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2021 Chemical Garden Membranes in Temperature-Controlled Microfluidic Devices Qingpu Wang and Oliver Steinbock

This is the final accepted manuscript, and the publisher's version of record can be found at https://doi.org/10.1021/acs.langmuir.0c03548

Follow this and additional works at DigiNole: FSU's Digital Repository. For more information, please contact [email protected] Chemical Garden Membranes in Temperature-

Controlled Microfluidic Devices

Qingpu Wang and Oliver Steinbock*

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida

32306-4390, USA

ABSTRACT: Thin-walled tubes that classically form when metal salts react with sodium silicate solution are known as chemical gardens. They share similarities with the porous, catalytic materials in hydrothermal vent chimneys and both structures are exposed to steep pH gradients that, combined with thermal factors, might have provided the free energy for prebiotic chemistry on early Earth. We report temperature effects on the shape, composition, and opacity of chemical gardens. Tubes grown at high temperature are more opaque indicating changes to the membrane structure or thickness. To study this dependence, we developed a temperature-controlled microfluidic device which allows the formation of analogous membranes at the interface of two co-flowing reactant solutions. For the case of Ni(OH)2, membranes thicken according to a diffusion-controlled mechanism. In the studied range of 10-40 °C, the effective diffusion

coefficient is independent of temperature. This suggests that counteracting processes are at play

(including an increased solubility) and that the opacity of chemical garden tubes arises from changes in internal morphology. The latter could be linked to experimentally observed dendritic structures within the membranes.

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INTRODUCTION

Chemical gardens are life-like precipitate structures which grow at the interface of two solutions if those reactants rapidly form an insoluble product.1-3 In the classic version of this process, a salt grain is placed into waterglass—essentially dissolved sodium metasilicate—and subsequently produces one or several hollow tubes as well as bulbous or vesicle-like structures. The thin wall of these structures consists of an outer layer of amorphous silica and an inner layer of metal hydroxide or oxide. Due to the buoyancy of the interior metal salt solution, tubes typically grow upwards reaching heights of several centimeters. While the latter process is driven by osmotic pressure and the resulting inflow of water, tube growth also occurs if the corresponding metal salt solution is directly injected into the waterglass.4 This injection method allowed the identification of distinct growth regimes as well as systematic measurements of shape parameters and growth speeds in terms of well-defined parameters.4 It also formed the basis of recent studies that investigated chemical garden growth under spatial confinement created by Hele-Shaw cells5-9, glass capillaries10, and microfluidic channels11-14.

Y-shaped microfluidic devices are ideally suited to study the slow, secondary thickening of the

chemical garden walls. This increase in the width of the metal-hydroxide precipitate membrane occurs typically in the direction of the metal salt solution and was first studied by measuring the weight increase of tubes.15 Later microfluidic studies11,16 created essentially linear membranes along the reactive interface of co-flowing, laminar streams of sodium hydroxide and metal salt solutions. These membranes increase their width w according to the simple time dependence

1/2 w ∝ (Deff t) , where Deff is an effective diffusion coefficient much smaller than the molecular diffusion coefficients of small chemical species in aqueous solution. One notable exceptions of

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this diffusion-controlled growth law is a type of hybrid membrane that shows bidirectional and rhythmic thickening dynamics17. We also note that flow over wavy membrane surfaces can enhance diffusive transport, which has been discussed as a possible factor in enhancing the bioenergetics of prebiotic chemistry and early life.18

The links between chemical gardens and the origins of life relate to possible scenarios in which life started in hydrothermal vent systems.19-24 These systems entail tall, chimney-like precipitate structures that form when hot, mineral rich water surges into the cold ocean. The dissolved minerals as well as simple hydrocarbon species, CO, and H2 result from the serpentinization of the rock underneath the ocean floor and determine the complex composition of the precipitates which include insoluble Ca, Ba, Fe, and Ni compounds.21,25 Many of these substances are catalytically active and might have facilitated the formation of more complicated molecules including sugars,26

27,28 29,30 31,32 33 amino acids, nucleobases, and pyrophosphate as well as CO2 reduction and the carbon fixation34. In addition, the porous precipitates provided spatial confinement without the need for lipid membranes and were subjected to steep gradients in pH and temperature. The combination of this geological source of free energy with a plethora of catalytic microreactors and appropriate reactants might have provided suitable conditions for the emergence of life on Earth four billion years ago and possibly on other planetary objects with a rock-water interface such as Enceladus and early Mars.35-38

