Nano, Micro, Macro: Adaptive Materials Laboratory

In collaboration with Iain Gordon Caleb Marhoover, + Penelope Phylactopoulos

A fire glows somewhere. Its heat turns a turbine, or pushes a piston, or melts ore into a serpentine flow. This combustion is but a precursor to consumption, enabling the creation of products as well as the transport of them. Trinkets and tchotchkes, however, are not the only byproducts of burning. A jet of particles, each forty times smaller than a grain of sand, spews forth from this fiery font. These curling carbon chains are carried quickly away by a draft, a breeze, or a jetstream. Suspended in a river of air, they circle the globe, clouding our horizons with haze. 

Just as the wealthiest nations have shifted much of their manufacturing capacity beyond their borders, they have also exported much of the associated pollution. Unfortunately, the winds do not respect geopolitical boundaries. Particulate matter (PM) pollution (2.5µm to 10µm) was responsible for nearly 3.5 million premature deaths worldwide in 2007. About 12% of these deaths were related to air pollutants emitted in a region of the world other than that in which the death occurred. Nearly 22% were associated with goods and services produced in one region of the world for consumption in another (Zhang et al. 2017). 

A satellite view of the Korean peninsula reveals the vast quantities of transnational particles transiting the Yellow Sea. China is currently the world’s largest emitter of anthropogenic air pollutants, and measurable amounts of Chinese pollution are transported via the atmosphere to other countries, including South Korea and even the western United States (Lin et al. 2013). PM2.5 pollution produced in China in 2007 is linked to nearly 65,000 premature deaths in regions outside of China (Zhang et al. 2017). Many of these occurred in South Korea, where roughly 50% of the ambient PM2.5 concentration can be traced back to northern China. 

On a typical day, about 25 million South Koreans inhale dangerous levels of airborne particulate matter. Particles smaller than 10µm are able to bypass the human body’s natural filtration mechanisms. If inhaled, they get lodged in lungs and other cardiovascular tissues. Over time, an accumulation of particles can lead to respiratory infections, heart disease, lung disease, and cancer. (Mosteller 2016)

Precautions taken to ward off this pervasive, invasive threat fall along a continuum of scales and fuel consumption. Individuals in urban environments, like the 25 million residents of Seoul, often wear fiber face masks whenever outdoors. Unfortunately, these have been shown to be only marginally effective at protection from particles of 2.5µm size (Shakya et al. 2017). Building scale filtration systems, High Efficiency Particulate Air (HEPA), are effective at removing PM2.5, but are costly to operate. An ASHRAE industry manual recommends expecting 50% of a typical building’s total fuel costs to come from the HVAC system, while 30% of the HVAC costs are due solely to air resistance created by the filters. Additionally, conventional building level systems recognize only the binary spatial states of indoor and outdoor, with one pool of air receiving maximum cleaning and the other receiving none. 

Our buildings behave as exosomatic lungs, an important task when the environment bombards us with hazards against which our bodies cannot defend. Luckily, the human respiratory tract is quite effective at filtration against particulates larger than 10µm. It achieves this using a layered entry sequence during inhalation, wherein the wetted surfaces of the nasal passages and windpipe capture and flush foreign material. In order for our buildings to address the need for air cleared of even smaller particles without enormous fuel costs, perhaps we might replace the ‘closed system’ paradigm of typical ventilation design with bioinspired ‘open system’ thinking. 

Moisture-based systems for removing particulate pollution already exists at the industrial scale, using high pressure, high flow wet scrubber systems to clean dirty gas streams. Re-imagining these systems to address the low pressure, low flow conditions within buildings requires a little creativity. Moisture is generally viewed as a harmful substance in the built environment – something that causes decay and mildew, and to be guarded against. These characteristics are present because we have commonly built in a way that is moisture intolerant. There are opportunities, however, to welcome moisture into our buildings as a performative filtration mechanism. 

One common source of moisture within the built environment is condensation in the form of dew. Dew forms on surfaces whenever they cool below the local dew point, or the temperature at which air becomes fully saturated with moisture. The formation of dew is often linked to diurnal cycles, usually forming at or in the early morning. In an analogous manner to wetted respiratory passages, dew has been found to be effective at cleansing the layer of air nearest the condensing surface. PM2.5 removal rates of 21.5% were observed in the near-surface layer on clear days, with rates declining during foggy and hazy conditions (Xu and Zhu 2017). 

This phenomenon can be incorporated into building design, where large area condensing surfaces can act to capture and flush pollutants with low energy input. The relatively low ambient PM2.5 removal rates observed by Xu and Zhu could be improved upon by nesting spaces within one another, such that air must encounter multiple condensing surfaces in order to enter the innermost rooms. This strategy already suggests certain building typologies and traditions may be better suited than others.

