Foam Drainage Investigated Using Terahertz Spectroscopy

The region of the electromagnetic spectrum known as terahertz (tera = 1012) has remained relatively unexplored due to the limitations of microwave and infrared sources.1–3 Microwave sources, which rely on electronic propagation of single- or low-order-guided waves, are only suitable below 100 gigahertz (giga = 109), while optically generated IR multimode beams such as CO2 lasers are unsatisfactory below 30 THz. However, new methods for the generation and detection of THz radiation have been revealed, including photoconductive antennae, electrooptic crystals, backward-wave oscillators, and optical mixing.4–6 These methods have been employed to generate and detect the frequencies from 0.1 to over 10 THz, ideal for detection of rotational and vibrational energy transitions in polar molecules such as water.

Figure 1 - Basic schematic of THz instrument design.

The commercially available terahertz time–domain spectrometer (THz–TDS) used in this study is shown schematically in Figure 1 (T-Ray 2000, Picometrix, an Advanced Photonix company, Ann Arbor, MI). A Vitesse diode-pumped, mode-locked Ti:sapphire laser produces <100-fsec pulses centered at 800 nm. These pulses are coupled into fiber optic lines to remove additional steering optics and provide ease in directing the beam. To correct for optical pulse dispersion by the fiber optic components of the instrument, a grating dispersion compensator imparts negative group velocity dispersion to the optical pulse. Since the pulse will naturally disperse while traveling in the fiber optics, it is essentially compressed negatively to balance the broadening. The laser pulse then passes through a beamsplitter, which directs one resultant beam to the THz transmitter and the other to the detector. Both transmitter and detector consist of photoconductive antennae, namely low-temperature grown (LT)-GaAs. When illuminated by the excitation beam, the transient current in the biased GaAs emitter generates THz frequencies. From the transmitter, the THz radiation is focused on the sample via a high-density polyethylene (HDPE) collimating lens, and the radiation transmitted through the sample passes through an identical HDPE lens and on to the detector. The computer controls a delay line, which varies the optical delay; this delay gates the interrogation of the THz pulse by the initial fsec laser pulse. Only when the THz pulse and laser are present on the antenna simultaneously is there a current produced; this current, as a function of delay time, is the time-dependent signal. Through Fourier transform (FT) analysis, the time-domain data can be expressed in the frequency domain, and absorbance peaks are evident.

Radiation absorption by water has been widely explored using THz systems.7–10 The sheer number of rotational and vibrational energy transitions available in H2O molecules results in broad absorbance peaks in the THz spectrum of liquid water, particularly in the frequency range of 0.3–2.0 THz. This phenomenon creates potential for analyzing water-containing samples having little or no absorptive characteristics (besides the water within). One particular system of interest is aqueous foams.

Foams are used in various applications, including oil recovery, brewing, personal care, medicine, and biotechnology.11,12 Structural properties of the foam in each application are critical to proper performance and stability. Due to the fragile and dynamic nature of foams, these properties and flow dynamics are difficult to measure. Temperature fluctuations and mechanical stresses may cause foams to degrade or lose functionality.12 Therefore, it is imperative to have an effective measurement and qualifying method that characterizes the foam, yielding drainage rate data. Foam properties such as rigidity, structure, and mechanical strength can be extracted from drainage rate profiles.13,14

Figure 2 - Interfaces present during drainage of water through foam structures.

Drainage is defined as the liquid flow between fragile film membranes (lamellae) via plateau borders under the influence of gravity and capillary forces.11 As the water drains from these lamellar regions into plateau borders, the foam gas bubbles become less stable and are increasingly susceptible to bursting. The Gibbs-Marangoni stabilization, however, plays a role in maintaining bubble life. Here, surfactant molecules diffuse to thinned areas, providing additional support before the bubble bursts. This stabilization makes it difficult to predict foam behaviors using models and theory. Figure 2 labels interfaces of a draining region consisting of the thin films (lamellae), a plateau border, and gas regions.

Several methods currently exist to study foams. Table 1 lists a fraction of these methods and touches on the positives and negatives inherent to each method.

Table 1 - List of methods used to characterize foams and brief advantages/disadvantages

Although the techniques listed offer important information, many are too costly, do not provide sufficient data, or are not feasible for fast-throughput environments. Also, it is during real-time drainage that important data are extracted. Therefore, a method that can collect real-time data of a draining foam system is required to sufficiently characterize the foam properties.

Foam studies have been made possible using the absorption of THz radiation by water in aqueous foams. Data have been obtained on the following systems: sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB), Triton X-114 (Rohm and Haas, Philadelphia, PA), Ultra Dawn® dish soap (Procter and Gamble Co., Cincinnati, OH), and Guinness beer (Guinness and Co., Dublin, Ireland). These samples are representative of anionic surfactant, cationic surfactant, nonionic surfactant, mixed surfactants, and protein surfactant solutions, respectively. Two different pore sizes were used with all the systems except the Guinness beer, and five scans were averaged for each system studied.

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