The precipitate membranes of chemical gardens are useful models for the study of specific processes in hydrothermal vents and applications as biomimetic structures39-41. They spontaneously generate gradients in pH and redox potential27,42 and are porous as well as catalytically active43-45. Various aspects of these common features have been studied through the lens of origins-of-life research such as the production of pyrophosphates in iron mineral 3

membranes31,32 and morphological similarities between chemical gardens and disputed

microfossils that were found in ancient rocks associated with hydrothermal vents46. However, to

date, the relevance of temperature effects has attracted little attention.47 To start closing this

knowledge gap, we here report an experimental study of the effects of temperature on the growth

of precipitate membranes based on the aforementioned microfluidic methodology.

EXPERIMENTAL SECTION

Chemicals and Materials. Nickel chloride (NiCl2·6H2O, Fisher Chemical), cobalt chloride

(CoCl2·6H2O, Sigma-Aldrich), cupric sulfate (CuSO4·5H2O, Fisher Chemical), zinc sulfate

(ZnSO4, Fisher Chemical), and sodium hydroxide (NaOH, Macron Fine Chemicals) are used as received. All solutions are prepared with nanopure water filtered by a Barnstead Easypure UV system (resistivity 18 M cm).

Pump Injection Experiments. For the experiments performed below room temperature, reactant solutions are kept in an ice bath for 30 min. For the experiments above room temperature, reactant solutions are thermostated in a 65 °C water bath at for 30 min. After thermal equilibration,

1.0 mL of 0.1 M NaOH solution is injected through a glass capillary (inner diameter: 1.0 mm) into a glass cylinder (inner diameter: 31 mm, outer diameter: 35 mm, height: 9.0 cm) containing 40 mL of 0.05 M metal salt solution (NiCl2, CoCl2, CuSO4, or ZnSO4). The pump rate is 8 mL/h in all the experiments. We record the temperature of the solution in the cylinder by using a thermocouple data logger (Pico Technology, JKE35/277) connected to a PC and PicoLog 6 software. The sensor was positioned about 1 cm below the meniscus of the metal salt solution and about 3 mm from the container wall.

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Microfluidic Experiments. The microfluidic device (Figure 1) consists of a cut parafilm

membrane (approximate thickness: 130 µm) sandwiched between two plexiglass plates measuring

8 cm × 10 cm (thickness 1.5 mm). Two holes with a diameter of 1.6 mm are drilled onto the top

plate and barb fittings (NResearch Inc., FITM 331) are secured onto the holes using an epoxy glue.

The three layers are then assembled, fastened with the help of several binder clips and glass slides

for even pressure distribution. The assembly is then heated on a hot plate at medium-low heat for

5 min to slightly melt the parafilm membrane. After cooling under ambient conditions, the resolidified parafilm acts as an adhesive that holds the two plates firmly together without requiring external fastening.

Figure 1. (a-b) Schematics showing the experimental setup of the temperature-controlled microfluidic device. The side view in (b) highlights the view window for the optical microscope.

This window is positioned right next to the temperature-regulated region. (c) Photograph of the microfluidic device and the resulting precipitate membrane (thin vertical line in the lower half of

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the photo). The large, gray dashed box shows the position of the metal block during the experiment and the very small red box indicates the microscopic view window.

We use a PC-controlled cutting tool (Silhouette Portrait) to cut the parafilm membrane into the desired pattern: a Y-shape17 with branches extending to both sides along serpentine tracks that consist of semicircular-ring segments connected by straight parts. The stem of the Y-shaped

pattern is 45 mm long, 2 mm wide, and approximately 130 µm high. Two syringes containing

0.50 M NaOH and 0.25 M NiCl2 solutions are connected to the barb fittings with plastic tubing

(Tygon, inner diameter 1/16’’). Reactant solutions are prepared in water degassed by prior boiling

to avoid undesired bubble formation in the microfluidic channel during heating. The syringes are slowly discharged using a programmable syringe pump (New Era Pump Systems, NE 2000) at a constant pump rate of 1 mL/h. Prior to the injection, we manually fill the microfluidic channel with water to avoid surface-tension induced perturbations. We also preheat or precool the system by placing a hollow aluminum block containing circulating water on top of the microfluidic device for 30 min. The water circulation is controlled by a refrigerated bath/circulator (NESLAB RTE-

111). Notice that for the investigated temperature range of 10 to 40 °C, small but noticeable pH

48 changes occur (e.g. 13.9 to 12.9 for 0.25 M NaOH as calculated from the relevant Kw values ).

