PHYSICAL REVIEW E 89, 013110 (2014) Head-on collision of dust-acoustic solitons in a strongly coupled dusty plasma S. K. Sharma, A. Boruah, and H. Bailung Physical Sciences Division, Institute of Advanced Study in Science & Technology Paschim Boragaon, Guwahati-781035, India (Received 25 July 2013; published 31 January 2014) The collision between two counterpropagating dust acoustic solitary waves in a strongly coupled dusty plasma has been observed. The measured velocity and width of the solitary wave agree with the solution of the Korteweg–de Vries equation derived by using the generalized hydrodynamic model. The two counterpropagating solitary waves of equal amplitude merge into a single pulse with twice the individual soliton amplitude and then pass through each other. The solitons suffer a small time delay in propagation after collision. The measured delay time obtained from their trajectories is also presented. DOI: 10.1103/PhysRevE.89.013110 PACS number(s): 52.27.Lw, 52.27.Gr, 52.35.Sb I. INTRODUCTION A dust acoustic wave is the analog of an ion acoustic wave in the very low frequency regime where the dust mass provides the inertia and electrons and ions provide pressure to sustain the wave [1,2]. The nonlinear evolution of the dust acoustic wave into solitons has been well studied theoretically in an unmagnetized weakly coupled dusty plasma [1,3,4]. The effect of strong correlation among the dust particles, which arises when the ratio of intergrain potential energy to the dust thermal energy becomes greater than unity (i.e., the coupling parameter Ŵ> 1) [5], on the propagation of the dust acoustic solitary wave (DASW) has also been investigated using various theoretical models (see Ref. [6] and references therein). However, there have been very few experimental investigations of the DASW in laboratories. Bandyopadhyay et al. [7] studied the excitation and propagation of the DASW and compared their results with the solution of the Korteweg– de Vries (KdV) equation derived for a weakly coupled dusty plasma [3]. In a monolayer hexagonal dust lattice (for which Ŵ>Ŵ c , where Ŵ c is the critical coupling parameter for dust crystallization), solitary waves and their interactions have been studied [8,9]. One important criterion for a solitary wave to be identified as a soliton is that it has to survive a collision (head-on or overtaking) with another solitary wave [10]. In this paper, we report an observation of a head-on collision between two counterpropagating dust acoustic solitons in a strongly coupled dusty plasma in the regime 1 ≪ Ŵ<Ŵ c . II. EXPERIMENTAL PROCEDURE The experiment is performed in a cylindrical glass chamber which is 100 cm in length and 15 cm in diameter. The chamber is first evacuated down to 10 −4 Pa and then is filled with argon gas to attain a working pressure in the range of 0.1–2 Pa. Discharge is produced by applying rf power (13.56 MHz, 2–10 W) as described in Ref. [11]. A rectangular graphite plate (G) of dimension 30 cm (length) × 14.5 cm (breadth) × 0.2 cm (thickness) is kept horizontally inside the chamber with vertical fencing at both ends. Gold coated silica particles of diameter 5 μm and density 2.6 g cm −3 are injected into the plasma by using a piezoelectric buzzer fitted below the plate (G). The particles acquire a large amount of negative charge in the plasma and levitate in the plasma sheath boundary region above the plate by balancing gravitational force (downward) and the sheath electrostatic force (upward). The particle cloud is 2 to 3 mm thick and levitates 0.8–1 cm above the plate. A horizontal sheet of green laser light (532 nm, 70 mW) is used to illuminate the particles, and their motion is recorded (from top) in a high resolution digital camera with a Nikkor microlens attached to it. The particle cloud undergoes a transition into a highly ordered crystal structure when the chamber pressure is raised above a critical value (3 Pa) as observed in Ref. [11]. In this experiment, the working pressure is maintained in the range of 0.5–2.0 Pa so that the particles remain in the fluidlike state, i.e., the coupling parameter Ŵ lies in the range of 1 ≪ Ŵ<Ŵ c . Typical values of the plasma parameters are electron and ion densities on the order of 10 8 cm −3 , electron temperature 5 eV, ion temperature 0.1 eV, dust charge on the order of 10 4 electron charge, and dust density (estimated from the interparticle distance) on the order of 10 3 cm −3 [11]. Dust particles are considered to be at room temperature. It is to be noted that the growth of submicron size carbon dust particles due to sputtering of the graphite target has been observed in Ar plasma at 100 W rf power and −300 V dc self bias at the graphite target [12]. However, for the present experimental conditions, i.e., low rf power (6 W) and grounded graphite plate, the possibility of the formation of submicron size particles due to sputtering from the graphite plate is very low. The dust acoustic wave is excited by applying a continuous small amplitude sinusoidal signal (1–20 Hz) to the exciter E1. The exciter is a 0.6 cm wide graphite section placed on the same horizontal plane of the grounded graphite plate (Fig. 1). A small positive dc offset is also applied to the exciter prior to the application of the excitation signal. This dc offset creates a potential deep filled with particles just above the exciter. The measured wave numbers of the observed waves at different frequencies of the applied signal are found to follow the linear dispersion relation of the dust acoustic wave in the long wavelength limit [1]. The velocity of the observed dust acoustic wave is found to be 5.0–6.0 cm s −1 for the pressure in the range of 0.5–2.0 Pa. Then, instead of a sinusoidal signal, a short negative pulse of amplitude of 5–20 V (100 ms duration) is applied to E1 to excite the DASW. In order to observe collision between two DASWs, the short negative pulse is applied simultaneously to the exciters E1 and E2 which are 5.7 cm apart as shown in Fig. 1. The dimension of E2 is the same as E1. 1539-3755/2014/89(1)/013110(5) 013110-1 ©2014 American Physical Society