispersible or not in water, depending on the grade of

CNC used (Beck et al. 2012).

A red-shifted pattern can be incorporated into the

structure of the final film when a CNC suspension is

evaporated to dryness from a container placed on a

heated pattern-forming object (PFO) or a PFO made

from a thermally conductive material and heated in an

oven. The resulting patterned area is thicker than the

surrounding film due to the longer chiral nematic pitch

of the CNC film structure. Conversely, a thinner, blue-

shifted pattern can be incorporated into the film by

decreasing the temperature of the PFO relative to the

surrounding suspension areas. The presence of a

plasticizer does not inhibit pattern formation.

Increasing the temperature of the PFO or of the

film-casting environment (and hence increasing

DTPFO) improves the distinctness of the pattern and

red-shifts its reflection wavelength. When the film is

cast by heating in an oven, the relative rates of heat

transfer to the suspension govern pattern formation in

CNC films by differential heating.

Pattern formation occurs during the final stages of

drying (from gel state to complete dryness). Thermal

pattern formation in CNC films is caused by differ-

ences in evaporation rates and thermal motion

between areas of different temperature in the drying

suspension. Further work is needed to fully compre-

hend the phenomena controlling the optical properties

of CNC films produced in this manner.

Acknowledgments The authors thank Craig Muirhead for his

valuable experimental contributions and Dr. Lyne Cormier for

helpful comments and suggestions. Two of the authors (SB, GC)

were supported by Industrial Research and Development

Fellowships from the Natural Sciences and Engineering

Research Council of Canada.

Fig. 10 CNC films formed by heating an Na-CNC suspension in a Petri dish on a metal ring at increasing casting temperatures. The

red-shift of the patterned area increases with increasing temperature

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