The progress of the precipitation processes within the microfluidic device is observed using an inverted microscope (Leica DM IRB). The microscope is connected to a camera (Nikon D3300), and photos are collected using a PC and digiCamControl software. For the Deff measurements, the microfluidic device is positioned to view the channel area right next to the thermostated metal unit

(Figure 1b). For dark-field-like microscopy, images are obtained at high ISO settings under diffuse illumination by room light only.

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XRD and ATR-IR Spectroscopy. For further characterization, the precipitate is extracted from

the device, rinsed in water, and dried under ambient conditions. The powder XRD (X-ray

diffraction) measurement was carried out using a Rigaku SmartLab diffractometer with a Copper

X-ray source. The ATR-IR (attenuated total reflection infrared) spectra were collected with a

JASCO 6800 FT-IR spectrometer. The samples consist of precipitate membranes produced in

multiple experiments.

Infrared Photography. For the temperature measurements of the microfluidic device, we use a

thermal imaging camera (Seek Thermal, Compact XR) connected to an Android smartphone.

RESULTS AND DISCUSSION

Firstly, we investigate the effect of temperature on the growth of chemical garden tubes. The precipitate structures form upon an upward injection of temperature-adjusted NaOH solution

(0.10 M) into a large reservoir of metal salt solution (0.05 M) at the same temperature. Notice that

this procedure is forming a reverse chemical garden49 that might be a closer approximation of the structures in hydrothermal vents in a slightly acidic ocean. Figures 2a-d show representative

precipitate structures in four different divalent metal salt solutions containing Co2+, Cu2+, Zn2+,

and Ni2+, respectively. Each panel consists of an image pair illustrating tubes formed at low and

high temperature. Either a white or black background is used to achieve a strong contrast under

diffused white light.

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Figure 2. (a-d) Photographs of precipitate structures formed during the injection of 0.10 M NaOH

2 into 0.05 M solutions of CoCl2, CuSO4, ZnSO4, NiCl2, respectively. Field of view: 1.9 × 5.1 cm .

(e) Time evolution of the temperature during the injection. The blue dashed and red solid borders/lines correspond to experiments with cold and hot solutions, respectively.

The low and high temperatures in these experiments are initially 11.9 and 51.2 °C, respectively.

However, since we do not control the temperature of the reactor system, the solution conditions slowly equilibrate to room temperature. The temperature measurements in Figure 2e quantify this equilibration process and yield average rates of -0.74 °C/min and 0.33 °C/min for the hot and cold conditions, respectively. The photos of the structures in Figure 2 were recorded within the first ten minutes of continuous injection and the tubes had gradually formed during this period (see

Movie S1). After this time, we find final temperatures of 15.4 and 42.3 °C, respectively.

A qualitative comparison of the tube structures at low and high temperatures (Figures 2a-d) reveals differences in size and opacity. Our results show that the tube structures are less opaque

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and, for Co2+ and Ni2+, wider at the low temperature. For elevated temperatures, we also noticed

the formation of chemically more complex products due to the oxidation of Co2+,16 the reaction of

Cu(OH)2 to CuO, and the amphoterism of Zn(OH)2. Amphoterism manifests itself in short tubes that abruptly end and fail to extend despite continued delivery of reactant solution. This behavior

50 is similar to earlier reported dynamics of VO(OH)2 tubes. The nickel-based tubes show the strongest difference in opacity and are not expected to undergo redox reactions; we hence select the nickel system as the target for the following microfluidic studies.

To overcome the limitations of nonconstant temperatures in our 3D experiments and to simplify the geometry of the precipitate structures for in situ measurements, we developed a temperature-

controlled microfluidic device. Based on our previous studies11,16,18, Y-shaped microfluidic

devices are well-suited to create a laminar co-flow of reactant solutions. We modified this design

by elongating the two arms of the Y-shaped channel and introducing a hollow metal block connected to a water bath circulator (Figures 1a,b). This allows a long travel time of 166 s for preheating or precooling reactant solutions before they come into contact.

