GoKawiilSiO 2 accounts for up to 60% of the Earth’s crust, primarily in minerals such as olivine and pyroxene1. Extraterrestrial bodies, including the Moon2, rocky planets3, chondrules and asteroids2, are derived from SiO 2 and other oxides (Al 2 O 3 , MgO and so on). Given their ubiquity, these materials play important roles in many natural processes. Of particular consequence is the exchange of electrical charge, or CE, that occurs when two oxides touch. For instance, electrostatic forces on CE-charged particles are implicated in the long residence times and extraordinary distances of fine sands (SiO 2 ) transported during Saharan dust storms4. In volcanic plumes, CE creates displays of ‘volcanic lightning’5,6 that, beyond spectacle, may have provided an energy source to convert primordial molecules into amino acids7. On the Moon, Mars and asteroids, CE presents substantial challenges during space missions8,9,13. Within stellar disks, attractive forces from CE are crucial for the early stages of rocky planet growth10, helping large objects form before gas drag causes them to spiral into their stars14,15.
Despite this universal relevance, we do not know why CE occurs in most situations, including the restricted case of insulator oxides. Furthermore, CE does not just occur between different oxides (for example, SiO 2 and Al 2 O 3 ) but also between samples of the same oxide (for example, SiO 2 and SiO 2 ). In the scenarios mentioned above, this ‘same-material’ CE is at play, despite any obvious symmetry-breaking parameter. Experiments show that work-function differences drive electron transfer between metals16,17,18, but no related mechanism has been demonstrated for oxides, different or same material. Different-oxide CE may correlate with acid/base properties, yet this again fails to explain same-material CE (refs. 19,20,21). Other experiments show that adding even a single monolayer of artificial molecular adsorbates can alter CE behaviour. For example, a fused silica (amorphous SiO 2 ) surface dressed with (γ-aminopropyl)dimethylethoxysilane charges positively to a ‘natural’ surface22 (that is, clean and stored in air). Similarly, soda–lime glass dressed with trimethylchlorosilane charges negatively to ‘natural’ soda lime23. This latter result23 is one of many that have pointed towards adsorbed water as an important factor in CE, with oxides and beyond24,25,26,27,28,29, although a definitive connection has remained elusive. Considering these factors, we might speculate that the symmetry-breaking factor responsible for same-material oxide CE resides in the ‘natural’ adsorbates that adhere to surfaces from their environment. Yet this raises questions. Do ‘natural’ adsorbates alone affect oxide CE at all? Which ones matter? And how can two samples in the same environmental conditions acquire differences in these adsorbates to break symmetry?
We explore these questions with the experiment shown in Fig. 1a–d. We use acoustic levitation to suspend a small (d ≈ 500 μm) spherical particle above a plate (Fig. 1a), both made from high-purity fused silica (Methods). We measure the charge of the sphere by applying an AC electric field sweep across the acoustic cavity (Fig. 1b) and fitting its trajectory to the equation of motion (Methods). To cause a collision, we briefly (roughly 25 ms) interrupt the acoustic field, letting the sphere fall and bounce off the plate (Fig. 1c). At the apex of its rebound, we reinitiate to ‘catch’ the sphere. Supplementary Video 1 further illustrates how the experiments work. We ensure that spheres/plates begin with zero charge through photoionization of the surrounding air (Fig. 1d and Extended Data Fig. 1). Spheres/plates are subjected to a standardized cleaning protocol: 30 min sonication in acetone, 30 min sonication in methanol, 30 min sonication in ultrapure water, 2 h baking at 200 °C and then storage for >72 h in the experimental chamber. Humidity (30 ± 1% relative humidity) and temperature (25 ± 1 °C) are controlled.
Fig. 1: Experimental set-up and baseline behaviour of nominally identical silica samples. a, We levitate a 500-μm silica sphere in an acoustic trap above a silica target plate. b, Charge is measured by applying a frequency-swept electric field, E(t), and extracting the trajectory of the sphere, y(t), using a high-speed camera. The amplitude and phase of y(t) at resonance depend, respectively, on the magnitude and sign of the particle’s charge, Q. c, Timed interruptions of the acoustic field trigger charge-exchanging collisions between the sphere and the plate, in which the sphere falls, bounces off the plate and is then ‘caught’ again. d, The entire system can be discharged using an X-ray photoionizer. e, Every sphere charges with a systematic sign and magnitude against its partner plate, but the slopes for an ensemble of sphere/plate pairs are spread randomly about zero. Source Data Full size image
Figure 1e shows the charge acquired over sequential bounces for an ensemble of sphere/plate pairs (see Methods for details on protocol and error bands). Each sphere charges with a definite sign and constant slope against its partner plate, yet for many pairs, the slopes spread randomly about zero. This behaviour—a systematic difference between any two samples but randomized across the ensemble—can be considered the baseline for what is to follow30. Paradoxically, it indicates that each SiO 2 sample behaves as a different material in relation to CE.
