Wood Identification: Wood discs (50 mm long) were made from freshly cut logs of Samanea saman (Jacq.) Merr., with their bark still attached from 2 m above ground level. The 20-year-old specimen was collected from Dhaka, Bangladesh (23°51´N, 90°24´E). Discs were then air-dried in the laboratory. From the air-dried discs, pieces of wood were separated, including sapwood and heartwood, and converted as shown in Figure 1. Microscopic slides and macerations were made according to standard techniques (Baas and Zhang, 1986). To confirm the deposits in the vessels, microtomed sections were stained with iodine and chrysoidin. This staining method, which has been proven to permanently stain starch in tree tissue, facilitated microscopic observation (Essiamah and Eschrich, 1985). The sections were stained with chrysoidin (1% chrysoidin in 70% ethyl alcohol) solution for 5 min and then were counterstained with Lugol’s solution for 7 min. The stained samples were mounted with euparal for microscopic observation. Samples for FESEM (field emission scanning electron microscope) observation were prepared according to Exley et al. (1977). At different resolutions and magnifications the wood was then examined at the accelerating voltage of 15 kV in a Hitachi S-4300 FESEM. Different microscopic measurements were made for 2 diffuse porous wood species, viz. Populus tomentiglandulosa T. Lee and Gmelina arborea Roxb. These measurements were made in order to determine which anatomical and microstructural features might be responsible for the variation in liquid penetration. The permeability of these 2 species was reported by Ahmed and Chun (2007), Choi et al. (2007), and Ahmed et al. (2007). In all, 100 measurements of various micro-structural features were made. The terminology used herein and the method used to determine quantitative features conform to the recommendations of the IAWA Feature List (IAWA Committee, 1989).
Block Preparation: Two 50 × 50-mm stakes were taken from outer sapwood and heartwood, and each was cut into 3 specimens (producing 6 specimens each 50 mm long), which were then further divided along their length into 3 blocks–a 5-mm block for moisture content measurement, a 40-mm block for treatment, and another 5-mm block microscopic examination. For safranine treatment the exact air dried sample size was maintained at 40 mm (longitudinal) × 10 mm (radial) × 5 mm (tangential). Samples for treatment were sealed with silicon resin, leaving their ends or radial faces open, allowing either longitudinal or radial flow (Figure 1). This was done to prevent leakage onto other surfaces. Preparation of Safranine Solution (1%) Into a 1-l volumetric flask, 10 g of safranine powder (Junsei Chemical Co., Ltd.) was placed and 500 ml of 50% ethyl alcohol was added. After mixing properly, distilled water was added to bring the volume to 1 l. Safranine solution was used because of its bright color and it facilitates observation of the penetration path in different cells. Measurement of Surface Tension of Safranine Solution If a liquid has a contact angle, θ, with a thin tube it produces an associated vertical force on the liquid, the capillary force of which is given by: 2πrγCos(θ) (1) where r is the radius of the capillary (cm) and γ is the surface tension (dynes cm−1). If the wetting angle, θ, is < 90°, the liquid will rise in a thin tube, whereas if the angle is > 90°, the liquid in the capillary will drop. This is due to the surface energy between the liquid and the walls of the capillary. If this energy is favorable, the liquid wants to increase its contact with the capillary surfaces, which induces the liquid to rise in the capillary tube. The height to which a liquid rises or falls is determined by a balance between the capillary force and the force of gravity, πr2hρg. When the liquid is in a stable condition (neither rising nor receding), these 2 forces are equal; so: γ(2π r) = ρh(π r2)g (2) where ρ is density (g cm-3), g is gravity acceleration (980.665 cm s-2), and h is height (cm) of the capillary. From equation (2) we can obtain the surface tension: γ = 1/2(ρghr) (3) Surface tension of the safranine solution was measured with the capillary rise method, using equation (3) at 24 °C. The unit was expressed in dynes cm-1. Observation of Liquid Impregnation Safranine solution penetration behavior was observed with an i-Camscope (model SV32). While observing safranine solution penetration, the room temperature was 24 °C and the wind speed was 0 m s-1. The coated sample was fixed on a petri dish and the safranine solution was poured onto it. Using the i-Solution v.2.5 image processing program, a 5-min safranine solution impregnation video file was captured, which contained 300 frames. Specific digital images at 1, 2, 3, 4, and 5 min were selected from the captured video frames with VitrualDub v.1.6.16 software. Then, penetration depth was measured in millimeters from the digital images with the i-Solution program. Three replications were conducted to measure penetration depth and for each replication, 7 measurements were made.