In our experiments, green precipitate forms at the interface of the two co-flowing solutions. The average fluid velocities are 0.53 mm/s in the individual channels and 1.07 mm/s in the combined reaction channel. The resulting fluid flow is laminar as indicated by small Reynolds numbers (Re

< 1). The reactant concentrations are stoichiometric (0.50 M OH- and 0.25 M Ni2+) with respect to the formation of Ni(OH)2. The precipitate membrane starts at the mixing point and extends down to the outlet along the middle of the stem of the Y-shaped device (Figure 1c). We did not observe differences in the width and overall appearance of the membrane along the channel. Exceptions, however, include the presence of undesired small air bubbles pinned at the channel walls, which decrease the effective channel width and consequently result in local perturbations. 9

With a steady injection of reactant solutions at 20 °C, the membrane wall thickens over time and reaches a width of w = 350 µm after 4 h (see Figure 3a and Movie S2 in the Supporting

Information). We emphasize that the membrane thickening occurs exclusively in the direction of the Ni2+ solution. This unidirectional growth phenomenon had also been reported for other metal hydroxide membranes and was interpreted in terms of membrane semipermeability.11,16,18

Figure 3. (a,b) Micrographs of the precipitate membrane after 4 h growth at 20 °C. Scale bars correspond to 100 µm (a) and 50 µm (b). (c) Area ratio of the dendrite along the membrane. The continuous red line is a fit to α A–exp(–ky) and yields k = 0.28 mm-1.

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Furthermore, we observed various microstructures within the nickel-based precipitate membranes (Figures 3a,b). At the early stage, the membrane shows band-like features that are likely related to the fast initial precipitation (Figure S1). These bands are oriented along the flow direction and have a spacing of less than 6 µm. Subsequently, a featureless layer increases the width by about 30 µm, followed by a thin granular layer. Then, dendrite-like microstructures

emerge from this layer at seemingly random positions. The majority of these dendrites continue to

grow as the membrane thickens, while others stop developing and are left behind. We also noticed very thin linear structures of different lengths that are oriented perpendicularly to the membrane’s boundary. These structures appear as black vertical lines in Figures 3a,b. They form during the active growth of the membrane and have an initial spacing of about 10 µm. However, only very few of these structures extend beyond a distance of 15 µm, with the longest one reaching a length of 139 µm.

In order to characterize the dendritic microstructures, we measure the area ratio α of the granular image areas (dendritic structures) within the analyzed total areas of the membrane. For this measurement, we collect micrographs at different positions along the channel after 4 h of injection.

These images are then carefully stitched together by cropping and visual alignment using

MATLAB, which produces a panoramic view of the membrane over a length of 25 mm. Figure 3c shows the measured area ratio α as a function of the position y along the channel, with y = 0 being

the mixing point. Each data value is calculated based on a 1 mm wide interval of the membrane

(see Figure S2). We find that α increases along the channel and saturates around an α value of

0.82±0.08.

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In addition, we noticed that the nickel-based membranes display a shiny and smooth surface after extraction from the microfluidic channel. Optical inspection reveals that the membrane is highly transparent (Figure 4a-c) as demonstrated by experiments in which we placed the membrane

between an optical target (glass slide with microscale markers) and the objective of the inverted

microscope. The membrane induces only very small distortions in the image of the calibration

pattern indicating a spatially homogeneous refractive index and the effective absence of scattering

sources. This surprising feature is further documented by the dark-field micrographs in

Figures 4b,c that are obtained for two different focal settings. The high optical clarity of the

membrane material of the dendrite-free regions is independent of temperature and unusual

considering that precipitate products are typically opaque including other membrane materials

studied by our group in earlier investigations.11,16,18,32

Figure 4. Micrographs illustrating the optical clarity of the precipitate membrane under bright-

field (a) and dark-field (b,c) illumination. The focal plane was on the optical target (a,b) or the

membrane (c). Scale bars correspond to 100 µm. (d) XRD pattern of dried, powdered membrane

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samples. (e) ART-IR spectra of a freshly made membrane (red) and one stored at ambient conditions for eight months (black).