Korean vernacular construction offers the example of the Hanok, a traditional home design which captures and channels the ‘waste’ heat from the kitchen fire in order to heat the home (See appendix B). The smoke is brought beneath the house, where it warms the floor surface from beneath before being exhausted through a vent along the wall. In a striking display of openness, this scheme allows environmental flows to penetrate a porous building in order to perform work. There is potential to build off this predisposition for open systems to encourage the functional flow of humidity and condensation within interior spaces which would, in contemporary buildings, be designed as airtight boxes. 

Review Of Literature And Precedents 

Industrial Particulate Removal

Wet-scrubbing and the humidity swing method are established and trusted in industrial applications (deVries 1981, Mussatti 2002, Haga et al. 2016) as effective methods for removing particulate pollution from high pressure gas streams. This method uses humidity to entrain PM as it passes through a chamber. Afterwards the humidity is removed from the air stream using a heat exchanger that condenses the water and flushes PM. In Haga 2016, it was observed that a redesigned mixing chamber and the introduction of a coarse mesh filter for catching droplets allowed for 99.9% PM removal at 10L/min flow. 

Filtration Properties of Condensation 

Dew has been observed to be a good indicator of near surface air quality (Xu and Zhu 2017). This article does not explore the possibility of utilizing their findings in a potential air cleaning method, but it is evident that with enhanced condensing surfaces we could effectively reduce PM concentrations (Tsuchiya et al. 2017, Park et al. 2016). Various surfaces are known to collect PM in polluted environments, like waxy leaf structures in urban settings due to surface adhesion and turbulent airflow which increases contact (Dzierżanowski et al. 2011).  Informed by these scientific precedents, our current proposal for sweaty interior surfaces aims to bring technologies from these other fields into nested, tempered living spaces. 

Photocatalysis

Volatile Organic Carbon (VOC) and PM both occur in interior living spaces, but VOC’s continue to be emitted by many of the products and finishes within our buildings long after construction has finished. For this reason we wanted to explore the potential to integrate strategies to mitigate both in our experimental studies. Photocatalysis is a proven method for neutralizing harmful VOCs, and has the potential to be readily integrated into our sweating surfaces model (Wooh and Butt 2017, Zhu et al. 2017). 

SLIPS

The slippery liquid-infused porous surface (SLIPS) technology is a hybrid material system in development for a variety of potential uses that takes advantage of nanostructures. These nanostructures are created on the surface of the material in question, making it hydrophobic. This structure is then infused with an appropriate lubricant to decrease the time in which water condensate-droplets evacuate the surface. This has exciting possible applications in many fields, including anti-biofouling, and more efficient heat exchangers (Wong et al. 2011).

Bumpy Surfaces

Inspired by the biological morphology of beetles, cactus’, and pitcher plants, bumpy surfaces could be an additional layer of the SLIPS technology by controlling the nucleation point and the evacuation path for dropwise water condensation. Bumpy surfaces can be made of a variety of materials, and due to the shape of the bumps allow for increased water collection in less than ideal contexts. There has been a sixfold higher exponent of growth rate observed over other surfaces in water collection, droplet growth and turnover (Park et al. 2016).

Design Studies

Embracing Humidity to Perform Filtration

To achieve the condensation on the surfaces necessary for the particulate collection, a temperature differential is needed. Consequently, a chilling source, whether passive or active, is needed for the temperature differential. The following are two methods of chilling:

Experimental Studies

Sweating Surfaces

In order to evaluate the capacity of differing surface treatments to influence the removal of ambient PM, we constructed a sealed acrylic testing chamber. It is a transparent cube with an internal volume of 0.027 m3 (30cm x 30cm x 30cm), made from laser-cut acrylic and sealed with a waterproof silicone sealer. The removable lid of the box is sealed by a foam rubber gasket and secured with four thumb screws. 

The front of the box also has a slot for inserting the sample sled while maintaining a seal. Each sample is 5cm x 5cm. The exterior side of the sample is chilled with a reservoir of ice that is kept at a constant level throughout the test. The humidity within the test chamber mirrored that of the office space in which the experiment was setup, hovering reliably at 30% RH. Inside the box, condensate produced by the sample is collected in a petri dish. 

An air quality sensor (FengSensor) sits within the box and records ambient temperature, relative humidity, CO2, CH2O, and PM2.5 at five minute intervals. Also within the box is a pollution source: a single match that is lit for 2 seconds, blown out, and then placed inside. 

