## 2 Related Work  
Opportunistic ad hoc networks are characterised by typically intermittent contacts of short duration between pairs of mobile devices. The message delivery probability depends on the contact duration, which is determined by the mobility of the users. The only way to increase this contact duration is by promoting interactions between users. However, people can hold back from cooperating with others for lack of trust or simply to save energy. In order to improve the delivery ratio, some authors like [19; 20; 21; 22] have proposed new collaboration mechanisms between mobile users. Regarding collaboration or cooperative approaches, we can find research works that encourage collaboration through credit systems. More specifically, in [23; 24], the authors affirm that, when a cooperative system offers security and reliability, mobile users are more stimulated to cooperate, especially when the network includes several unknown users. In [25], we can find another method to stimulate the collaboration with the simple idea of giving more credits to those who send more information in a cooperative way. They conclude that a credit-rewarding system stabilises the network, increasing data traffic and cooperation. In this context, some nodes can refuse to cooperate because of security reasons. It is also possible that malicious and/or selfish nodes could cause a negative impact on network performance. Many interesting proposals focused on this topic. In [26], a novel approach is described to detect and avoid selfish nodes using an improved watchdog mechanism [27] that detects selfish nodes with greater accuracy and speed than classical watchdog methods. The authors of [28] considered a decentralised environment and heterogeneous wireless networks. They proposed a distributed algorithm approach with an analytical basis that enables the interaction in the opportunist network according to their needs, while _pseudo-encouraging_ collaboration through punishment to selfish nodes. Considering that a cooperative system is a social opportunistic network where the message diffusion level is crucial, in [29; 30], the authors examine a utility-based cooperative data dissemination system where the utility of data is defined based on the social relationships between users. They studied and validated the performance of this system through an analytical model, allowing characterisation of the data diffusion process. Furthermore, they analysed the behaviour of the system with respect to key parameters such as the definition of the data utility function, the initial data allocation on nodes, the number of users in the system, and the data popularity. In this context, the authors of [31] used theoretical analysis applied to social networks to classify and study some diffusion schemes based on the _homophily_ (social networks phenomenon) by combining node relationships and their interests in the data. They noticed that, in order to improve diffusion performance, a node should first diffuse, when meeting a friend, the most similar data according to their common interests, while, when meeting a stranger, it should first diffuse the most dissimilar data according to their common interests. A recent paper [32] evaluates the impact of human behaviour in the opportunistic forwarding of messages. Experimental results using the ONE simulator show that, as nodes start to move less (i.e., increasing pause time), fewer messages are expected to be delivered. Since nodes start to encounter each other at a less frequent rate, the number of replications is also expected to decrease. Regarding latency, messages are to experience longer waiting times prior to delivery. An analytical model based on Delay Differential Equations is proposed for the authors of [33; 34] to evaluate through simulations the diffusion of messages in social groups taking into account the transmission time of messages. The idea of actively involving users with tasks of minimum impact on their regular activities is also employed by crowd-based participatory systems [35; 36].