The XRD pattern in Figure 4d shows broad features around 30°, 43°, and 60°. The absence of

sharp peaks indicates that our sample is predominantly amorphous. Note that the large quantity of

sample required for this measurement was obtained from around ten experiments performed over

a period of several months. Accordingly, we cannot fully rule out that the chemical composition

of the membranes had changed over time; however, the overall appearance as evaluated by optical

microscopy was similar to that of freshly prepared samples (Figure S3). In addition, the attenuated

total reflection infrared spectra (ATR-IR) of the fresh (stored for less than 48 h) and old samples

(entire material eight months old) are nearly identical (Figure 4e). The large absorption band

centered at 3376 cm-1 can be assigned to the stretching vibration of hydrogen-bonded hydroxyl

-1 groups in Ni(OH)2. A sharp O–H stretch peak at 608cm is ascribed to the hydroxyl lattice

vibration.51,52

In the following, we systematically study the effect of temperature on the Ni(OH)2 membrane

growth in microfluidic devices. To validate the efficacy of the employed temperature-control

method, we characterized the reactor system for a water bath temperature of 40 °C. The photo of

the experimental setup in Figure 5a shows the thermostated metal block positioned over the inflow

portion of the microfluidic device, the narrow reaction channel near the center of the image, and

the Petri dish used for collecting the outflowing solution (cf. Figure 1). The other frames in

Figure 5 are infrared images recorded with a vanadium-oxide-based microbolometer. The dark

blue and white/red color regions in the images correspond to the lowest and highest temperatures,

respectively (see caption of Figure 5 for details). Frame b shows the infrared images in an area

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comparable to frame a. Frames c-f show a magnified view of a region around the small reaction channel (red box in frame a). Over a course of 60 min, both the changes in temperature and the temperature gradient are negligibly small. The region of interest for the membrane growth experiments is indicated by the dashed rectangular box in panel f. We conservatively estimate these two quantities as ∆T (1 h) < 2 °C and ∆T/∆y < 0.4 °C/mm where y is the vertical image coordinate. We note that our heat measurements characterize the surface temperature of the device.

In addition, all subsequent analyses will employ a very short length of the membrane (< 2 mm) within the white region of the heat maps. These measurements show that the temperature control is effective for our microfluidic system (see also a related analysis for 1 °C in Figure S4).

Figure 5. Efficacy of controlling the microfluidic device temperature. (a) Photograph of the microfluidic device and the thermostating metal unit. Scale bar: 2 cm. (b-f) Heat maps recorded

30 min (b,c), 35 min (d), 60 min (e), and 90 min (f) after positioning the thermostated metal block onto the device. (c-f) are magnified views of a region in (a,b) centered near the view window (red

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box). The color bar indicates local temperatures according to the extreme values Tmin = 21, 20, 24,

23, 24 °C and Tmax = 32, 34, 34, 35, 33 °C in (b-f), respectively. Approximate field of view: 10.3

× 14.6 cm2 (b), and 2.7 × 3.8 cm2 (c-f).

Figure 6a shows a typical time-space plot of the thickening precipitate membrane at 20 °C. The

image data for this analysis is obtained 20 mm away from the mixing point (viewing window in

Figures 1c and 5f) and collected at 10 s intervals for 4 h. From these images, we extract spatial

color profiles across the membrane. Each profile spans 1185 pixels and corresponds to a 500 µm

× 0.4 µm band (e.g. a vertical line in Figure 3a). The profiles are then stacked sequentially into the

time-space plot which shows the aforementioned unidirectional thickening. Next, we mark the

coordinates of the upper membrane border (contact line to the Ni2+ solution) in terms of specific

RGB values (120 < red < 140, 120 < green < 140, 125 < blue < 135). These color ranges are based

on manually selected points along the visually determined border and yield a set of (w, t) values.

In Figure 6b, we superpose this data set onto the original time-space plot as red markers. For further

analysis, we average the data for each time to yield a function ( , t) which describes the growth

of the precipitate membrane over time. These results are then fitted to a square-root function

( ) = . We find good agreement between our experimental data and the fit (Figure 6c),

which confirms that the membrane growth is diffusion-controlled. In addition, we measured the time evolution of the membrane’s width at different temperatures (Figure S5) and found no clear trend between the width of the membrane and the position in the viewing window.