Samples were generously prepared by Solomon Adera, of the Aizenberg Group, the Wyss Institute, and Harvard’s SEAS (See Appendix A). The following samples were were tested: 

Unfortunately, off-gassing from the acrylic and silicone prevented accurate measurement of VOC reduction by samples coated with the photocatalyst TiO2 (titania), provided by Tanya and Elijah Shirman of the Aizenberg Group, the Wyss Institute, and Harvard’s SEAS.

Baseline tests performed without any pollution source or sample heat sink established a steady state PM2.5 value of 12 µg/m3 within the test chamber. A subsequent series of three tests with a pollution source but no condensation established a typical drawdown time of 12 hours for the ambient PM2.5 levels to return to pre-test values. This return to baseline indicates that all particles released by the match had been sequestered by the internal surfaces of the test chamber. 

Image of experimental setup with sensors inside

Experimental results show that samples with high effusivity (Aluminum) were able to significantly reduce the PM2.5 drawdown time, while samples with low effusivity (Glass) were not. Qualitative observations confirmed that the Aluminum samples were able to produce significant amounts of condensate while the glass samples were unable. 

In a foundational paper for this study, the effectiveness of dew in the removal of PM from air is measured in three different weather conditions and at three particulate sizes. They measured the removal efficiency of the dew before sunrise on non-rainy days by measuring the weight of PM in the air versus the weight of PM in collected dew. This study was conducted in situ in urban settings in China, so this method was the only way to understand the effectiveness of dew in the open air (Xu and Zhu 2017).

Final Proposal

The environmental situation in South Korea demands attention. Industry and geography have conspired against the health of the citizens, and feeble regulatory legislation has had little impact on improving the situation. A global inventory of air quality indicators listed South Korea as 173 of 180 countries surveyed (Mosteller 2016). The Seoul Times reported on the study with the (only slightly) hyperbolic headline “Worst Air in the World.”

In order to address this issue, we propose a building typology, in which moisture is incorporated so that it can perform air filtration. The design is heavily influenced by Korean and Japanese vernacular buildings, in which environmental flows are permitted to penetrate the structure, and multiple layers modulate privacy, program, and climate (See Appendix B). 

Aluminum bumpy surfaces are installed on a series of exterior, semi-exterior, and semi-interior walls. Once on the wall, the high effusivity surfaces are thermally coupled to a heat sink, either passive (massive walls that provide chilling power via diurnal temperature swings) or active (capillary hydronic mats circulate chilled water, possibly geothermally coupled). As condensation collects on the surfaces, it drips down and is collected in a channel that could then be drained in the interior courtyard and into soil. Along their journey, the water droplets capture and flush harmful particulate matter from the ambient air at a low energy cost.  

In the best case scenario, the condensing surfaces are able to remove approximately 20% of particulates from the near surface layer of air (Xu and Zhu 2017). Additional collection efficiency gains can be made by increasing geometric complexity and exposed area, creating turbulent flow within the surface layer. This mechanism enables trees to reduce ambient airborne PM10 by 2-10% (Dzierzanowski 2011). In order to take advantage of these significant, yet subtle phenomena, we must both increase the total surface area of our condensing surfaces, increase the nucleation of droplets with bumpy surfaces (Park 2016) and give them a surface morphology that encourages adequate air mixing.

The plan is separated into three distinct areas, modeled after the Japanese zashiki. In order from outermost to innermost, we have the engawa, the covered courtyard, and finally the interior. Air quality is improved as one proceeds deeper within the house, which sensitive individuals (like the elderly) or activities (like sleeping) would primarily occupy. 

In addition to the bumpy surfaces, the proposed design incorporates skylights with TiO2 treated glass that acts as a photocatalytic surface to neutralize interior VOC’s.

Future Works

Based on our newfound understanding of how the materials and processes explored in our experiments relate to nested spaces in a vernacular architecture context, the door has now been opened to future experiments that may shed more light on our research thus far. The question at hand goes from “How do buildings achieve high interior air quality?” to  “How many layers are necessary to obtain a fully purified atmosphere?.” Understanding that full purification is not realistic or necessary, future research must work backwards from an accepted level.  Performance criteria and programmatic layering need to be explored in the newly complex interior-exterior transitional relationship. Further exploration is needed to understand at what layer these material applications perform at their highest level and at what depth does the system have the largest benefit to the inhabitants. Nuances in climate zone and program adjacencies will need to be tested, defined, and implemented as well as new standards of health, comfort and building technology if we are to grow out of the established vernacular.

The implementation of Sweating Surfaces is not limited to large scale examples of highly polluted urban landscapes.  Rural village with inadequate electricity and heavy smog events are just as if not more applicable. While not strictly reserved for passive applications, this is a hybrid material system that functions very well with little energy input and potentially, given the extant literature, on a wide range of scales.

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