To test whether atmospheric adsorbates underlie this behaviour, we alter samples not by addition of artificial molecules22,23 but instead by subtraction of those naturally present. We do this with two common treatments: exposure to low-power plasma or mild baking. Both are widely used for removal of adsorbates, for example, in ultrahigh vacuum (UHV) or cleanrooms31. The procedure is shown in Fig. 2a. Two samples are prepared jointly with the standard protocol and their ‘baseline’ CE is measured. Then, one sample (S or P) is retrieved and either plasma treated (5 min) or baked (typically 2 h at 200 °C). The CE measurement is repeated immediately thereafter.
Fig. 2: Controlling charging behaviour by removing ‘natural’ adsorbates. a, We introduce a bias by exposing one sample, plate (P) or sphere (S) to either heat or mild plasma, with the intention of removing naturally occurring adsorbates. b, Spheres treated with plasma always charge negatively to their partner plate, regardless of how they charged before. Conversely, if the plate is treated with plasma, its partner sphere will charge more positively. c, Baking affects the samples in a similar way: baked samples always charge more negatively, to the point of polarity reversal if the temperature/duration of the bake is sufficiently high/long. d, Even a gentle bake of 75 °C for 15 min has a measurable effect, which is cumulative if done successively. Source Data Full size image
Figure 2b shows results for the case of plasma. The blue curves correspond to the baseline charging of two spheres against their partner plates. In both instances, CE happens to be positive at approximately 105 e per collision. After plasma treating a sphere, it charges negatively (about −105 e per collision). Conversely, treating a plate results in its partner sphere charging more positively (about 5 × 105 e per collision). Baking has the same effect: baking a sphere causes it to charge more negatively, whereas baking a plate causes its partner sphere to charge more positively (Fig. 2c). Temperature and duration matter: baking at 300 °C for 2 h almost always results in negative charging; baking at 200 °C for 2 h usually does; but even 75 °C for 15 min has an effect, which is cumulative if done successively (Fig. 2d).
These data contradict widespread arguments implicating adsorbed water as the symmetry breaker in oxide CE. Generally, it is argued that a hydrophilic surface charges positively to a hydrophobic one by means of donation of OH− (refs. 23,25). Plasma and baking make samples more hydrophilic and adsorbed water reaches equilibrium quickly owing to its abundance in air32,33. Yet we observe that these surfaces charge negatively, not positively. Figure 2 therefore offers provisional insight into our first question; naturally occurring adsorbates do indeed matter in same-material oxide CE but it is not clear that water is the symmetry breaker.
To search for other candidates, we perform time-of-flight secondary ion mass spectrometry (ToF-SIMS) on a silica plate after our standard cleaning protocol (Fig. 3a). Far from just SiO 2 and H 2 O, the surface hosts a cocktail of molecular and atomic species, with adventitious carbon—that is, naturally occurring carbonaceous molecules from the environment—as the marquee ingredient. Detected species include small moieties such as CH 3 (m/z = 15) but also larger ones such as C 3 H 6 , C 4 H 10 and C 6 H 10 (m/z ≈ 42, 58 and 82, respectively). We use spatially resolved ToF-SIMS to image two of these, C 3 H 6 and C 4 H 10 , which reveals extensive coverage (Fig. 3b). If we plasma treat or bake, these species are greatly reduced (see also inset to Fig. 3a and Extended Data Fig. 2). If we remove the sample from UHV, store it in air for several hours and image again, some return. In contrast to water, which readsorbs to an air-exposed32,33 or even UHV-exposed34 surface essentially instantly, these data suggest hours-scale dynamics in the readsorption of adventitious carbon. This allows us to make a prediction about the CE behaviour after baking or plasma—if it is connected to adventitious carbon, it should slowly change with time.
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