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Figure 6. (a) Time-space plot of the growing membrane constructed from a time-lapse video

recorded at 20 ºC. (b) The same data as in (a) with superposed data on the upper border defined in

terms of specific color values. (c) Square-root fit (black curve) and the experimental data (red dots). (d) Effective diffusion coefficients Deff measured at different temperatures. The error bars correspond to the standard deviation values from seven measurements for each temperature.

Next, we systematically vary the temperature while keeping both reactant concentrations constant. The effective diffusion coefficient Deff is measured at different temperatures from 10 to

40 °C. For each temperature, we construct seven time-space plots for different positions of the microscopic images and calculate the averaged Deff values with their standard deviations. Our results (Figure 6d) show that the Deff is nearly independent of the reaction temperature. The dashed

-7 2 line indicates an average value of Deff = 0.93×10 cm /s which is at least 100 times smaller than the diffusion coefficient of low molecular weight species in this temperature range. Notice that molecular diffusion coefficients increase with temperature which makes our finding surprising. 16

Motivated by the existence of large temperature differences in the chimney-like precipitates of

hydrothermal vents53, we have investigated thermal effects on the formation of chemical gardens.

The injection of NaOH into a large reservoir filled with metal salt solutions revealed that an

increase in reaction temperature results in precipitate tubes that are more opaque than their low-

temperature counterparts. For chemically simple cases, such as Ni(OH)2 structures, this

observation indicates an (i) altered wall structure or (ii) increased wall thickness. Our microfluidic

studies, however, show that the growth rate of Ni(OH)2 membranes is independent of temperature

for the investigated range between 10 and 40 ºC. This finding weakens the possible relevance of

explanation (ii). Before discussing the 3D case in more detail, we first consider the simpler

situation of membranes in microfluidic systems.

The temperature-independence of the studied membrane thickening is surprising as diffusion

accelerates with increasing temperature T. In crystalline solids, for example, diffusion coefficients

D tend to follow Arrhenius behavior in which the activation energy corresponds to an energy

barrier for positional changes of the diffusing particles.54,55 In the liquid phase, self-diffusion is

often discussed in terms of the Stokes-Einstein equation D = kBT/6πηa where kB, η, and a are the

Boltzmann constant, the dynamic viscosity, and the molecule’s hydrodynamic radius, respectively.56 While this equation seemingly suggests a proportional dependence between D and

T, the temperature dependence of the viscosity has to be considered, which for water decreases by about 50% over the relevant temperature range.57 In addition, molecular dynamics simulations of

the diffusion of hydroxide ions in water revealed striking deviations from a simple proportional

dependence reporting an increase in D by a factor of 1.3 as the temperature is increased from 10

to 40 ºC (see Figure S6 in the Supporting Information).58,59 The observed lack of a temperature

17

dependence of Deff, hence, suggests the presence of a second effect that counteracts the increase

in diffusivity.

One likely explanation is the increase in solubility of Ni(OH)2 with increasing temperature. The

2+ - 0 precipitation reaction Ni (aq) + 2OH (aq) Ni(OH)2(s) is indeed exothermic (∆H = -15.7 kJ/mol

60 at 25 °C) , which implies that the solubility of Ni(OH)2 increases with increasing temperature.

Experimental data by Chickerur et al.61 confirm this thermodynamic expectation and report a

-12 -12 change of Ksp from 1.5 × 10 at 25 °C to 2.9 × 10 at 40 °C.

Other factors that could counteract the accelerated diffusion include the temperature dependence

48 of the dissociation of water and variations in the membrane properties. The Ni(OH)2 precipitate

membranes are amorphous (see Figure 4d) and porous as reported by earlier gas physisorption

studies62 (carried out on dried samples obtained at slightly different reaction conditions). If

changes in porosity would hinder the diffusive flux of OH- ions at high temperatures, this effect

would provide an alternative explanation for the temperature-independence of Deff. Porosity

measurements on the hydrated material are, however, difficult and will require experimental

methods63,64 that to date have not been applied to precipitate membranes. Lastly, the reaction rate

can be expected to increase, but this increase is irrelevant as even at low temperature the reaction

is extremely fast compared to the time scale of the experiment and usually assumed to be

instantaneous as justified by our measurements in Figure 6 that indicate diffusion control.

Returning to the opacity differences of the three-dimensional chemical garden tubes grown at

different temperatures (Figure 2), we suggest that changes to the internal morphology (Figure 3)

are a likely source. Our experiments show that dendritic structures are more abundant at high

temperatures (Figure S7) causing enhanced light scattering and hence an increased opacity. Other

factors could include temperature-dependent changes in the density difference between the two 18

reactant solutions and resulting changes in the buoyancy-driven fluid flow. The value of ∆∆ρ is

5.3 × 10-4 g/cm3 for the investigated temperature range (see Table S1 for more details).65 Although

small, this density difference could trigger fluid flow that might contribute to or possibly account

for the observed morphological differences. In addition, temperature-dependent viscosity changes

and differences in the overall growth dynamics that are difficult to quantify, might also cause

optical scattering due to self-healing fission and fracture lines.

Considering the importance of thermal effects and temperature gradients in prebiotic

hydrothermal vents—which provide a source of free energy and also induce thermophoretic

separation and possibly RNA/DNA replication66,67—more work is needed to characterize the

complicated effects of temperature on inorganic precipitates. Furthermore, Braun and others

pointed out that RNA replication required not only temperature gradients but also habitats that

locally allowed for persistent accumulation. They showed that such an accumulation can occur in

closed, elongated compartments subjected to localized heat flux.68,69 Also the activity and the

specific products of catalytic reactions could have been modulated in temperature gradients to

yield greatly varying prebiotic environments in closest vicinity to each other.44

We have shown that microfluidic devices provide a useful platform to unravel certain aspects of

these underlying mechanisms. Future studies should expand our approach to other precipitates and,

the technically more challenging, study of cross-membrane temperature gradients. In a

microfluidic setting similar to the one introduced here, the application of two different influx

temperatures will necessarily induce a decrease of this cross-membrane temperature gradient along the reaction channel. While this decrease can be minimized by the use of a differently shaped, thermostating unit, it can also be considered a useful unfolding of different temperature conditions

19

in space. In other words, one microfluidic experiment would analyze a wide range of temperature

gradients along the reaction channel.

Temperature gradients in the context of three-dimensional chemical gardens and hydrothermal vent precipitates also strongly depend on the specific geometry of the involved precipitate membranes and even for cylindrical membranes, the heat exchange will vary with tube radius. In analogy to hydrothermal vents, we performed a few preliminary experiments in which a hot NaOH solution was injected into a cold NiCl2 solution (Figure S8). The resulting tubes were very similar to those obtained by cold-into-cold injection, probably due to the rapid cooling of the narrow tube structure. Considering the wide range of shapes in chemical gardens and hydrothermal vents and the complex involvement of buoyancy-driven flow, it is reasonable to begin systematic investigations with measurements of the nearly constant width membranes formed in microfluidic channels that offer simplified and well-defined experimental conditions.

CONCLUSION

In summary, we have investigated thermal effects on the amorphous structures formed during the precipitation of Ni(OH)2. This reaction can form hollow tubes that in a microfluidic device are reduced to linear membranes. In the investigated temperature range of 10 to 40 °C, the membrane thickness increases under diffusion control. The effective diffusion coefficients are much smaller than the values expected for free diffusion and show no temperature dependence. We interpret the latter finding as the result of an effect that counteracts the anticipated increase in diffusivity. A likely candidate is the higher solubility of Ni(OH)2 at elevated temperatures. Future studies should evaluate the relevance of thermal effects—and specifically cross-membrane gradients—on other materials and also with regard to prebiotic processes. 20

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge.

Additional micrographs of the membranes, method description for calculating the area ratio α,

temporal evolution of the membrane width at different positions and temperatures, literature values

of the OH- diffusion constant at different temperatures, area ratio α at different temperatures,

photographs of precipitate tubes formed under different conditions, and the densities of reactant

solutions (PDF).

Time-lapse movie of growing 3D precipitate structures at different temperatures (MP4).

Time-lapse movie of a growing precipitate membrane in the microfluidic device (MP4).

AUTHOR INFORMATION

Corresponding Author

* Email: [email protected]. Phone: +1 850-644-4824.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This material is based upon work supported by the National Science Foundation under grant no.

1609495 and NASA under grant no. 80NSSC18K1361. We thank Dr. Xinsong Lin and Dr. J. S.

Raaj Vellore Winfred for assistance with powder XRD and ATR-IR measurements.

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